IR Opto Detector

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     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 
     - 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]



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