Hey guys,
My question can pertain to any 0-3.3v or 0-5.0v analog signals, and averaging them together. But more specifically, I am using phototransistors to create the analog signals.

I have this simple circuit which uses a phototransistor to create an analog signal that is fed into an analog to digital converter (ADC). Here it is:

I am wondering how I can use multiples of these circuits, and feed all of the Vouts into a single Vout that is the average of them? Then this averaged analog signal is what would go into the ADC.

What is the process for doing this?

Is this possible without using op-amps? Or is this necessary to do something like this? I don't want to add unnecessary complexity to the circuit, but if thats how its gotta be then so be it.

Thanks and any help is greatly appreciated!

 

If you connect all the photo-transistors to a common (single) load resistor then the output voltage will be the sum of all the signals.

 

If you connect all the photo-transistors to a common (single) load resistor then the output voltage will be the sum of all the signals.

I'm having a hard time picturing what you are describing. Are you saying that I take the "Vout" of each circuit, connect them all together, then run that through the "common single load resistor" that I will now have the sum of all of the signals? Isn't that just connecting them all in parallel?

 

Your photo-transistors probably present a high-impedance source. Check the source impedance requirements of the ADC. A buffer stage may be needed.

 

Check this out:

http://www.allaboutcircuits.com/textbook/semiconductors/chpt-8/averager-summer-circuits/

 

I'm having a hard time picturing what you are describing. Are you saying that I take the "Vout" of each circuit, connect them all together, then run that through the "common single load resistor" that I will now have the sum of all of the signals? Isn't that just connecting them all in parallel?

Yes, you just use one RL for all the sensors.

Yes, that is connecting them in parallel, but since they are basically high-impedance current-output devices, the one resistor will give the output voltage for all the currents summed together.

 

In principle what crutschow says is true. The one detail missed is that photo-transistors have a very wide variation in sensitivity (gain). This woudl require that all of the inputs be calibrated to sum together in equal weights.