AnalogRead moving from 0V to 5V and back

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dear community

i'm working on fifth arduino project , guess left beginner status behind me, although getting crazy actual problem , hope opening eyes simple hint... :-)

i ordered simple 4x4 membrane keypad (http://www.zerko.ch/downloads/27899-4x4matrixmembranekeypad-v1.0.pdf).
i created voltage divider connecting 4 resistors (10k, serial), on 1 side 5v, other side gnd. i'm getting beside 5v 3.7v, 2.4v, 1.1v between resistors.

i connected 4 rows of membrane keypad different voltages, colums analog inputs 0-3.
then used sample read voltage (arduino.cc/de/reference/analogread), works great far...

there 1 problem, when not press button, voltage increases 0v 5v every few seconds , decreasing 0v again after second on 5v????????

i tried 2 different boards, same problem. when connect analogpin gnd getting clear solid 0 (sensor value, not yet converted voltage) on 1 board, on other i'm getting arround 23 (from 1023 max).

i double-checked soldering... can bad soldering causing that?

thanks lot in advance

dave

you have ground analog inputs pull down resistor, example 100kohm in case. otherwise analog inputs float , down based on electrical fields induced wires.


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