I have my students build a simple colorimeter using LEDs for the light source and have been using simple photoresistors as the detectors. I would like to add a blue LED as this range of the visible spectrum is more useful, but the simple photoresistors are not at all sensitive to those short wavelength photons. Does anyone have a suggestion for my use. I would prefer something that requires my students to build their own amplification circuit.

 

Photo-transistors ?? IMHO, you don't need the speed of photo-diodes...

IIRC, simple photo-resistors are more sensitive towards the red end of visible spectrum, happily detect infrared. Also, IIRC, they tend to be temperature and history sensitive, too.

 

I found this generic datasheet and assuming blue LEDs are about 450-470 nm the sensitivity should be 20-40% in this frequency range. Looks like just more gain is required.
What circuit are you using at the moment?
http://ronja.twibright.com/datasheets/cds-resistor-pgm.pdf

 

The CdS resistors I use have a peak sensitivty at 540nm, but the characteristic spectrum drops sharply (http://www.token.com.tw/pdf/resistor/cds-resistor-pgm.pdf).

Are PV cells worth looking at - they have a broader spectrum (depending again on type).

Ciao Tim

 

Remember that a LEDs themselves can be used as photodetectors. AND they are already setup for their own wavelength.

So use a blue LED to detect a blue LED's light.

 

Remember that a LEDs themselves can be used as photodetectors. AND they are already setup for their own wavelength.

So use a blue LED to detect a blue LED's light.

Exactly, and they work rather well. First time I heard of this I didn't believe it, had to hook a few different ones up to prove it to myself.