Open Collector Output Pins

Thread Starter


Joined Apr 3, 2019
Hello dear fellows, I have a really important problem.

I need to connect a Stepper Motor on a board, which has only open collector output pins. I need to activate and deactivate those pins through the computer software.
To begin with, the output pin I need to connect is open collector type. I had read that this means that the output is "floating" until it is pulled to ground when the output is ON.

This output pin is working with 5 Volts. I had read on the internet that an open collector output pin needs an external power source, so I feed it with a 5 Volt power supply.

By feeding the output pin with 5 Volts from an external power supply, the Stepper Motor works all the time without stopping and I cannot control it (turn it off or on) through the computer software.

I put also a pull-up resistor, but the Stepper Motor was working again without stopping.

>>> But the strange thing is that, when I turn of the external power supply, the Stepper Motor was working. That is because the output pin was giving 0.5 Volts to the circuit which made the Stepper Motor works, but with not a stable continuous rotation. It was rotating like crazy stopping and starting all the time. With this modification, I was also able to activate and deactivate the output pin through the software.

>>>> What I think it is needed now, is to increase the 0.5 Volts that the output pin is giving, to 5 Volts in order for everything to work properly. How is this possible to happen? <<<<

Sorry for not analyzing the circuit on a great depth, but the circuit is much bigger and confusing. The problem I have is specific and it is described above.

Thank you very much.


Joined Mar 10, 2018
Post a schematic.

An open collector output is on when a logic "0" is written to its
output register, off when a "1" is written to it.

Regards, Dana.


Joined Mar 19, 2019
Depending on the device, some are addressable by software to declare pullup or pulldown at their normal state. What board? What software? Many details missing here???