Simple e-stop circuit

Question

I have a motor controller that has an electronic stop function where if I put 5v on a pin the motor runs. Once that 5v gets interrupted (goes to ground) the motor stops immediately.

On my device I have what’s called a “safety edge” - basically a bumper. At normal times the bumper is simply a wire with a 8k ohm resistor. If the bumper gets pressed the resistance goes to 0 ohm.

I have a feeling this is super basic but what’s the circuit that allows me to feed my controller with 5v and then cut off that 5v if my number goes from 8k ohm to 0 ohm (gets pressed)?

Answer 1

Well at least on paper this appears to work using a voltage divider and a pull up resistor configuration.

Answer 2

The obvious thing to do would be to disconnect the switch from where it connects to +5V and the resistor and connect it to ground instead. The input would be pulled up to 5 V through the resistor and when the switch is closed it would ground the input.

If you can't modify the switch in this way you would need some electronics to sense when the switch is closed and pull the input low.

Here is a circuit that does this. The 8000 ohm resistor in the bumper and the 470 ohm resistor form a voltage divider such that when the switch is open there is not enough voltage to turn on the transistor. When the switch closes the voltage is enough to turn on the transistor, pulling the output low.

This is just a generic example circuit, the exact implementation would depend on the input requirements of the controller. Also since this is safety related, any of this is done at your own risk.

^{simulate this circuit – Schematic created using CircuitLab}