I have built the circuit as per picture below.

I have used the components specified other than the IR LED and Sensor (both of which I have tested elsewhere and are functioning). The parts I have used instead are: LED - L-34F3C 3mm and Sensor L-32P3C.

When the 12v power is on, the red LED is always on, regardless of the detection by the sensor.

Any ideas what I have done wrong or how to modify the circuit?

 

The first thing to do is to short out the sensor T1. Just short both legs of T1 together with a small screwdriver. The LED L2 should go off.

Does it?

If it does, then the fault is in T1 or L1. Don't know much about these components. One way to tell if the I.R. emitter L1 is working is to look at it with the camera on a mobile phone. These cameras detect I.R. light.

If the LED T2 does not go off when you short T1 then the fault is in transistor T2 or the components/wiring around it.

 

Thanks.

I will try that tonight. I know the IR Led works (camera trick last night), so that leaves either sensor, transistor or bad wiring.

Fingers crossed...

 

Edit: same reason posted by other member(JDT).

Any way good luck.

 

When shorting the sensor it does not turn the light off.

I am thinking it is the transistor which may be at fault, can somebody clarify my thinking below (I am new to this and read much but not really sure).

I have a 3904 transistor that should switch the LED on when a current is received on then "base" terminal. Therefore when there is no base current it should be "off". I have disconnected the base, so only collector and emitter are connected (collector to the cathode of the LED and emitter to ground), meaning the LED should be off. However it continues to light.

Therefore the problem lies with the 3904 transistor?

Thanks again.

 

Short out the base and emitter pins of the transistor. If the LED still does not switch off you have a faulty transistor or it is connected incorrectly.

 

Excellent thanks - the 3904s terminals were the reverse of the datasheet - helpful!

Solved.

 

I went to the RadioShack's website and the IR emitter/detector pair that corresponded to the part number 276-142 were not side looking types as shown in the diagram you have attached. Instead they appeared to be the standard T-1 3/4 style LED package. It also looks like the phototransistor is actually a photodiode instead. That explains the labeling as "cathode" on the detector in the diagram you posted.

There are several things that must be properly adjusted to make this simple IR emitter/detector circuit operate at its optimum. I think that you are going to find that it will be to your advantage to aperture the detector. Aperturing will prevent the detector from responding to IR light that may be present in the normal room lighting. Two other issues that must be carefully checked are distance between the emitter and detector as well as the alignment of the emitter with the detector. The IR beam from the emitter and the angular range of the IR detector are very narrow. This forces one to be sure that the two are aligned properly for the detector to be able to "see" the emitter.

hgmjr

 

Thanks for the advice, that has tuned it up nicely.

Appreciated.