This simple project resulted from a letter written in Scale Rails Magazine (Volume 75, Number 3, Issue 812 - November, 2008). In a letter written in responose to a another letter describing a circuit which generates a grade crossing signal, Kenneth W. Levine wrote about an alternative simple, more reliable and very stable circuit which generates alternating grade crossing signals based on a common Quad-LM339 Comparator. I found the circuit to be not only simple but elegant in it's design. The signal generated is very stable and flexible as the circuit accepts a wide range of input voltage.
The circuit presented by Mr. Levine generates an alternating grade crossing signal which oscillates at about 1.5Hz. It drives two LEDs which can be placed in a model grade crossing to serve as a realistic model. As mentioned above the circuit is based on a Quad-LM339 Comparator. However it uses only 3 of the 4 available comparators on the chip. I thought an interresting enhancement to this design might be to also add an output signal which could also trigger a bell. The circuit would then have the ability to not only produce flashing signal lights but also sound a bell. Cool!
The bell signal is a short pulse (approximately .1 sec) which is intended to drive a small bell through a coil.
Links / Downloads
Mouser EZ-Buy Project (Parts)
BatchPCB Order Link (PCB)