Phototransistor applications?

[RussMartin]

Does anyone have experience driving either an analog or digital PIC input with a phototransistor?

I'm wondering whether I'll need to add an amplifier stage, either a transistor or an op amp.

[mackrackit]

I have used them for digital with out an amp.

Anode at 5VDC
Cathode to PIC pin with a 10 meg pull up resistor.

Have not used one for analog though.

[aratti]

Lately I have used the TSL250R for analog reading. It works pretty well.

http://www.taosinc.com/productdetail.aspx?product=63

Al.

[AMay]

Last time I investigated photo transistors, I found that they (at least the ones I acquired) had a very sharp change in status (off - on.) Not good for controling on at dark lights, as adjustment for desired light level next to impossible using the usual adjustment to the other comparator input. I did not try adjusting the bias, which might have worked?

They would have worked great to pull a PIC pin low.

[RussMartin]

Last time I investigated photo transistors, I found that they (at least the ones I acquired) had a very sharp change in status (off - on.) Not good for controling on at dark lights, as adjustment for desired light level next to impossible using the usual adjustment to the other comparator input. I did not try adjusting the bias, which might have worked?

They would have worked great to pull a PIC pin low.

Well, the typical phototransistor has two modes, kind of like most garden-variety transistors: Switch mode and active mode. Switch mode is easy to work with, but active mode means you are dealing with current that ranges over four or five orders of magnitude, from a few microamps to milliamps or even tens of milliamps.

Driving a digital input in switch mode is no problem; it's just like saturating a transistor. Active mode, especially in low light levels, requires some massaging--playing with the emitter resistor (in a common collector configuration), then amplifying.

What I was trying to do was end up with a light sensor that gave me two things at the same time, an analog signal (intensity/signal strength) and a digital signal (is light present, yes or no, regardless of intensity).

[Byte_Butcher]

Use it in active mode with an emitter resister and feed a ADC. ?
What PIC are you using? does it have an ADC?

[nemesis]

I am glad someone asked, I was just starting to look in to how to do this same task.

I just picked up a few of these. I was wondering the same thing. I found a few circuits on the net. Here is a few:

http://www.uoguelph.ca/~antoon/circ/sensor2.htm
http://www.acroname.com/howto/photor...oresistor.html

Please post back if you come up with anything.

[mackrackit]

I just picked up a few of these. I was wondering the same thing. I found a few circuits on the net. Here is a few:

http://www.uoguelph.ca/~antoon/circ/sensor2.htm
http://www.acroname.com/howto/photor...oresistor.html

Please post back if you come up with anything.

Resistor?
Transistor?
Whats the difference?

[nemesis]

Yeah those links were for resistors, his question is about transistors. My mistake. Maybe the resistor photocell might be easier than active mode on the photo transistor. Maybe that can produce the same results with intensity... Seems a little easier.

-Nemesis

[RussMartin]

Use it in active mode with an emitter resister and feed a ADC. ?
What PIC are you using? does it have an ADC?

For that group of trials I was using the 12F683 which, yes, does have ADCs.

The problem, as I wrote before, is the range of the phototransistor response. Assuming an emitter resistor of 1 megohm and a current of 1 microamp, this puts 1 volt on the resistor. (The dark current is in the nanoamp range.) Given a 5 volt supply, 5 microamps (not very much light!) is about as far as you can go.

The smallest "slice", assuming 8-bit conversion, is 19.6 millivolts. Since the phototransistors I was working with can provide current ranging from a microamp to 5 or more milliamps (essentially 5 orders of magnitude), obviously something has to give--either the low end or the high end.

[Acetronics2]

Hi, Russ

Could you tell us what you want to do ???

Phototransistors sensitivy range can be greatly stretched ... but the circuit used ( yes ...OPAs needed ) will depend upon the light variation frequency to be detected.

sooooo ... without knowing what to achieve ... tons of solutions !!!

Alain