# Crossing controller picking up track noise



## mikek (Dec 29, 2013)

I've built a circuit board using IC chips (74ls123 voltage comparator for photosensors, a 556 to control the servos, a 555 to control the flashers, and a dpdt relay between the sensor chip and the servo chip to determine up or down on the gates). The circuit operates fine away from the track, but every time I connect or even get near the track and run a loco, the circuit picks up noise and acts up. Just turning on the track controller is picked up, triggers a reaction from the 74ls123. I have put .1uf disc caps on the track bus and only feed battery power to the crossing controller. I tried snubbers, and a home-made Butterworth filter. Still the controller picks up the noise. My test track is a circle. DC. 
Any clues?


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## wvgca (Jan 21, 2013)

the 555 / 556 inherently generates what can be called a 'supply current glitch' due to it's design, around 150ma during output transitions, a relatively large bypass cap should be added across the supply close to the ic ... low power versions such as the TLC555, and LMC555 have better spurious noise rejection .
if on a 'bread board' overly long connecting jumpers can add false 'noise' triggers .. both on the timer and detector sections


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## mikek (Dec 29, 2013)

Thanks so much. That's the one place I had not placed a bypass cap, near the power pin of the 74ls123 IC. I'll do that. Mike


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