This circuit converts infrared light into sound. Modulated IR light, like that from remote controls, IR received by the phototransistor and is amplified by the LM386 IC. The IC in turn drives a small 8 Ohm speaker. Unmodulated IR light, like that from an incadescent light bulb, produce no sound so the phototransistor also turns on an LED indicating the presence of IR light. _________________ [IR Photo Trans.] 9Vdc—-[20K RES]–+–[emit coll]–+–[20K RES]—-Gnd | [_ base___] | | +———————+ +–[10K Ohm Pot]–Gnd | ^ | +————————+ | | LM386 | | +—u—+ | | nc|1 8|nc | +——-|2 7|nc | Gnd|3 6|9Vdc | Gnd|4 5|—–[+100uF-]–[8 Ohm Skeaker]–Gnd | +——-+ | +—————————————-+ | [ base ] +——[emit coll]—–[270 Ohm]—9Vdc | [Gen Pur NPN trans.] | +——[LED |>| ]—-Gnd
NOTES: - All of the parts for this circuit are available from your local Radio Shack for a few dollars. RS also has a great little case for the project (# 270-294), but you better build it small. - My prototype used the phototransistor from the index sensor of a junked floppy disk drive. Make sure you get the transistor and not the IR LED, they look very simular. - The transistor driving the LED can be any general purpose NPN device. 2N2222 and 2N3904 are a couple of common parts that will work. - The LM386 amplifies the incomming signal by about 20 times. The potentiometer adjusts the level of the signal feeding the amplifier. Adjust the pot until amplifier starts to feed back and then back it off a little. See the data sheet for the LM386 for details about increasing the gain. - This circuit was designed as a quick, simple tester to check IR remote controls and as such was designed to use as few parts and cost as little as possible. The design could be improved for audio fidelity, sensitivity and gain. I'd love to see any improvements you make.
- John R. Schuch schuch[at]phx.mcd.mot.com