IR sensor

It depends on the sensor.

In many cases you'll get a very small current or voltage that you have to amplify.

 

Often you have 2 devices that look like LEDs.

One is an IR LED, the other is a phototransistor or photodiode.

When the photodiode sees IR light, it produces a small voltage/current

When a phototransistor sees IR light, the transistor turns on (maybe just a little)

It is probably more likely that you have a phototransistor.

If you have a circuit that uses (say) an LDR to detect light and dark, you *may* be able to replace the LDR with the pototransistor. The resistor that forms the other leg of the voltage divider will probably have to change -- generally by making it much larger. If the circuit uses an op-amp (as opposed to transistor(s) then this is far more likely to work.

Beware that the photopransistor will see IR from more than just the LED. The sun is a particularly good source of IR, so you need to shade it from that.

Phototransistors are much faster than a CDS cell (and photodiodes are much faster again). Often you modulate the signal to the photodiode so it can easily be detected from the amplified output of the phototransistor.

 

A photo diode/transistor operates just like a transistor without a base connection
that is.... with a normal transistor you supply a small current to the base and that turns the transistor on and it conducts between collector and emitter. with a photo transistor
there is no base, instead the presence of light turns the transistor on and it conducts between its collector and emitter. A photo diode works on pretty much the same principle
... the diode is biassed on in the presence of light and will conduct

Dave