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Hello again, Arduino forums! Today I'm using a set of phototransistors (five of them) on a breadboard to sense light. I've set up the circuits correctly, but I'm using a common ground for all five phototransistors - that is messing up the readings a lot. I have determined this by changing my circuit setup on another breadboard, singling out the fact that the common ground is the real problem here. I tried using the digital pins as grounds (set to INPUT, write pin HIGH) with no luck. 

There are only three ground pins on the Arduino I'm using. How do I effectively "ground" all five phototransistors so I can get accurate readings from them? There is probably an easy answer to this but I'm new to the world of circuitry.

Thanks,

-Kevin

[Reply #1 on:]

I tried using the digital pins as grounds (set to INPUT, write pin HIGH) with no luck. 

Why? A digital pin that is set up as an input will not act like a ground but will be a high impedance.

There are only three ground pins on the Arduino I'm using. How do I effectively "ground" all five phototransistors so I can get accurate readings from them? There is probably an easy answer to this but I'm new to the world of circuitry.

Choose any one of the Arduino's ground pins and connect that to all the ground connections of your five phototransistors. If you're using solderless breadboard, there are connections along the edges that will do this for you.

[Reply #2 on:]

Anachrocomputer, thanks for the response. I've tried that setup with the separate grounds connected to the parts on the side of the board, connecting all of those to a ground wire that leads to the breadboard. The results are not accurate; for example, a sensor that does not exist may read 1000 counts on the A/D converter when nothing else is happening in the circuit.

[Reply #3 on:]

A sensor that does not exist may well make the analog input read 1000, because it's floating. That's not a problem with grounding, that's expected behaviour. If you want an analog input to read 0, you must connect it to ground, either directly or via a pull-down resistor.

The ground lines on the breadboard should give you good grounds. Remember that some breadboards have the edge lines split in two at the middle, and need a short jumper wire to link the two halves.

If you're still having trouble, it would help if you could post a circuit diagram of what you're building.

[Reply #4 on:]

Thanks! I've pinpointed the problem, it is in another part of the system :/