Help With Ringing on trigger circuit

[halltech]

Hi! I've been having a bit of trouble with ringing on a input signal to a uc.
The setup is as follows:

The circuit is just basically converts the +12V signal to a 5v level, the input signal is from an infrared sensor (Telemecanique Osiconcept) which is connected to the uc via approx 15m of unscreened cable.
During testing I have the sensor and cable around on the floor and when I turn the lights on in the office I get a high frequency ringing on the signal line with then triggers the uc unintentionally.
As the infrared sensor is used as control signal, I don't want to have false triggers caused by noise.
The uc and other circuity is all power by a 12 battery with the 5V supply a switching regulator running at 250khz

Any help would be appreciated.
Cheers
Cameron

Attached is the trigger circuit schematic and a pic of the oscilloscope output

 

[keith1200rs]

Problems with grounding an 0V connections are possible but I notice that the transistor has not base-emitter resistor. It will therefore trigger with quite small input voltages. Normally the resistor to ground would be directly from the transistor base to form a potential divider and hence reduce the sensitivity.

Keith

 

[halltech]

Cheers, Will modify the circuit.