Hi all,

I’m new to electronics, but having a great time so far.

I’m building up my understanding by creating a bunch of very simple circuits so that I’ll know what I’m doing when the circuits get more complicated. I’m stuck on one of those simple circuits, and I’m hoping you’ll be able to help me figure out what I’m doing wrong.

The idea is simply an LED that lights up when an IR detector receives light from an IR emitter. That’s it. Nothing fancy. But I can’t seem to get it right. Here’s what I've sketched up as a schematic:

I’m pretty sure I’m using adequate resistance in each place because I can test each loop on its own and each component holds fine.

When I wire this up on a breadboard, the output LED (the one on the bottom) is on all the time, whether the IR LED switch is open or closed.

Do I have the logic wrong? If so, what am I not understanding? If the logic is right, any idea what might be happening?

Thanks.

 

When I wire this up on a breadboard, the output LED (the one on the bottom) is on all the time, whether the IR LED switch is open or closed.

Welcome to the wonderful world of electronics. Your logic and line of thinking is correct so far. Keeps up the good work.

The opto transistor is also sensitive to ambient lights besides IR light, so even without the IR LED ON, a small current current "i" still flows and this is sufficient to bias ON the transistor and light up the LED.

You can reduce this "unwanted" current by two simple ways. The first is to add an extra resistor from the base of the transistor to its emitter, thus bypassing this small "unwanted" current as shown below. The second method is to place an IR filter over the poto-transistor to reduce the influence of ambient lights.

Also you must absolute sure that light from the LED(the normal one, not the IR one) does not shine into the photo transistor or else a regenerative feedback circuit will result, kind of like audio feedback with microphone in a hall.

 

It would be helpful if you could annotate your schematic with values for the resistors and the voltage source.

hgmjr

 

What happens if you switch off all other light sources and test the circuit in a pitch dark room?