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Topic: Voltage drop across silicone rubber keypad button (Read 514 times) previous topic - next topic

mattbott

I'm try to sense key presses on a music keyboard. The keyboard is set up like a 11x12 diode matrix. I have two MCP23017s and the Adafruit library for the chips. One for 12 outputs and one for 11 inputs. I was all set up with some simple test code, but I ran into some problems.

I started troubleshooting and found when I put 5V into one side of the "switch," (the PCB traces that get connected with the silicone button), close the switch with the silicone button thing, I only get about 0.2 volts on the other side of the switch.

Am I missing something here? Or do these types of switches always cause this much voltage drop?

If I'm not missing something, how do I get around it? 11 amplifiers of some sort for the 11 "rows" of buttons that should be returning voltage?

Thanks/

I can put up some pictures if I'm not making sense, so just ask =]

tinman13kup

Where are you sourcing the 5v from? It's either a 5v rail or the ic. One side of the switch will have the 5v, the other will connect to ground (or hi-z input). If you are using the ic as the source, then it makes sense that when the button is pressed, the voltage drops to 0v. That is an active-low signal.

Post a picture of the circuit and code you are using. It would help a lot in figuring out what you have
Tom
It's not a hobby if you're not having fun doing it. Step back and breathe

Wawa

I started troubleshooting and found when I put 5V into one side of the "switch," (the PCB traces that get connected with the silicone button), close the switch with the silicone button thing, I only get about 0.2 volts on the other side of the switch.

Am I missing something here? Or do these types of switches always cause this much voltage drop?
Rubber/carbon switches, unlike metal contacts,  do not have a very low contact resistance.
The voltage you see is likely from the "pull up resistor + switch" voltage divider.
0.2volt should not be a problem, since everything below 0.2*VCC (1volt) is recognised as LOW by the chip.
Not sure about your diode matrix. Post a diagram.
Leo..

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