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In the Circuit shown below, I am trying to achieve the following (I know my circuit is not correct, it is an attempt).

Problem: WHen INPUT_N = 0V, the input to the microcontroller does not go low.

INPUT_N is at 12V when inactive and needs to be sensed using a microcontroller running on 3.3V.

Question: If I remove the Diode, there will be leakage from the 12V rail (i.e. INPUT_N when inactive) to the 3.3V rail right???

What would be a good way to isolate these two?

A Proposal of the circuit for the question

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Please specify values for R1 and R2. Depending on these values, the voltage at the junction could be higher (so the diode always conducts when input is inactive) or lower (diode never conducts, input is disconnected from MCU) than 3.3 Volts. – Anindo Ghosh Mar 6 at 8:24
One other thing that might be worth adding to the question to be sure is what the 12V is connected to. If something like monitoring a motor or as part of an automotive system you may see much larger voltages spikes. – PeterJ Mar 6 at 9:15

1 Answer

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Remove the connection to 3.3V and simply connect the 12V input to a voltage divider to feed the 3.3V input. For example, 12V input to 9K ohm resistor to 3K ohm resistor to ground. Connect the micro's input to the junction of the two resistors. When the input is 12V, the micro will see 3V (assuming that the impedance of the micro's input is large enough to not disturb the divider).

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