AC-coupled Light Receiver

This is a detailed plan for a particular circuit. It shows the typical amount of design for a small circuit.

This is a design for an AC-coupled light receiver. It has not been built or tested. The phototransistor specified in the schematic has infrared sensitivity and also visible-light sensitivity, but probably less for green and blue. In bright office light, the current is .22mA. If you use a 9V battery, let the output resistor be 2k to save the battery. If you use a high-current, 12V battery, you can reduce the output resistor to 470 ohms to let the earphone power be higher.

This is a fully analog circuit. If your light source is weak, the analog output can be amplified by any audio amplifier or the microphone input of a computer sound card.

The AC-coupled design means that a large amount of "DC," steady light does not interfere with a weak, AC component of light. This design benefits greatly from use of lenses, even cheap lenses from $1 magnifier glasses at Dollar Tree, 3 or 3.5 power. If you make an audio light transmitter with an LED, use a lens on the transmitter, too.

This is a PCB layout for the light receiver. It has a lot of parts labels and is 1.7" x 1.4". Without labels, it can be smaller. This design does not need any jumper wires. The output connector pad is set up for an 1/8" audio jack, panel mount, that has a metal ground contact. The transistor pad at upper left is there just to show where collector, base, and emitter are. There is a parallel arrangement of two, 100k resistors and two, series arrangements of 1k and 10k resistors. This is because the instructor stocks only 16 resistor types, to limit the dollars tied up in inventory. Resistors bought in, like, 600 quantity of a single part number is $.02.

The circuit design skills for this type of analog circuit can be learned by middle schoolers, at the earliest. It requires coaching or reference to a particular type of book, like one that is sometimes at Radio Shack. (Forrest Mims author of Getting Started in Electronics and Engineer's Mini-Notebook.) The trickiest thing is selecting the "bias" currents for the two transistors, to avoid draining the battery too quickly. Digital design that uses flip flops is a more advanced topic. There are a lot of ways that digital can fail, such as electrostatic discharge and clock ringing. Digital state-machine design is in the area of undergraduate education (electrical engineering), and digital software can do a lot of it but requires learning and purchasing. Netting this out: analog design for hobby work can start in middle school but any field of circuit design benefits from mentoring by an experienced designer.