[125] Effect of Light on Diodes

Date: August 1st, 2008 | Comments : [4] | Categories: DIY.

Silicon and Germanium diodes

It is a well known fact that light can have an impact on the conduction of some types of diodes. LEDs have been used as photodiodes in certain circuits.

I theorized that light could be used possibly control the clipping point and/or the knee of the diode conduction curve. A quick search through my parts bin yielded some glass 1N4148 silicon diodes and a few clear 1N34a germanium diodes, which were perfect for the experiment.

I arranged a selected diode on a breadboard with a red LED positioned to illuminate the glass body of the diode. A small black opaque 35mm film can was used as a cover to shield the setup from ambient light. The LED could be activated from outside the film can shield by applying power to the breadboard.

With the LED off, I measured the forward voltage (Vf) of the silicon diode at 550mv. Then I powered up the LED and measured the Vf… still 550mv.

Next I put the germanium 1N34a into place in front of the LED. It measured 265mv with the LED off and 265mv with the LED on. So far, it’s a bust.

I tried both diodes with a bright yellow LED, then a green one and finally a really bright blue LED. In no case did the Vf change from the 550mv and 265mv values! I switched meters and measured the Vf with a different meter and then with a diode tester. No change.

Bottom line is that light does not seem to have any influence on the conduction of the diode.

Not all good ideas work out   Smiley


4 Responses to “Effect of Light on Diodes”

[491] yamhill Says: 10:20 am, August 1st, 2008

I’m curious what would happen if you used LEDs as the clipping diodes and then exposed them to light from other LEDs.

[492] Tedmich Says: 10:40 am, August 1st, 2008

The PAIA Quadrafuzz (Anderton) used LEDs to clip:
never tried light on them to see effect…

and a shorter wavelength is apparently needed for irradiation, see: http://itp.nyu.edu/physcomp/sensors/Reoprts/LEDAsPhotodiode

[496] Martin Says: 12:27 pm, August 8th, 2008

As far as I know light changes the parasitic capacitance of diodes. This change of capacitance is what is used in applications that use diodes as light sensor. So the forward voltage will not change with different light intensities. The capacitance is quite small (pF) so I don’t expect any noticeable change in sound at the clipping stage of stompboxes.

[791] Mike Duncan Says: 9:30 am, September 14th, 2009

as far as i know i’ve only heard of using a photo sensor in conjunction with an LED in an enclosed space like the one you’ve created, very close together. the light the led puts off is picked up by the photo sensor and that data from the sensor is used to define the amount of x…


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