I am using the Arudino Uno board as a trigger/delay pulse generator to drive a laser. I read the input pulse train sequence from a test point on the external device using pin 12 set as INPUT. The Arduino and the external device share the same ground.
Now, when the power is on, everything works fine. But when I power off the Arduino board, it seems that pin 12 starts to distort the signal on the test point of the external device which wrecks up the device's normal operation. This only happens when the Arduino is grounded. When I power On the Arduino or disconnect its ground from the common ground, the external device works fine again. As a result, I cannot have the Arduino connected to the device when it is powered off.
If I check the resistance between pin 12 and the ground, it is infinite when the power is ON and about 16MOhm when it is OFF.
Does anyone have an explanation/cure for that?