Is it photodiode or phototransistor?

[Willen]

I found it on a destroyed boared (junk box). On the board these two conponents are soldered face to face so I guessed it was a protection from fire (smoke detectors). Unluckly I don't have the board to see its basing. So asking directly with all of you.

I searched but they say sometime 'photodiode' and sometime 'phototransistor' to same picture. Actually I have no idea how to test photodiode and phototransistor to find different between them.

The component is working very very nicely for 'Crystal Radio' rectifier (instead of germenium diode) while the component is exposed to light.

 

[jpanhalt]

On the board these two conponents are soldered face to face so I guessed...

Is that an image of the actual item(s) you have or something you found that looks like it/them?

• Please post an image of the actual item(s).
• Please show the other component.
• Please post the link to the image you have posted, if it is not the actual item.

John

 

[Willen]

I have exactly same component (same housing/case) so I didn't captured an image of my real component. Attached picture from Google search (link below) and they included the picture in the 'phototransistor' category. But they also talked about IR diodes, photo diodes below in text. And also, I heard that phototransistors are slow and photodiodes are fast. My component is working as a Crystal Radio rectifier to detect MW band and I think it is fast work, so I simply assume that it is a photodiodes.

https://www.directindustry.com/prod/sharp-microelectronics-europe/phototransistors-29672-249412.html

 

[alec_t]

If the device has only 2 legs then it is acting as a photo-diode.

 

[jpanhalt]

Phototransistors can have just 2 legs.

A photodiode will act like a diode when forward biased. A transistor (in the dark) will not show diode conduction from collector to emitter.

The photodiode also produces a voltage with only light. I don't believe but am not sure a phototransistor also produces a voltage:
https://physlab.lums.edu.pk/images/1/10/Photodiode_circuit.pdf

John

 

[dr pepper]

If theres 2 facing logic says one is a photodiode and the other is a infra red led.
Use a mulitimeter on diode test and check them both, the one that registers around 2v is the led, you can then if you like connect the led to a 9v battery via a 1k resistor and 'look' at the led with your camera phone or whatever, you should be able to see it lit up. You can then 'shine' the led on the phototrany (or photodiode) while measuring its resistance with said meter, it should drop the more its illuminated.

 

[MrAl]

Hi,

It could also be a PIN diode. They work as fast photo detectors too.

 

[Willen]

you can then if you like connect the led to a 9v battery via a 1k resistor and 'look' at the led with your camera phone or whatever, you should be able to see it lit up.

Can a infra red LED give light just with battery supply? Infra has high frequency, so don't we need Oscillator?

 

[audioguru]

Color has frequency but not caused by an oscillator.
Ultra-violet has a very high frequency above light frequencies so it is invisible. Purple is a little lower and has a high light frequency. Green has a medium light frequency. Red has a low light frequency. Infrared has a frequency lower than visible light and has heat frequencies. "Infra" in a dictionary means "below".

An infrared LED produces infrared radiation when it has current from a powered series resistor the same as an LED produces visible light.

 

[alec_t]

Infra has high frequency, so don't we need Oscillator?

Perhaps you're thinking of the modulation frequency (commonly 38kHz)?

 

[Willen]

Yes. If LED can rediate itself at 'Infra red's' frequency, why we use oscillator like 38KHz? For modulation purpose? Which kind of modulation commonly used?

 

[Willen]

*****

 

[alec_t]

Amplitude modulation of the IR is used to give a signal distinguishable from background interference.

 

[Willen]

Ok, Once I tried to make a stereo encoder for FM tx. I didn't get 38KHz crystal. Can I use remote controll system of TVs to get 38KHz pulse for stereo encoder? any idea?

 

[alec_t]

I don't see any way you could hack a TV remote to make a stereo encoder.

 

[audioguru]

Some TV remotes have the IR LED modulated by 36kHz, 38kHz, 40kHz or a few other frequencies.
The IR receiver ICs are also made with those same frequencies.
Some compact fluorescent light bulbs operate at 38kHz so the IR receiver ICs expect bursts of 38kHz from an IR remote. If the receiver receives continuous 38kHz then its AGC attenuates its reception to avoid interference.

 

[dr pepper]

Toneburst 38khz is used to reduce the effects of noise caused by fluorescent lights.
36, 38 and 40 khz are also used, depends on the protocol used.