IR Detector - Understanding multimeter results

[oliverj4455]

Hi there everyone,

I am trying to make some IR sensors (http://www.societyofrobots.com/schematics_infraredemitdet.shtml) but first I need to test the resistance of my IR detector. So I plug it into a multimeter and on all settings I get a 1 returned on the screen i.e. infinite resistance (relative). My multimeter goes up to 2M Ohms. When I shine an IR LED onto the detector the multimeter (still in 2M Ohms setting) jumps down to -0.550 Ohms. Then I move the LED away slowly and the multimeter reading gets closer to zero, continue moving it away and it quickly rises through th positive numbers up to a value of nearly 2. I connected the leads from the multimeter the other way around and just get a continuous reading of 1.

Can anyone help me make sense of this reading ro perhaps suggest a resistor value that I should use?

Thanks,

Oliver

[Soeren]

Hi,

The phototransistor is working in photovoltaic mode, hence the negative reading when you shine a light on it.
A suggested value is allready in the text (10k), but if you just wanna discriminate between absolute dark and light, anything between a few kOhm to a few MOhm should be useable.