I am working on a project at the moment which requires a microcontroller to pull a signal low for two seconds. This signal is from a break away board which will be connected to the microcontroller by a screw connector. I am using a PIC microcontroller (18f25k22) running at 5 volts. The break away runs on 12 volts and has a pull up resistor on the signal input. I have built a circuit using two transistors to control the 12 volt signal, but I don't know if setting the pin low on the microcontroller will pull the signal low on the break away board.
The driver circuit:
This circuit is on the pcb containing the MCU. Point E goes to a connector which is used to connect to the break away board. Point RC4 is connected to the MCU pin. My idea was to hold the circuit high until I needed to pull it low, in which case I would set the pin low to turn off the transistors and cut power. The MCU circuit and the break away share a common ground and the whole thing is powered by 12VDC. I have a voltage regulator to power the MCU.
So I ask; will this work and pull the output low? Or is there a better way to do it?
Best Answer
That will not work.
You say you have a pull up that you need to pull low.
In the way your schematic is drawn, whatever you do to turn Q8 on, will make it pull your signal high, as in, connecting it to 12V.
What you need is actually a simpler open collector (or open drain if using a fet) output:
simulate this circuit – Schematic created using CircuitLab
When MCU pin is high, it turns Q1 on, making it conduct and connecting the signal to ground. When you turn it off (MCU low), Q1 stops conducting, and the pull up pulls the signal back to 12V.