Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Some LED detects light!

Status
Not open for further replies.

Willen

Well-Known Member
According to few reading and few experiment, I know that each and every junctions produce light (tiny amount in transistor junction, huge amount in LED) during operation. So transistors and ICs are packed by black epoxy to save from light affect. So we can get some unusual benefit from transparent LEDs or dioodes (like 1N4148).

I experimented with a green light as a light detector, shown below. How actually it works? Sure not like a LDR. I think it passed few volt and current above than 0.6V (forward voltage across the LED) so PNP conducts. Few more analysis needed.
 

Attachments

  • LED light detector.JPG
    LED light detector.JPG
    19.9 KB · Views: 136
Last edited:
I remember an old radio that had the old OC71's germanium tr, The owner complained that when he opened the back the volume shot up, checking it showed some of the black paint had flaked off the OC71 and was acting as a LDR.
Max.
 
LEDs will work as solar cells and generate current. They are quite small so not much current can be obtained, but with a high impedance voltmeter you can measure the voltage quite easily when the LED is in sunlight.

I have even lit an LED with the current generated by two others.
 
I remember an old radio that had the old OC71's germanium tr, The owner complained that when he opened the back the volume shot up, checking it showed some of the black paint had flaked off the OC71 and was acting as a LDR.
Max.

They did a paintless version, called an OCP71 - as it was considerably more expensive, you could get a razor blade and make your own with a little scraping :D
 
Hi MikMI and all,

I Read again some articals about LED light detector. Semiconductor and light has very close relation. I knew that LED creates voltage as solar cell, wow one LED is making 1.5V very easily!

My simple analysis of the circuit:

I guessed in the OP image, during focusing on light, the forward green LED creates +1.5V less than the emitter on the 'base' (probably 0.6V less than emitter?) so positive emitter of the PNP starts to conducts to the negative base. Thus it passes current through E to C to operate, isn't it?
 
Last edited:
I saw a circuit with a 8 pin micro and one LED. It made a night light. 100 times a second the LED current was turned off. After mS delay the LED voltage was measured to determine how much light was in the room. The LED both made light and detected light.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top