negative voltage threshold detector

[bertjeivens]

Hello all,

I'm learning electronics by doing a small project but i have hit a small bump.
I need to simulate a valve which is controller by a intelligent remote controller. At work we are going to test 15 of these controllers at the same time. To avoid to have to install 15 valves I would like to simulate them.
I have 3 elements that are locally on the valve 1*10k pot and 2 piezo electro valves for the compressed air.
So all i need to do is detect the pulses on the piezo elements to check wether the controller wants to open or close and then use a digital pot to simulate the position of the valve accordingly.

I thought I had it figured out to detect the pulses from the controller (+22V) by clipping but now that i'm doing some testing I noticed it are actually pulses of +22V or -22V depending on the direction. I wonder how i could detect the negative voltage on an arduino, I just want to use a DI?

I was thinking about using a optocoupler and using the ground connection of the digital pot but as i dont know how the controller exactly works i'm afraid of sending too much current there.

I dont want the solution but just a pointer to the best route to take. I have the feeling I'm overlooking something abvious.

Thanks,
Bert

[Zero999]

There are many ways to detect pulses below the power supply rail.

You could connect a potential divider between the MCU's +V pin, the input and the signal you're detecting. With a +V of 5V, a divider ratio of five, will mean the MCU input will drop below 0V when the input falls below -20V. Don't worry if the MCU input goes slightly below 0V, the protection diodes will prevent any damage, providing the current is low enough.