Everything for Electronics

From the Q&A

With TJ Byers

Trolley Tricks


I use a Miniatronics RU1-1 point-to-point automatic reversing unit on my model train layout to control a trolley running up and down Main Street. Problem is, when the trolley gets to each end of the line it reverses instantly and takes off in the opposite direction. Can you suggest a way to kill power to the tracks for a few seconds each time the track voltage is reversed?

— Don Hicke
San Diego, CA


The time delay is the easy part. The hard part is triggering the timer from a reversing DC voltage. After considering the alternatives, I decided to monitor the voltage across the RU1-1 reversing relay rather than the voltage across the trolley track itself. For one, the relay voltage is either on or off, and second, it’s a constant 12 volts as opposed to a variable voltage that the speed controller places on the RR tracks. That done, I now needed to translate the relay voltage (or lack of) into a negative-going pulse to trigger the 555 timer. This I did using a pair of inverting logic gates.

When the gates are cascaded, a negative transition occurs alternately at the outputs of the inverters each time the relay is energized and de-energized. These outputs are then summed and fed to the time-delay’s input trigger (pin 2). The result is the circuit shown below.

The 1N4148 diodes, 100K pull-up resistors, and .01 µF caps are required to prevent the outputs from interacting with each other. Now each time the relay changes state — reversing the polarity across the tracks — the 555 activates the time delay relay, which cuts power to the tracks for the prescribed time. This is about 12 seconds for the values shown; increasing or decreasing the value of the 100 µF cap changes the delay time accordingly.