Blue LEDS to Show Train Location

by Charles R. Hentz

During open house with crowds or scenery blocking our view of the trains, we needed to know where each train was located when out of view. To solve this we used several Circuitron DT-4 Rolling Stock Detectors which use photo sensors between the rails. What is really nice about this board is the ability to adjust for the amount of light above each sensor. If a person casts a shadow on the sensor, it will not be triggered if adjusted for the true dark of a train.
When a train is over the sensor, the DT-4 produces a low on the output. A blue LED on our panel would light up as long as the sensor was dark. However, the sensors were further apart than the length of a train. So the operator would glance at the blue LEDS and not see any lit. In order to use as few sensors as possible and still know the last location of the train, we needed each blue light to stay on until the next sensor was dark.
To accomplish this required using the 555 timer integrated circuit as a bilateral switch. A low from the DT-4 would trigger a high at the 555 timer output and reset the previous timer back to a low. Since our line of Blue LEDS on our panel had been wired with a common positive feed, we needed a low to light each one. That is why we also used an 4049 inverter integrated circuit to change our high output from the 555 timers to a low to activate our Blue LEDS. We incorporated the 556 dual timer to save space.