Simple Optical Receiver Project #1

Overview

This is a simple phototransistor-based optical sensor based on the circuit in the book The Basement Bugger's Bible, by Shifty Bugman.  It's designed to only receive noncarrier-based optical signals with their modulation(s) falling within the normal audio "hearing" range of 20 to 20,000 Hz.

The circuit is fairly simple and consists of using the phototransistor light sensor described earlier in this issue to feed a common-emitter 2N3904 transistor pre-amplifier which is configured to reduce gain below 72 Hz and amplify everything up to around 7,000 Hz by around 20 dB.  The overall transistor gain will depend on the setting of the 10 kohm "Gain" potentiometer (audio taper).  Everything is run from a standard 9 volt battery.

The output of the transistor feeds a Motorola MC34119 or JRC NJM2113 low-noise audio amplifier for an additional 20 dB of gain.  You can configure that amplifier for more gain by adjusting the feedback resistors as discussed in the datasheet, but this is not recommended for low-noise operations.  The MC34119 or NJM2113 is highly preferred over the common LM386 due to its lower noise characteristics.  Note that the final audio output from the MC34119 or NJM2113 must be isolated from ground.  This means you may need to add a nonconductive washer around the collar of the 1/8" output jack.  Radio Shack does still carry the proper 1/8" stereo jacks which doesn't require this additional isolation.  It may also be a good idea to run the MC34119 or NJM2113 off its own 9 volt battery for additional performance in low-noise operations.  This prevents any voltage spikes from the audio output amplifier from feeding back into the phototransistor or the pre-amplifier stages.

Connected to the input from the phototransistor front-end, there is an (optional) op-amp circuit in series with a 0.33 µF capacitor.  This op-amp circuit is called a "gyrator" and is a simple way for a couple of op-amps to simulate a fairly high-value inductor.  For this circuit, we'll be simulating an approximate 21 henry inductor.  When in series with the 0.33 µF capacitor, those two circuits form a resonant tank circuit which will shunt any 60 Hz signal to ground.  This is very simple way to attenuate some of the 60 Hz AC light hum that optical sensors tend to pick up.  A 20 kohm multiturn potentiometer is used to tune the notch circuit for maximum null.

You can replace the 0.33 µF capacitor with a 0.47 µF capacitor if you need to null out a 50 Hz signal.  This pseudo-series LC resonant tank circuit doesn't have a very large "Q," so the 60 Hz hum isn't completely eliminated, but it still does a very good job of knocking it down to something which isn't totally annoying when listening through headphones.  Tune the notch by pointing the light sensor near an incandescent light bulb powered off 60 Hz 120 VAC mains.  Avoid fluorescent lights as they tend to emit a strong signal at 120 Hz.  To make the notch null out a 120 Hz signal, you can replace the 0.33 µF capacitor with something around 0.082 µF.  You may wish to add a selector switch for each of these capacitors.

All the resistors in this circuit should be 1% metal-film and the coupling capacitors should be high-quality, low-leakage film types, like polystyrene.

The PC board area around the the phototransistor's input should be clean of any solder flux and a high-quality Teflon dielectric BNC jack and connecting cable should be used to minimize any additional leakage currents.

Note that this circuit is not designed to work with photodiodes.  That will be an upcoming project.

Pictures & Construction Notes

Overview of the phototransistor-based optical receiver circuit board.

The input from the phototransistor is on the middle-left.  The 60 Hz notch op-amp circuit is along the bottom, with the large green capacitor on the lower-left being the series 0.33 µF capacitor.  The blue multiturn potentiometer is for tuning the notch.

The JRC NJM2113 audio amplifier is along the upper-right of the circuit board.  The 10 kohm "Gain" potentiometer in the transistor's collector hasn't been installed yet.

The two 2.2 ohm resistors on the output of the NJM2113 audio amplifier pad output impedance a bit as this chip likes to see a higher impedance than 8 ohms.

Alternate view of the circuit board.

Try to use high-quality, low-leakage, non-polarized film capacitors in any coupling stages.  Standard electrolytics can be used in bypass operations.

Also note the use of a large ground plane and clean, isolated areas around the input pre-amplifier transistor to help minimize leakage current.

Installing the circuit board in a case from an old printer switch.

The input from the phototransistor sensor head is on the left via a panel-mount BNC jack.

Next to that is an optional infrared emitter dithering circuit using a 5 kohm potentiometer.  The DC output is via a panel-mount F jack.  An integrated switch on this potentiometer controls power to the dithering circuit.  This dithering circuit is the same one as described earlier.

Next to that is the 10k ohm potentiometer for gain/volume control.  Note the 2200 pF capacitor is soldering directly across the pins of the potentiometer and the three connecting wires are twisted together.

On the bottom right is the 1/8" stereo headphone jack.  Note that the jack's ground pin isn't used as this is connected internally within the headphones.

Alternate view of the completed optical receiver circuit.

The two potentiometers also control the +9 VDC power to their respective circuits.

Front-panel overview.

Phototransistor input is on the left, then the dithering DC output, and the dither control and gain/volume potentiometers.

A power indicating LED and 1/8" stereo headphone jack are on the right.