Nuts and Volts - August 2008

Q^&A

■ WITH RUSSELL KINCAID

In this column, I answer questions about all aspects of electronics, including computer hardware, software, circuits, electronic theory, troubleshooting, and anything else of interest to the hobbyist. Feel free to participate with your questions, comments, or suggestions.

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WHAT’S UP: Join us as we delve into the basics of electronics as applied to every day problems, like:

✓Fly Zapper

●✓Vacuum Tube Circuit

✓Solar Tracker

RAILROAD CROSSING LIGHT CONTROLLER

QI am a volunteer worker/helper at a small local history museum located in Tullahoma, TN. We have a two arm RR crossing light that has been modified with small 110V holders and lamps. I would like to buy/build a controller to alternately light the lamps so that when one goes out, the other will come on. Do you happen to have any ideas?

— Greg Klein

AThis calls for a flip-flop. I have used a 555 oscillator driving an SN74LS74 and two solid-state switches (see Figure 1). The oscillator is designed to run at 2 Hz, but you may want to make the frequency variable by replacing R2 with a rheostat connected 500K pot. The SN74LS74 is a dual D type flip-flop and I have connected them in parallel to give more drive to the solid-state switch. The output is arranged to pull down because TTL does not pull up very well. The switch is an opto triac rated at two amps, so it will operate a 100 watt lamp with no problem. The rocker switch

mounts in a rectangular hole; you will have to file it to fit, and the toggle switch mounts in a round hole — just drill it. A five volt power supply is needed and the power is low, under 1/2 watt. If you don’t have a spare kicking around, this $8.70 unit from Mouser will do: 826-DA4-050US.