Your right mackrackit,

But this cart is only used outside.

BTW, the cart has already an "electonic" speed indicator (looks like a cheap gadget).

The sensor's output must be an open-collector type contact (I might be wrong).

I tried to capture the signal in parallel with the sensor but I must do wrong an the only result I get is to make the signal crash completely (no speed is indicated anymore). I use an OpAmp for this but there is surely a better way...