How do I use a microcontroller and transistor to control resistance on a wire?

Question

I need to programmatically pull a wire down to ground with a 1k resistor. I put the following schematic together on a breadboard and then programmed a GPIO pin on a STM32 microcontroller to alternate the +3.3V GPIO pin high and low every 1 second.

When I probe the resistance from "probe" to ground, sometimes it will alternate from no contact to 1kohm, other times it will alternate from no contact to 0 ohm, sometimes it will just stay at no contact. I'm clearly doing something wrong, since if I probe any other point, I'm getting the correct values.

^{simulate this circuit – Schematic created using CircuitLab}

The device I'm trying to interface with measures the resistance to ground on a wire to tell whether a device is turned on or not. If the device is on, it expects 1kohm to ground. If it's off, it expects it to be floating.

Answer 1

Bipolar transistors are controlled by the current between the base and the emitter. In your circuit, with the PNP transistor, you have no control over what potential is the emitter at, and thus you have no way of reliably turning the transistor on and off using a voltage source (GPIO) referenced to the collector.

Instead, you need to change the circuit such that the emitter will be connected to the node you control. You can achieve that with an NPN transistor - see the circuit on the left. The base resistor R1 is required to limit the base current, since the B-E connection is "just" a diode junction.

^{simulate this circuit – Schematic created using CircuitLab}

I've tried that circuit with a 3.3V square wave, and it works as expected with a multimeter: the multimeter alternates between a reading slightly above 1kOhm, due to the collector-emitter saturation voltage, and open circuit (OVLD).

To get a more accurate resistance output, if such may be necessary, use an NMOS mosfet transistor instead. Even with a 2N7000, the error on probed resistance should be 1% or less. A more modern mosfet transistor, like BS170 or BSS138 will work even better and introduce essentially no resistance error.

For a mosfet, the gate resistor R1 is optional and not necessary in this circuit.

In either circuit we're using low power transistors, and the maximum probe current should be limited to 200mA max. This will always be the case, since we need to limit the probe voltage to about 20V so as not to exceed the voltage ratings of the transistors.