An RV "Generator Start Battery" regulator/controller for use with a LiFePO4 power system

I was recently retrofitting my brother's RV's electrical system with LiFePO₄ batteries (ReLi³on RB-100's).  This retrofit was done to allow much greater "run time" at higher power loads and to increase the amount of energy storage for the solar electric system while not adding much weight, not needing to vent corrosive fumes.  (These types of batteries, LiFePO₄,  are very safe - e.g. they don't burst into flame if damaged or abused.)

While I was doing this, I began to wonder what to do about the generator "start" battery.

Charging LiFePO4 batteries in an RV The voltage requirements for "12 volt" Lead-Acid batteries are a bit different from those needed by LiFePO4 "12 volt" batteries: Lead acid batteries need to be kept at 13.2-13.6 volts as much as possible to prolong their life (e.g. maintained at "full charge" to prevent sulfation). LiFePO4  batteries may be floated anywhere between 12.0 and their "full charge" voltage of around 14.6 volts - but they will not be fully recharged unless they are held near the high end of this range. Routinely discharging lead-acid batteries below 50% can impact their longevity - and they must be recharged immediately to prevent long-term damage. LiFePO4  batteries may be discharged to at least 90% routinely - and they may be left there, provided their voltage is not allowed to go too low. Lead acid batteries may be used without any management hardware:  Maintaining a proper voltage is enough to ensure a reasonable lifetime. LiFePO4 batteries must have some sort of battery management hardware to protect against overcharge and over-discharge as well as to assure proper cell equalization.  Many modern LiFePO4 batteries (such as the "Rel3ion" devices used here) have such devices built in. Conventional RV power "converters" are designed to apply the proper voltage to maintain lead-acid batteries (e.g. maintain at 13.6 volts.) Because LiFePO4 batteries require as much as 14.6 volts to attain 100% charge (a reasonable charge may be obtained at "only" 14.2 volts) connecting them directly to an existing RV with this lower voltage means that they may never be fully-charged!  Modern, programmable chargers (e.g. inverter-chargers, solar charge controllers) have either "LiFePO4 " modes or "custom" settings that may be configured to accommodate  the needs of LiFePO4 batteries.  While the lower voltage (nominal 13.6 volts) will not hurt the LiFePO4 batteries, they likely cannot be charged to more than 40-75% of their rated capacity at that voltage.  (approx. 13.6-13.7 volts is the lowest voltage were one can "mostly" charge a LiFePO4 battery.) Because of Peukert's law, one can only expect 25-50% of the capacity of a lead-acid battery to be available at high amperage (e.g. 0.5C or higher) loads. With LiFePO4 batteries, more than 80% of the battery's capacity can be expected to be available at similar, high-amperage.  What this means is that at such high loads, a LiFePO4 battery can supply about twice the overall power when compared with a lead-acid battery of the same amp-hour rating.  At low-current loads the two types of batteries are more similar in their available capacity. In short:  Unless an existing charging system can be "tweaked" for different voltages and charging conditions, one designed for lead-acid batteries may not work well for LiFePO4 batteries.  In some cases it may be possible to set certain "equalize" and "absorption" charge cycle parameters to make them useful with LiFePO4s, but doing this is beyond the scope of this article.

Originally the RV had been equipped with two "Group 24" deep-cycle/start 12 volt batteries in parallel (a maximum of, perhaps, 100 amp-hours, total, when brand new, for the pair of "no-name" batteries supplied) to run things like lights, and the pump motors for the water system, jacks and slide-outs and as the "start" battery for the generator.  Ultimately we decided to wire everything but the generator starter to the main LiFePO₄ battery bank.

Why?

Suppose that one is boondocking (e.g. "camping" away from any source of commercial power) and the LiFePO₄ battery bank is inadvertently run down. As they are designed to do, LiFePO₄ battery systems will unceremoniously disconnect themselves from the load when their charge is depleted to prevent permanent damage, automatically resetting once charging begins.
 
If that were to happen - and the generator's starter was connected to the LiFePO₄ system - how would one start the generator?

Aside from backing up the towing vehicle (if available), connecting its umbilical and using it to charge the system just enough to be able to get the generator started, one would be "stuck", unable to recharge the battery.  What's worse is that even if solar power is available, many charge controllers will go offline if they "see" that the battery is at zero volts (e.g. when they are in that "disconnected" state) - even if the sun is shining, preventing charging from even starting in the first place!

What we needed was a device that would allow the starting battery be be charged from the main battery, but prevent it from back-feeding and being discharged.


Note:

It is common in many RVs for the generator to not charge its own starting battery directly, via an alternator.  The reason for this is that it is assumed by the makers of the generators and RVs that the starting battery will be charged by the towing vehicle and/or via the RV's electrical system via its AC-powered "voltage converter", powered from "shore" power or via the generator's AC output.

But first, a few weasel words:

• Attempt to construct/wire any of the circuits only if you are thoroughly familiar with electronics and construction techniques.
• While the voltages involved are low, there is still some risk of dangerous electric shock.
• With battery-based systems extremely high currents can present themselves - perhaps hundreds or even thousands of amps - should a fault occur.  It is up to the would-be builder/installer of the circuits described on this page - or anyone doing any RV/vehicle wiring - to properly size conductors for the expected currents and provide appropriate fusing/current limiting wherever and whenever needed.  If you are not familiar with such things, please seek the help of someone who is familiar before doing any wiring/modifications/connections!
• This information is presented in good faith and I do not claim to be an expert on the subject of RV power systems, solar power systems, battery charging or anything else.
• You must do due diligence to determine if the information presented here is appropriate for your situation and purpose.
• YOU are solely responsible for any action, damage, loss or injury that might occur.  You have been warned! 

Why a "battery isolator" can't be used:

If you are familiar with such things you might already be saying "A device like this already exists - it's called a 'battery isolator'" - and you'd be mostly right - but we can't really use one of these devices because LiFePO₄ batteries operate at a full-charge voltage of between 14.2 and 14.6 volts, and the battery isolator would pass this voltage through, unchanged.  If you apply 14+ volts to a "12 volt" lead-acid battery for more than a few days, you will likely boil the away electrolyte and ruin it!

What is needed is a device that will:

• Charge the generator start battery from the main (LiFePO₄ ) battery system
• Isolate it from the main battery, and 
• Regulate the voltage down to something that the lead-acid chemistry can take - say, somewhere around 13.2-13.6 volts.

In this case the main LiFePO₄ battery bank will be maintained via the AC-powered (generator or shore) charging system and/or the solar power converters at its normal float voltage, so it makes sense to use it to keep the start battery fully-charged.

The solution:

After perusing the GoogleWeb I determined that there was no ready-made, off-the-shelf device that would do the trick, so I considered some alternatives that I could construct myself.

Note:  The described solutions are appropriate only where the main LiFePO₄ bank's voltage is just a bit higher (a few volts) than the lead-acid starting battery:  They are NOT appropriate for cases where a main battery bank of a much higher voltage (e.g. 24, 48 volts, etc.) is being used to charge a "12 volt" starting battery.

Simplest:  "Dropper diodes":

Because we need to get from the nominal 14.2-14.6 volts of the LiFePO₄ system down to 13.2-13.7 volts it is possible to use just two silicon diodes in series, each contributing around 0.6 volts drop (for a total drop of "about" 1.2 volts) to charge the starting battery, as depicted in Figure 1, below.  By virtue of the diodes' allowing current flow in just one direction, this circuit would also offer isolation, preventing the generator's battery from being discharged by back-feeding into the main battery.

To avoid needing to use some very large (50-100 amp) diodes and heavy wire to handle the current flow that would occur when the starter motor was active - or if the start battery was charging heavily - one simply inserts some series resistance to limit the current to a few amps.  Even though this would slow the charging rate somewhat, the starting battery would be fully recharged within a few hours or days at most - not a problem considering the rather intermittent use of the starting battery - more about that later.

Figure 1.
This circuit uses a conventional tungsten-filament "1157" tail/turn signal bulb (NOT an LED replacement!) with both filaments tied together, providing more versatile current limiting.  Please read notes in the text concerning mounting of the light bulb.
The diodes (D1 and D2) should be "normal" silicon diodes rather than "Shottky" types as it is the 0.6 volt voltage drop per diode that we need to reduce the voltage from the LiFePO₄ stack to something "safe" for lead-acid chemistry.  If one wished to "tweak" the voltage on the starting battery, one could eliminate one diode or even replace just one of them with a Shottky diode to increase the lead-acid voltage by around 0.2-0.3 volts.
The use of a current-limiting device like a tungsten light bulb allows lighter-gauge wire to be used to connect the two battery systems together.
Click on the image for a larger version.

In lieu of a large power resistor, the ubiquitous "1157" turn signal/brake bulb is used as depicted in Figure 1.  Both filaments are tied together (the bulb's bayonet base being the common tie point) providing a "cold filament" resistance of 0.25-0.5 ohms or so, increasing to 4-6 ohms if a full 12 volts were placed across it.  The reason for the use of a light bulb will be discussed later.

Although not depicted in Figure 1, common sense dictates that appropriate fusing is required on one or both of the wires, particularly if one or more of the connecting wires is quite long, in which case the fuse would be placed at the "battery" end (either LiFePO₄ or starting battery) of the wire(s) to provide protection should a fault occur between that source and the charge controller:  Fusing at 5-10 amps is fine for the circuit depicted.

This circuit is "good enough" for average use and as long as the LiFePO₄ bank is floated at 14.2 volts with occasional absorption peaks at 14.6 volts, the lead-acid starting battery will have a reasonably long life.

A regulator/limiter circuit:

As I'm wont to do, I decided against the super simple "dropper diode and light bulb" circuit - although it would have worked fine - instead, designing a slightly fancier circuit to do about the same as the above circuit, but have more precise voltage regulation.  While more sophisticated than two diodes and a light bulb, the circuit need not be terribly complicated as seen in Figure 2, below:

Figure 2:
The schematic diagram of the slightly more complicated version that provides tight voltage regulation for the starting battery.  As noted on the diagram, appropriate fusing of the input/output leads should be applied!
This diagram depicts a common ground shared between the main LiFePO₄ battery bank and the starting battery, usually via the chassis or "star ground" connection.  In the as-built prototype, Q2 was an SUP75P03-07 P-channel power MOSFET while D1 was an MR750 5 amp, 50 volt diode. A circuit board is not available at this time.
NOT SHOWN is the fusing of the input and output leads, near-ish their respective batteries/source connections, with 10 amp automotive fuses.
Click on the image for a larger version.

How it works:

U1 is the ubiquitous TL431 "programmable Zener".  If the "reference" terminal (connected to the wiper of R5) of this device goes above 2.5 volts, its cathode voltage gets dragged down toward the anode voltage (e.g. the device turns "on").  Because R4, R5 and R6 form an voltage divider, adjustable using 10-turn trimmer potentiometer R5, the desired battery float voltage may be scaled down to the 2.5 volt threshold required by U1.

If the battery voltage is below the pre-set threshold (e.g. U1 is "seeing" less than 2.5 volts through the R4/R5/R6 voltage divider) U1 will be turned off and its cathode will be pulled up by R2.  When this happens Q1 is biased on, pulling the gate of P-channel FET Q2 toward ground, turning it on, allowing current to flow from the LiFePO₄ system, through diode D1 and light bulb "Bulb1" and into the starting battery.

By placing R1 and R2 on the "source" side of FET Q2, the circuit is guaranteed to have two potential sources of power:  From the main LiFePO₄ system, through D1, and from the starting battery via the "backwards" intrinsic diode inside Q2.  The 15 volt Zener diode (D2) protects the FET's gate from voltage transients that can occur on the electrical system.

Figure 3:
The completed circuit, not including the light bulb, wired on a small
piece of perforated prototype board.
A printed circuit board version is not available at this time.
Click on the image for a larger version.

Once the starting battery has attained and exceeded the desired float voltage set by R5 (typically around 13.5 volts for a "12 volt" lead-acid battery) U1's reference input "sees" more than 2.5 volts and turns on, pulling its cathode to ground.  When this happens the voltage at the base of Q1 drops, turning it off and allowing Q2's gate voltage, pulled up to its source by R1, to go high, turning it off and terminating the charge.

Because the cathode-anode voltage across U1 when it is "on" is between 1 and 2 volts it is necessary to put an additional voltage drop in the emitter lead of Q1, hence the presence of LED1 which offsets it by 1.8-2.1 volts.  Without the constant voltage drop caused by this LED, Q1 would always stay "on" regardless of the state of U1.  Capacitor C1, connected between the "reference" and the cathode pins of U1 prevent instability and oscillation.

In actuality this circuit linearly "regulates" the voltage to the value set by R5 via closed loop feedback rather than simply switching on and off to maintain the voltage.  What this means is that between Q2 and the light bulb, the voltage will remain constant at the setting of R5, provided that the input voltage from the LiFePO₄ system is at least one "diode drop" (approx. 0.6 volts) above that voltage.  For example, if the output voltage is set to 13.50 volts via R5, this output will remain at that voltage, provided that the input voltage is 14.1 volts (e.g. 13.5 volts plus the 0.6 volts drop of diode D1) or higher.

Because Q2, even when off, will have a current path from the starting battery to the main LiFePO₄ bank due it its intrinsic diode, D1 is required to provide isolation between the higher-voltage LiFePO₄ "main" battery bank and the starting battery to prevent a current back-feed.  Were this isolation not included, if the main battery bank were to be over-discharged, current would flow backwards, through FET Q2, from the generator starting battery and discharge it, possibly to the point where the generator could not be started.

Again, D1's 0.6 volt (nominal) drop is inconsequential provided that the LiFePO₄ bank is at least 0.6 volts above that of the starting battery, but this will occur very frequently if the charge on that bank is properly maintained via generator, solar or shore power charging.  A similar (>= 5 amp) Shottky diode could have been used for D1 to provide a lower (0.2-0.4 volt) drop, but a silicon diode was chosen because it was on hand.

Testing the device:

Assuming that it is wired/built correctly, connect a variable power supply to the input lead to simulate the LiFePO₄ battery bank.  Setting the voltage a volt or two higher than the expected float voltage (e.g. 14.5-16 volts) adjust R5 to attain the desired start battery float voltage (13.50-13.7 volts is recommended - I use 13.55 volts) as measured on either side of "Bulb1".  Adjust the power supply voltage up and down a bit (e.g. below 12 volts and up to 17 volts) and if working correctly, the output voltage from the circuit should be rock-steady as long as the input voltage is about 0.6 volts above the set output voltage.

Now short the output leads (e.g. the "positive" output lead should be going through "Bulb1") and the light bulb should illuminate fully - assuming that your variable voltage supply is capable of supplying the 3-ish amps needed for the lamp.  Measuring directly at the circuit board's "ground" (common "battery negative") terminal and at the connection between Q2 and "Bulb1" you should still have the voltage set by R5 within a few hundredths of a volt.

Note:  If you were to measure connect the negative lead of the voltmeter to the power supply or the shorted output leads the measured voltage would be a bit lower owing to voltage drop along the wires.

Shorting the output leads and measuring the voltage as done in the previous step demonstrates two important design points:

• That the voltage at the output of Q2 remains steady from no-load to maximum current conditions.
• That the light bulb is properly acting as a current limiting device.

While doing this "short circuit" test, make sure that the heat from the light bulb rises away from the circuit board itself and that the means of mounting it is capable of withstanding the bulb's heat without burning or melting anything.

Connecting the device:

On the diagram only a single "Battery negative" connection is shown and this connection is to be made only at the starting battery.  Because this circuit is intended specifically to charge the starting battery, both the positive and negative connections should be made directly to it as that is really the only place where we should be measuring its voltage!

Also noted on the diagram is the assumption that both the "main" (LiFePO₄ ) battery and the starting battery share a common ground, typically via a common chassis ("star") ground point which is how the negative side of the starting battery ultimately gets connected to the negative side of the main LiFePO₄ bank:  It would be rare to find an RV with two battery systems of similar voltages where this was not the case!

Finally, it should go without saying that appropriate fusing be included on the input/output leads that are located "close-ish" to the battery/voltage sources themselves in case one of the leads - or the circuit itself - faults to ground:  Standard automotive ATO-type "blade" fuses in the range of 5-10 amps should suffice.  In order to safely handle the fusing current and to minimize voltage drop while charging the connecting wires to this circuit should be in the range of 10 to 16 AWG with 12-14 AWG being ideal.

What's with the light bulb?

Figure 4:
The circuit  board mounted in an aluminum chassis box along with the
light bulb.  Transistor Q2 is heat-sinked to the box via insulating hardware
and the board mounted using 4-40 screws and aluminum stand-offs.  The light
bulb is mounted to a small terminal lug strips using 16 AWG wire soldered
to the bulb's base and the bottom pins:  A large "blob" of silicone (RTV)
was later added around the terminal strip to provide additional support.
Both the bottom of the box (left side) and the top include holes to allow
the movement of air to help dissipate heat.  Holes were drilled in the back
of the box (after the picture was taken) to allow mounting.
This box is, in this picture, laying on its side:  The light bulb would be
mounted UP so that its heat would rise away from the circuitry via
thermal convection.
Click on the image for a larger version.

The main reason for using a light bulb on the output is to limit the current to a reasonable value via its filament.  When cold, the parallel resistance of the two filaments of the 1157 turn-signal bulb is 0.25-0.5 ohms, but when it is "hot" (e.g. lit to full brilliance) it is 4-6 ohms.  Making use of this property is an easy, "low tech" way to provide both current limiting and circuit protection and, when the filament is cold (e.g. charging battery "mostly" charged), increase the amount of charging current that can flow.  Taking advantage of this changing resistance of a light bulb allows higher charging current that would be practical with an ordinary resistor.


In normal operation the light bulb will not glow - even at relatively high charging current:  It is only if the starting battery were to be deeply discharged and/or failed catastrophically (e.g. shorted out) that the bulb would begin to glow at all and actually dissipate heat.  

Limiting the charging current to just a few amps also allows the use of small-ish (e.g. 5 amp) diodes and reduce the heat that could be dissipated during regulation allowing the use of an aluminum box as Q2's heat sink, but more importantly it allows much thinner and easier-to-manage wire (as small as 16 AWG) to be used since the current can never be very high in normal operation.  Limiting the charging current is just fine for the starting battery due to its very occasional use:  It would take only an hour or two with a charge current to top off the battery after having started a generator on a cold day!

As noted on the diagram and in previous text the light bulb must be mounted such that its operating temperature and heat dissipation at full brilliance will not burn or melt any nearby materials as the glass envelope of the bulb can will easily exceed the boiling temperature of water!  With both the "simple" diode version in Figure 1 and the more complex version in Figure 2 it is recommended that the bulb is mounted above the circuitry to take advantage of air convection to keep the components cool as shown in Figure 4.  If a socket is available for the 1157 bulb, by all means use it, but still heed the warnings about possible amount of heat being produced.

In operation:

When this circuit was first installed, the starting battery was around 12.5 volts after having sat for a week or two (during the retrofit work) without a charging source and having started the generator a half-dozen times.  With the LiFePO₄ battery bank varying between 13.0 and 14.6 volts with normal solar-related charge/discharge cycles, it took about 2 days for the start battery to work its way up to 13.2 volts, at which point it was nearly fully charged - and then the voltage quickly shot up to the 13.55 volts as set by R5.  This rather leisurely charge was mostly a result of the LiFePO₄ bank spending only brief periods above 13.8 volts.

Even though this doesn't very quickly charge the battery under normal conditions, as we'll see below, this isn't really important.

How much of the starting battery's capacity is being used?

If one were to assume that the generator was set to run once per day and pull 100 amps (a current likely seen on a very cold day!) from the battery for 5 seconds this would represent (100 amps * ( 5 sec/3600sec )) = about 0.14 amp-hours: - This happens to be about the same amount of energy as is contained in 4 fresh hearing-aid batteries or about 1/10th of the capacity of a single AAA cell!

From this we can see that this "100 amps for 5 seconds" is an average current of just over 5 milliamps (1/200th of an amp!) when spread across 24 hours - a value likely comparable the self-discharge rate of the battery itself.   By these numbers you can see that it does not take much current at all to sustain a healthy battery that is used only for starting!  Because this battery is never used for running things like lights or motors, it really never gets abused by being deeply discharged.

A standard group 24 "deep cycle starting" battery was used since it and its box had come with the RV.  In this particular application, for generator starting only, a much smaller battery - such as one used for starting 4x4s or motorcycles - would have sufficed and saved a bit of weight and space.

The advantage of the group 24 battery is that it, itself, isn't particularly heavy and it is readily available in auto-parts, RV and "big box" stores everywhere.  Because it is used only for starting the generator, it need not have been a "deep cycle" type, but rather a normal "car" battery - although the use of something other than an RV-type battery would have necessitated re-working the battery connections as RV batteries have handy nut/bolt posts to which connections may be easily made.


Final comments:


There are a few things that this simple circuit will not do, including "equalize" the lead acid battery and compensate for temperature - but this isn't terribly important, overall in this application.


Concerning equalization:

Even if the battery is of the type that can be equalized (many sealed batteries, including "AGM" types - those mistakenly called "gel cells" - should never be equalized!) it should be remembered that it is not the lack of equalization that usually kills batteries, but rather neglect:  Allowing them to sit for any significant length of time without keeping them floated to above 2.17 volts/cell (e.g. above 13.0 volts for a "12 volt" battery) or, if they are the sort that need to be "watered" and not keeping their electrolyte levels maintained.  Failure to do either of these will surely result in irreversible damage to the battery over time.

It is also common practice to adapt the float voltage to the ambient temperature, but even this is not necessary as long as a "reasonable" float voltage is maintained - preferably one where water loss is minimized over the entire expected temperature range.  Again, it is more likely to be failure of elementary battery maintenance that will kill a battery prematurely than a minor detail such as this.

Practically speaking, if one "only" maintains a proper float voltage and keeps them "watered" the starting battery will likely last for at least the 3-5 year expected lifetime, particularly since, unlike battery in standard RV service, this starting battery will never be subjected to the deep discharge cycles which can really take a toll on a lead-acid battery.  While an inexpensive, no-name "group 24" battery, when new, may have a capacity of "about" 50 amp-hours, it won't be until the battery has badly degraded - probably to the 5-10 amp-hour range - where one will begin to notice starting difficulties.

Important also is the fact that the starting battery in this RV is connected to part of the main LiFePO₄'s battery monitoring system (in this case, a Bogart Engineering TM-2030-RV).  While this system's main purpose is to keep track of the amount of energy going into and out of the main LiFePO₄ battery, it also has a "Battery #2" input connection where one can check the starting battery's voltage - always a good thing to do at least once every day or two when one is "out and about".

Finally, considering the very modest requirements for a battery that is used only for starting the generator, it would take only a very small (1-5 watt) solar panel (plus regulator!) to maintain it.  While this was considered, it would have required that such a solar panel be mounted, wires run from it to the battery (not always easy to do on an RV!) and everything be waterproofed.  Because the connections to the main battery bank were already nearby, it was pretty easy to use this circuit, instead.

[End]

This page was stolen from "ka7oei.blogspot.com"

A daylight-tolerant TIA (TransImpedance Amplifier) optical receiver

While the majority of my past experiments with through-the-air free-space optical (FSO) communications were done at night, for obvious reasons, I had also dabbled in optical communications done during broad daylight, with and without clouds.

The challenge of daylight:

Clearly, the use of the cloak of darkness has tremendous advantages in terms of signal-noise ratio and practically-attainable communications distances, but daylight free-space optical communications has some interesting aspects of its own:

• It's easier to see what you are doing, since it's daylight!
• Landmarks are often easier to spot, aiding the aiming.
• Even in broad daylight, it is possible to provide signaling as an aiming aid, such as a mirror reflecting the sun - assuming that it is sunny.
• The sun is a tremendous source of thermal noise, causing dilution of the desired signals.
• Great care must be taken when one wields optics during the day:  Pointing at the sun or a very strong specular reflection - even briefly - can destroy electronics or even set fire to various parts of the lens assembly!

As you might expect the biggest limitation to range is the fact that the sun, with its irradiance of around 1kW/m³ (when sunny) can overwhelm practically any other source:  This is why the earliest "wireless" communications methods often used reflected sunlight, notably the Heliograph, where a mirror was "modulated" with telegraph code, or the "Photophone", a wireless audio transmitter using reflected light, an invention of Alexander Graham Bell from the 1870s with earlier roots - a device that Bell himself considered to be his greatest invention.

Figure 1:
Optical communications during daylight.  In the center of this contrast-enhanced picture (the red spot) is the light from an optical transmitter using a 30+ watt LED at a distance of 13.25 miles (21.3km).  This image is from my own "modulatedlight" web site.
Click on the image for a larger version.

Means of detection:

While the modulated speech may be produced in any number of ways (vibrating mirror, high-power LED, LASER) some thought must be given on the subject of how to detect it.  While the detector itself need not be spectacularly sensitive due to the nearly overwhelming presence of the thermal noise from the sun, it is worth making it "reasonably" sensitive so that this is not the limiting factor.  An example of an un-sensitive optical receiver (e.g. one that is rather "deaf" and itself is not likely to be sensitive enough even for daylight use) is a simple circuit using a phototransistor as depicted below:

Figure 2:
A simple phototransistor-based receiver (top):  Note that there's an error in the diagram in that it shows a photodiode instead of a phototransistor.  This circuit was built by Ron, K7RJ, simply to demonstrate the ability to convey audio a short distance:  It is (intentionally) not optimized in any way and is not at all sensitive!  A similar, but slightly better circuit was found on the Ramsey Electronics LBC6K "Laser Communicator", which was also quite "deaf".  See the article Using Laser Pointers for Free-Space Optical Communications - link that more thoroughly explains this issue.  This image is from my own "modulatedlight.org" web site and used with the permission of Ron Jones, K7RJ.
Click on the image for a larger version.

The circuit in the top half of Figure 2 (above) depicts one of the simplest-possible optical receivers - and one of the "deafer" options out there.  In this case a biased phototransistor is simply fed into an LM386 audio amplifier and the signal is amplified some 200-fold (about 46dB.)  As noted in the caption, this was a "quick and dirty" circuit to prove a concept and was, by no means optimized nor does it take maximum advantage of the potential performance of a detector.

As it turns out a phototransistor isn't really the ideal device because it is, by itself, intrinsically noisy.  Another, more practical issue is that its active area is typically quite small which means that it won't intercept much light on its own.  Of course, any half-hearted attempt to use any device for the detection of optical signals over even a rather short distance of a few hundred meters would include the use of a lens in front of the detector - no matter its type:  The lens will easily increase the "capture area" by many hundred-fold (even for a small lens!) and will effect noiseless amplification with the added benefit of rejecting light sources that are off-axis.  With the tiny active area of a phototransistor it can be difficult to properly and precisely focus the distant light onto that area and it is likely that unless very good precision in both alignment and focus can be maintained, the "spot of light" being focused onto the phototransistor will be larger than its active area, "wasting" some of its light as it "spill"s over the sides.

One of the biggest problems with a circuit like this is that there will be a level of light at which the phototransistor saturates, and when this happens the voltage across its collector and emitter will go very close to zero and the received signals will disappear, possibly "un-saturating" only briefly during those points in the modulation where the source light happens to go toward zero, resulting in badly distorted sound.  In broad daylight the phototransistor may be hopelessly saturated at all times unless an optical attenuator (e.g. neutral density filter) is used to reduce the total light level and/or more current is forced through it.

Introducing the TransImpedance Amplifier (TIA):

A much better circuit is the TransImpedance Amplifier, a simple circuit that proportionally converts current to voltage.  With this circuit (see Figure 3) one would more likely use a PIN Photodiode, a device akin to a solar cell, in which the output current is pretty much proportional to the light hitting its active area. This is quite unlike the manner in which a phototransistor is typically used where in the former case the impinging light causes a voltage drop across the device.

Figure 3:
A simple transimpedance amplifier.
(Image from Wikipedia)

In this circuit the junction between the inverting (-) and noninverting (+) inputs of the op-amp "wants" to be zero, so as the current from the photodiode increases in the presence of light, its output voltage will increase, sending a portion of that current through feedback resistor "Rf" until the overall voltage is zero.  What this means is that the output voltage, Vout, is equal to the current in the photodiode multiplied by the magnitude of resistance, Rf - except that the voltage will be negative, since this is an inverting amplifier.

As an example, assume that Rf is set to 1 Megohm.  Assuming no leakage and a "perfect" op-amp we can determine that if there is -1 volt output, we must have 1/1000000^th of an amp (e.g. 1 microamp) attributed to Ip, the photodiode current.  This sort of circuit is often used as a radiometric detector - that is one in which its output is directly proportional to the amount of light striking the photodiode' surface, weighted by intervening optics and filters and the spectral response of the detector itself.

For more about the Transimpedance Amplifier circuit, visit the Wikipedia page on the subject - link.

This is OK when the photodiode is in complete darkness - or in near-complete darkness, but what about strong light?  We can see from the above example that if we have just 10 microamps - a perfectly reasonable value for a typical photodiode such as the BPW34 in dim-to-normal room lighting - that Vout would be -10 volts.  If this same circuit were taken outside, the diode current could well be many hundreds or thousands of times that amount and this would "slam" the output of the op amp against a power supply rail.

A daylight-tolerant TIA:

One of the typical means of counteracting this effect is to capacitively couple the photodiode to the op amp so that only changing currents from a modulated signal get coupled to it, blocking the DC, but there is another circuit that is arguably more effective, depicted in Figure 4, below.

Figure 4:
A "Daylight Tolerant" Transimpedance amplifier circuit.
In this circuit the DC from the output is fed back to "servo" the photodiode's "cold" side so that its "hot" side (that connected to the op amp's inverting input) is always maintained at the same potential as the noninverting input, eliminating the DC offset caused by ambient light.  The disadvantage of this method is that it does not lend itself well to reverse-biasing the photodiode to reduce its capacitance, but between the high intrinsic thermal noise levels of daylight and the related photoconductive shunting of the device due to high ambient light, this is largely unimportant.  For the photodiode the common and inexpensive BPW34 may be used along with many other similar devices.
Click on the image for a larger version.

This circuit is, at its base, the same as that depicted in Figure 3, with a few key differences:

• An "artificial ground" is established using R101 and R102, allowing the use of a single-polarity power supply.  This artificial ground is coupled to the actual ground via C102 and C103 making it low impedance to all but DC and very low AC frequencies.
• The voltage output from the transimpedance amplifier section (U101a) is feed back via R104 to the "ground" side of the photodiode (D101) to change its "ground".  If there is a high level of ambient light, the voltage at the "bottom" end of D101 (at D101/C107) goes negative with respect to the artificial ground, setting the DC voltage at the non-inverting input of the op amp to zero, cancelling it out.
• R104 and C106 form a low-pass filter that passes the DC offset voltage to the bottom of D101, but blocks the audio.  In this way the DC resulting from ambient light that would "slam" the op amp's output to the negative rail is cancelled out, but the AC (audio) signals remain.  The time constant of this R/C network is slow enough to be "invisible" all but the very lowest (subaudible) frequencies, but more than fast enough to track changes in ambient light.
• By not placing any additional components between the "hot" end of the photodiode and the op amp, the introduction of additional noise from the components (including microphonic responses of the coupling capacitor) is greatly reduced.

In the above circuit the values of R103 and C104 would be chosen for the specific application.  In a circuit that is to be used at very high light levels where high sensitivity is not very important a typical value of R103 would be 100k to 1 Megohm:  Do not use a carbon composition but a carbon film or (better yet) metal film resistor is preferred for reasons of noise contribution.  While tempting to use, a variable resistor at R103 is also not recommended as these can be a significant source of noise.  If multiple gain ranges are used, small DIP switches, push-on jumpers or even high-quality relays - wired to the circuit - could be used to select different feedback resistances, knowing that these devices have the potential of introducing noise as well as additional stray circuit capacitance.  (Such a relay/switch would be wired on the "output" side of the op amp/relay of the feedback resistance(s) and proper component selection, layout and appropriate shielding would need to be considered.)

C104 is used to compensate for photodiode and other circuit capacitance and without it the high frequency components would rise up (e.g. "peak"), possibly resulting in oscillation and general instability.  Typical values for C104 when using a small-ish photodiode like the BPW34 are 2-10pF:  Using too much capacitance will result in unnecessarily rolling off high frequency components, but will not otherwise cause any problems.  A small trimmer capacitor may be used for C104, either "dialed in" for the desired response and left in permanently or optimally adjusted, measured, and then replaced with an equal-value fixed unit.

Again, the reason why the ultimate in high sensitivity is not required on a "Daylight Tolerant" circuit is that during the daytime, the dominant signal will be due to thermal noise from the sun - a signal source strong enough that it will submerge weak signals, anyway:  It need be sensitive enough only to be able to detect the sun noise during daylight hours.

The op amp noted in Figure 4 is the venerable LM833, a reasonably low-noise amplifier and one that is cheap and readily available (and actually works well down to 7 volts - a bit below its "official" voltage rating allowing the above circuit to powered from a single 9 volt battery) but practically any decent low-noise op amp could be used:  Somewhat better performance may be obtained using special, low-noise op amps, but these would be "overkill" under daylight conditions.

For nighttime use - where better sensitivity was important - a standard "TIA" amplifier that omits the DC feedback loop to cancel out the DC (potentially noise-contributing) components along with higher values of Rf would offer better performance, but for much better low-noise performance (e.g. 10-20dB better ultimate sensitivity) under low-light conditions than is possible with standard components at audio frequencies in a TIA configuration the "Version 3" optical receiver circuit described on the page "Gate Current in a JFET..." - link is recommended, instead.


Additional web pages on related topics:

• Daylight Optical Experimentation -  This page describes experiments done in broad daylight over 13 mile (21km) free-space links.
  
• Modulatedlight.org - This web site has a wide variety of information related to amateur, free-space optical, through-the-air communications.
• Optical Receivers for Low-Bandwidth, Through-the-Air Communications - This is a reference article that gives additional detail about the design of the circuits discussed in this article.
• Using Laser Pointers for Free-Space Optical Communications - This describes how one might use low-power laser pointers for low-rate optical communications, the practicalities of various circuits, the methods and the realities.
• The Modulated Light DX page - This page has several articles describing the practical aspects of free-space, through-the-air optical communications including various atmospheric effects.

The above web pages also contain links to other, related pages on similar subjects.


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