Photo Sensor Power Control Circuit Help

Thread Starter


Joined Jan 12, 2011

I'm working on a motor control circuit using a photo sensor. It's a two part reflective sensor with an IR LED and photo transistor. I've got the photo sensor working correctly, I can power and LED with it. (Jumpered the +18V to the collector with a ~700 Ohm resistor and the emitter runs through the LED to -18V)

I've also got my motor setup running off of the power transistor (18V draws up to 2A). When I jumper tht +18V to the base using a 330 Ohm resistor the motor powers up fine.

However, when I connect the emitter of my photo transistor to the base of my power transistor, it won't turn on the motor.

I'm sure that it has something to do with the current/voltage that the photo transistor is passing versus what the power transistor needs to "activate" but I'm pretty new and this and stumped at this point. I've attached the two data sheets for the photo sensor and power transistor that I am using.

Any advice on what I can do to fix this would be really helpful.




Joined Sep 9, 2010
Your hunch is correct, and you need another transistor to provide enough current to the base of the power transistor.
Picture 1.png
Sorry about the partial circuit but I hope something is better than nothing. Notice the use of the current limiting resistor on the first transistor of the darlington pair. Just use a value that, assuming a short, limits the current to the first transistor to some level below its max spec, ~100mA in the example.