I tried this with an op amp set up non-inverting with just the right amount of gain to get voltages in the 0-5V range or so. It more or less worked, but would've been nice if I'd set up the circuit so the minimum voltage of the photodiode produced 0V output -- or something much closer to 0V than I was getting.
I've no doubt there are far better ways... (it's not like I know what I'm doing after all ) am curious to see if anyone posts up ideas on how to do this.
I am guessing that part of the trick is in understanding the electrical behavior of a photodiode... how much current does it put out, how much voltage, what happens with a load, etc... ?
A photo-diode can be used in two ways:
1) With a reverse bias voltage it leaks a small current when exposed to light.
2) With no bias voltage it generates a small voltage and current like a tiny solar cell.
A transistor needs a high base current. Use a high input resistance non-inverting opamp circuit as an amplifier instead.
Yes, I'm currently using an opamp and it works perfectly. This is purely for my own interest.
I really want to experiment using an application of amplifying a small voltage (eg from a photodiode) using a transistor of some sort (yes amplifying voltage) - purely for my own interest. Any suggestions?
We are talking about the extremely small current from a photo-transistor. The fairly high base current of a transistor is almost like a dead short to it.
Maybe if you add a second transistor or more to make a darlington transistor or make your own opamp that the photo-diode will not be loaded too much.
You can use one transistor an an emitter-follower to amplify the current then use a second transistor as common-emitter to amplify the voltage.
Or you can use an opamp to do both.
This is an entirely wrong way of considering the situation.
A photo transistor is placed in a situation where the device changes resistance according to the light falling on it.
Place the photo transistor between base and positive rail of a 6v supply and use any small signal transistor with 2k2 to 10k load resistor.
The output voltage (from the collector) will be HIGH when no light is detected and very near 0v when light is detected by the photo transistor.
Guess what?
The TSL251 is not a photo-diode and it is not a photo-transistor.
It is a complete IR receiver with a photo-diode, an opamp and a very high value feedback resistor.
It is very sensitive and with a 5V supply its output goes from 3mV (dark) to 3.5V (lighted).
It is spec'd with a 10k load.
This is from its datasheet: