# How to invert an open drain output?

I am trying to use a output signal to reset a counter, but the signal I have is "low when counting" and I need "low during reset".

I want to "invert" an open drain output to a new signal so that when the output is on (connected to ground), the "inverted" signal has voltage or is not connected and when the output is off (not connected) the signal is connected to ground. My source voltage is 12v in an automotive system.

I have been struggling with this for a while and I must be getting confused with the negative logic because each solution I have tried gives me the same signal. I would love to do this with resistors and one or more of these BJT transistors: 2N3904 2N3906 S8050 2N2907 S8550 2N2222 BC337 C1815 BC327 A1015 since that is what I have available already and would work best in the enclosure space I have.

Output: I have a hardware component (LCR-II) with output Aux 1 which is an open drain output common to the negative power input line. Sinking capability up to 1 Amp. Maximum circuit voltage is 12v.

Input: I have another piece of hardware (a digital counter model CUB5B000) with a user input that is internally pulled up to +12 V with 10 K resistance. The input is active when it is pulled low (<1 .0 V)

I usually just work in software, so I'm apparently rusty from my University electronics -- thank you!

Between the work I already did and the answer from @Elliot-Alderson, here is my current attempt at a solution:

simulate this circuit – Schematic created using CircuitLab

• Actually, I'm confused. You say the signals are "common to the negative power input line", but the document you linked to says the only power input is +9 to +28 V. Commented Sep 5, 2019 at 17:32
• @ThePhoton I think the OP sees the power supply like someone might, used to working on cars. The battery has a negative and a positive. So I think the OP's writing merely means that the open drain output is referenced to the negative side of the power supply.
– jonk
Commented Sep 5, 2019 at 18:17
• Ross, are you willing to directly connect the negative side of your output hardware's power supply to your digital counter's chassis and/or ground reference? Or would you prefer some form of galvanic isolation to keep the two devices insulated from each other?
– jonk
Commented Sep 5, 2019 at 18:20
• @ThePhoton Thank you. I quoted "common to the negative power input line" simply because I was trying to be complete, found on page 6 of the LCR-II installation manual. I am sure it is really meaning ground. Both of these devices will run from +9 to +28V DC, but they are being powered by +12V. The common/ground side of the counter and the LCR-II are connected to the vehicle chassis ground (and thus the 12V battery's negative terminal). Commented Sep 5, 2019 at 18:52