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simple phototransistor circuit

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tony ennis

New Member
I pulled what I think is a phototransistor out of my junk bag.

It's a small clear flat rectangle with 3 leads.

I hooked it up with +5v on one leg, and an LED -> 100 Ohm resistor -> gnd on the other. The LED came on when I applied the power. I covered the phototransistor completely and the LED didn't go out.

I didn't hook the middle lead up to anything.

The guy that gave me the bag o' parts thinks the phototransistor is a PN116PA or PNA1605F. I can't find a datasheet.

I'd like to make a circuit that if the phototransistor is cut off from light, the LED goes out. I think what I have would do that if I understood how the phototransistor worked, especially the middle lead. Any hints?
 

audioguru

Well-Known Member
Most Helpful Member
I think you have the photo-transistor connected like a forward-biased diode.
 

nike6

Banned
you'd have to use amplification, but it's not difficult.

one 2n3904 is enough. I've tested that on breadboard a while ago, and the LED responds to brightness.

you do not need A/D or OPAMP, not compulsory.
 

tony ennis

New Member
Heh, I am a total n00b, bear with me.

I measured 3.2v across the collector and emitter. So it seems that though the phototransistor prefers infrared, it does respond to all visible light.

I expected 1v across the LED leads assuming a 2.2 forward voltage. I am not seeing it. I tested the LED and it works.

Ok, I set the multimeter to 2000m DC. I assume that means 'use this setting for up to 2000 millivolts.' (It's my first multimeter, the instructions were useless.) If that's the case, then I think I am seeing 20mV across the LED.

Hmm. I covered the phototransistor up completely and the voltage didn't change a bit. More reading is necessary. But now it's time for bed.
 
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duffy

Well-Known Member
Check the 'photocurrent' spec on the phototransistor, it probably mentions something in the microamp range. Unless you are using a photodarlington, it won't be able to switch enough current to light an LED, which requires ten milliamps or so to light.
 

audioguru

Well-Known Member
Most Helpful Member
You might need to shine upclose a very bright laser on the photo-transistor for it to conduct enough.
 

tony ennis

New Member
In fact, I did zap it with a laser. No difference. I think the LED needs to come out of the circuit until I get sane readings. My savvy friend recommends a much larger resistor than 100 Ohms, too. I didn't quite understand, but something about the phototransistor being a 'current based' device. So if the resistor is larger, then the voltage will go up too, as per Ohms law.

I have more reading to do.
 

tony ennis

New Member
The problem was that I misunderstood the diagram in the datasheet that showed which leads were the emitter and collector. The view is from the bottom, not the top as I had assumed.

I was able to coax 5v out of it (with a 5.x v input) by zapping it with a laser and putting a 14k resistor on it. Of course, I was unable to get an LED to do anything given the lack of current.
 

nike6

Banned
did you try to hook it up to a regular transistor?

often these transistors are 2-pole devices, maybe have internal connection.
 

tony ennis

New Member
Yeah, I connected the emitter from the phototransistor to the base of a typical transistor. I put the LED and resistor on the collector side of the transistor. I grounded the emitter. Then I hooked the LED counter thing I posted about a few days ago to this, on the collector side. Now when I shine the laser on the photodiode, the LED flashes and the counter increments.
 

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