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Infrared phototransistor


f4cepl4nt
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Ok, i built this infrared emittor using three 1.6v 50mA 100mW infrared leds, with a 100ohm resistor, and a 9v battery all in series. I did all the math and i bought the parts, soldered them and glued down the connections so that they dont move aroudn a whole lot, and it works great. Each led (when on) is completely clear unless i look at them from the front, in which case i see a dim red glow. On web camera, these lights appear to be bright purple when turned on.

Now, for the receiver. I bought an OP550A phototransistor, and for some reason it doesnt conduct when i shine the infrareds near it, and it still doesnt work even when ive got the leds touching the phototranssitor. I'm sure this has something to do with the way I've hooked the phototransistor up, but heres a simple schematic for the heck of it:

*note* the phototransistor is the OP550A in case you didnt read that above

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Hi f4,
Your problem is shown on the phototransistor's datasheet. It is a low current light sensor, not a high current motor driver. With a very bright light it conducts only 10mA. It is shown to saturate fairly well when it conducts only 0.1mA.
Your motor might draw 100mA or more. You need to measure its current.

Add a transistor that is driven by the phototransistor and has a current rating high enough to match the current rating of your motor. Then it will still probably need to have the IR LEDs very close to it without additional amplification. ;D

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Wow man, thanks for that. I didn't realize it only conducted 10mA under bright light. But I was browsing some websites and they had circuits similar to the one you described : a phototransistor drivin a transistor that is rated high enough to get a nice current flowing. Thanks for the help, its much appreciated.

Also, if you dont mind, what do you mean by 'additional amplification'. I never knew light could be amplified =O

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what do you mean by 'additional amplification'. I never knew light could be amplified

With a very bright light up close, the phototransistor conducts only about 10mA. A transistor can amplify the 10mA to about 800mA. If you want a range of maybe 10m, then the phototransistor will conduct only a few uA and additional amplification of its output can be done with an opamp. ;D
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