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[on:]

Hi

I have built 15 DIY force sensors: copper fabric/velostat/copper fabric sandwich, one side to 5v, the other to pin via pulldown 220R. If I connect them all in series, ie 15 x 220R (common ground) running off a single 5v pin, should I expect the same response off each sensor? Or will the current vary (reduce) from sensor 1 to sensor 15. Should I use 15 digital pins (OUTPUT, HIGH) to power each sensor with its own 5v?

Thanks

[Reply #1 on:]

I have built 15 DIY force sensors: copper fabric/velostat/copper fabric sandwich, one side to 5v, the other to pin via pulldown 220R. If I connect them all in series, ie 15 x 220R (common ground) running off a single 5v pin, should I expect the same response off each sensor?

No, because you will now have 15 times the resistance in the circuit. The 220 Ohm resistor will now be much too low for the circuit to work.

Or will the current vary (reduce) from sensor 1 to sensor 15.

Current will be the same in all the sensors, because they're in series.

Should I use 15 digital pins (OUTPUT, HIGH) to power each sensor with its own 5v?

No, just use the 5V power supply pin(s).  Probably best to power each sensor with 5V separately and use 15 separate 220 Ohm resistors, and then you'll have 15 analog signals but only six analog inputs.

Look up analog multiplexing for a way to handle that.

[Reply #2 on:]

Hi, and thanks for the very prompt response.

You suggest the 220R is too low when using multiple FSRs; what value can you suggest?
I only have one 5v output pin (Mega2560) - the reason I suggested 15x5v from the digPins was, if one circuit works, then duplicate it 15 times to ensure success?? Here's a sketch of my current circuit:

[Reply #3 on:]

That circuit looks good -- connecting all the 5V connections together to a single 5V pin.

In the case of trying to use a single resistor, I'd suggest 15 times 220 Ohm, or 3.3k Ohm, but in that case, you would not be able to read changes in each sensor separately. That is, you would not know which sensor had changed resistance, only that the overall resistance had changed (you'd only have one analog input, after all).

[Reply #4 on:]

Hi
I really need discrete input from each sensor (I'm building an accessible digital musical instrument). A single version of this circuit (ie, 1x220R pulldown) gives me a serial monitor reading of approx 0 - 620, when I start to duplicate and add sensors, the readings for each sensor become c. 300 - 900. My question really is, can I use 15 resistors, a single 5v pin and a common ground (to allow discrete readings from each sensor) and if so what resistor value? Forgive the n00bosity,

many thanks
Brendan
ps perhaps you might suggest a more robust solution

[Reply #5 on:]

To do what you want, i.e. to read each of the 15 sensors individually, you'll need the circuit that you drew with the 15 220 Ohm resistors. Otherwise, you won't be able to sense the resistance changes independently (one sensor will affect reading for all the others).

Is there any reason you can't build the version with 15 220 Ohm resistors?

[Reply #6 on:]

Ok
now I understand, my OP was a little misleading (suggesting only one 220R); correct, I am using 15x220R, but my readings changed and I assumed there was a error in my circuit. My other error was assuming that sensor1 would get 5v (being physically closest to the 5v pin) and sensor 15 would get something less than 5v, being furthest away.

Thanks
Brendan

[Reply #7 on:]

My other error was assuming that sensor1 would get 5v (being physically closest to the 5v pin) and sensor 15 would get something less than 5v, being furthest away.

As long as they're wired together with copper wire (i.e. not using conductive plastics, etc.) then they'll all be at 5V. I think my misunderstanding was when you said "in series", which will not result in all the "top" ends of the sensors being at 5V. Look up the difference between a "series" circuit and a "parallel" circuit. In this case, you need a parallel circuit, so that all the sensors are independent.