How it is possible to use LEDs as detectors?

[ark5230]

I noticed that LED can be used as detector ! This I noticed about Red LED, I checked for IR LED, it works !!
Does it mean the source and detector are not much different ?

 

[Pjdd]

Re: LED used as detector

It's an established fact that an LED can act as a photodiode, as can other diodes and transistors. This is not surprising since dedicated photodiodes also use basically the same photosensitive semiconductor materials and P-N junctions.

 

[ark5230]

Re: LED used as detector

So is it that for practical purpose more or less they can be used in place of one another !!

 

[FvM]

Re: LED used as detector

If you don't rely on minimal noise, maximum sensitivity and tightly specified component parameters, LEDs may be an interesting sensor option. A number of circuits has been published that's using LEDs as both light source and ambient sensor in time multiplex.

 

[Pjdd]

Re: LED used as detector

So is it that for practical purpose more or less they can be used in place of one another !!

Yes, if you keep in mind that an LED will not be as efficient as a dedicated photodiode. Long long ago, some transistors were housed in small glass cases that were painted black on the outside to prevent ambient light from interfering with normal transistor action. Scraping the paint off was an easy way to turn them into phototransistors!

Here's one example of how I used a home-made phototransistor in a practical application. I live in a remote place where many things that people in metros and advanced countries take for granted are often major issues, and I've often had to improvise with what's available. Long after the age of glass-cased transistors but also long ago, I had to work on a cinema projector in which the sound reproduction had badly deteriorated. The vacuum tube photocell that's used to extract the sound from film had weakened with age. Ordering a replacement would have taken a long time. So I drilled a hole at the top of a metal-cased transistor and used it in place of the vacuum cell. I had to rig up a mounting bracket, modify the power supply and bias circuits, and also made changes to the optical system. It worked beautifully and they used it until the theater closed down some 20 years later.

The transistor I used for the cinema projector was a 2N3019 with a TO-39 case. I've used larger TO-3 transistors in some other applications. The larger chip inside is more sensitive but slower, which is why I didn't use it for the projector as it would have degraded the high frequency performance.