catching pulse below ground

[yougarage]

Hi, I have a variable reluctance sensor(a pick up) monitoring a half moon wheel.One wire of the sensor is grounded. It's output is a positive pulse when the pickup meet the full edge of the wheel, and a negative pulse when it meets the other edge. I need to pick these signals to feed a mcu, I know how to condition the positive one, but how can I pick the negative one? Ground is shared between the mcu and one side of the pickup.I tought of a pnp transistor, do you suggest anything better?

 

[D.A.(Tony)Stewart]

It requires a small coupling capacitor with equal pull-up/down resistors like 10k and appropriate Schmitt trigger. This biases the signal to V+/2

If this is new to you, here are some details.

 

[BradtheRad]

This sounds like a case of level-shifting an AC signal into the positive region.

A potentiometer of the right value can act as a pull-up resistor:

 

[yougarage]

Thanks for your replies
both solutions look interesting, however the first avoid a current load to the sensor, right?

 

[BradtheRad]

Thanks for your replies
both solutions look interesting, however the first avoid a current load to the sensor, right?

Correct. My method has a few uA travelling down through the sensor. If your sensor cannot sink current then you need to feed its signal to a device which is voltage controlled.