Connecting phototransistors or photodiodes in series

Question

I am designing a velocity measuring device and want to use 2 strings of 30 photo-transistors in series.

The strings are 15mm apart and are illuminated by separate collimated light sources. The output of each string is a negative going pulse when any one or more of the photo-transistors in the string are blocked. This output goes to a comparator and then to a processor.

I am having problems with the slow and varied rate of transition when different PDs are blocked. This is affecting the velocity measurement. The rise time can vary from a few hundred micro-seconds to over a millisecond. I am considering putting a trans-impedance amplifier on the output of each string, so that I can create an output to the comparator and processor as soon as the first PD is blocked, and for the reaction time for the two strings to be equal. Or nearly equal.

I have chosen a TI device, OPA2380 for the 2 channels. I would like to ask for any comments on the design, hopefully to prevent me falling into a hole.

Answer 1

I agree with Oli, but lacking other info, here's a few suggestions.

"String of phototransistors" suggests they are wired in series. If they are all normally illuminated and hence "on", then when one is interrupted the chain goes to high resistance state. You presumably have a resistor or current source in series with this, and this is all wired across a voltage supply to create a voltage divider which your comparator is monitoring?

If that's the case, then here are some possible issues:

• the blocking of the phototrans doesn't occur as immediately as you perhaps believe
• the transition from low res to high res vs position of the obstruction isn't the same from one phototrans to another
• the phototrans fields of view may differ.
• Different phototrans may be seeing differing amounts of ambient light, which means the amount of current still flowing when ostensibly "off" may differ.

A further problem is that the transition you are looking at is one in which the series resistor (or current source) is charging the capacitance of the wiring, the phototransistors and the comparator input. That's normally a fairly small capacitance (so would charge quickly). But to get the phototrans chain to saturate (maximally conduct, produce minimal drop) when unobstructed, you may have chosen a relatively high series resistance. That high resistance which will slow that transition, and also make it sensitive to different degrees of offness of the blocked phototrans.

The larger problem is trying to deal with a bunch of phototrans in series. You may well be better off dealing with each phototrans separately. You could connect individual phototrans+resistor divider direct to a digital gate (cheap, and can include hysteresis to avoid noisy transitions) like an 74HC14, or 4093 (tie input pairs together). You can AND or OR these together to get aggregate results. You may be able to place these "signal conditioning" components close to the phototransistors to avoid running so many wires to your main electronics.

This strategy allows you to deal just with the characteristics of individual phototransistors, and to isolate anomalies, without having to deal with the effect of the rest of the chain on the individual detectors.

Hopefully this answer is not too far off base, considering we don't really know your setup. :-)

Answer 2

Photodiodes/phototransistors are current sources and you can't place those in series, just like you can't place voltage sources in parallel. Each photodiode will try to push its current through the wire and different currents will fight each other. The result is that the net current is undefined, and possibly wildly varying.

Normally you would place the photodiodes in parallel and connect them to a common resistor. The voltage across that resistor is then dependent on the sum of the individual currents.