ericgibbs
How do you measure the resistance of the light detector, when dark and light?
I am a novice at this.
hi,
If its an LDR [light dependent resistor] use a ohm meter across the LDR, shine a torch/bright light onto the LDR.
The resistance indicated by the meter will fall to a low value,,, note it.
Now cover the LDR so no light, the resistance will rise, note it.
These are the light/dark limits.
If its a PD [photo diode] connect the cathode to say +5V via a 10K resistor and the anode of the PD to 0V.
Measure the
voltage limits across the PD as you did the LDR.
If its a PT [photo tansistor] , connect the collector to +5V via a 10K and the emitter to 0V, measure the voltage across the PT as you did for the PD.
The 2N7000 circuit I posted is working just fine on the bench, using an LDR.
The 2N7000 costs about 50cUS.