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Phototransistor Answered

I am trying to use a phototransistor to detect an object passing between the diode and transistor.  The problem I am having is that the PT is always on - even without the photodiode.  Are these things that sensitive that even a florescent light will trigger it?  If so, how do you prevent it from staying on?  I have never used these things before and my electronic design is pretty limited.




I believe I used a 390 ohm in the first circuit. It was going to give me around 10 mils through the LED. On the circuit I found on the web, there was no resistor. The collector was tied to 9V and the emitter was tied straight to the base of the xsistor. You can see it here





I have tried a couple. The first was having the PT collector tied high thru a resistor and it's emmiter grounded. The collector was also tied to an LED and series R to high (9V and then 5V) The thinking was that when the PT turned on, the LED would light as a test. The second attempt was having the PT emitter drive the base of an NPN that had the LED on its collector. The PT collector was tied high. I saw this circuit on the web as a test for remote controls. Both cases had the LED always on.

Hi Les, what resistor value did you use ? PTs can be very sensitive, making the resistor smaller makes them LESS sensitive.

Have you tried shielding the PT with e.g. a small tube?


I have not. I shielded the PT with my hand to block any direct light to no avail. I need a way to reduce the gain on these things - they are extreamly sensitive.