Yes you are probably correct. The relay is probably used for inputI interpret the OP as wanting to use a formC relay contact to an Input of a ucontroller.
The OP proposes to hook 5V to NO, Gnd to NC, and connect Com to the uC Input.
That is not the way I would do it. As the relay armature is moving from one state to the other, the uC input becomes disconnected.
I would use only the form A relay contact (NC to gnd and Com to uC input; NO open). I would put a pull-up resistor (10K to 100K) from the uC pin to +5V.
Yes you are probably correct. The relay is probably used for input
A relay switch may be considered as any other mechanical switch. They have problems like switch bouncing, and some approach to debouncing should be taken https://www.google.com/search?q=switch+debouncing Another important thing is that a then a MCU input is not connected to anything (floating) it will most probably give high level (+5 volt). But noise may cause it to switch from this level. So a floating input shall be regarded as undetermined.
The time constant in question the effective CMOS input resistance (leakage) times 100nF after the relay contact breaks and only has to hold until it makes on the other side. The 100Ω is there just to limit the inrush current while charging/discharging the capacitor.Will that time constant be big enough, Mike? What is the relay switching time likely to be (recent reading for pcb latching relays gives ~3ms)?
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