# Averaging Multiple Analog Signals Together (0-3.3v or 0-5.0v) - Using Phototransistors

#### Mahonroy

Joined Oct 21, 2014
332
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!

#### crutschow

Joined Mar 14, 2008
26,073
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.

#### Mahonroy

Joined Oct 21, 2014
332
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?

#### Alec_t

Joined Sep 17, 2013
11,817
Your photo-transistors probably present a high-impedance source. Check the source impedance requirements of the ADC. A buffer stage may be needed.

#### HitEmTrue

Joined Jan 25, 2016
32

#### crutschow

Joined Mar 14, 2008
26,073
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.

#### RichardO

Joined May 4, 2013
2,271
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.