$18.00 on eBay

Track Tester showing the voltage and polarity

Close-up of TACK TESTER

Back of tester showing the microcontroller
and surface-mount components

Our kits has more features and costs $14.50 posted.

THE LEDs
The voltage on the track is shown by a set of 4 LEDs. You don't need to know the exact voltage but if the voltage is higher than 18v you need to be aware as this will damage some electronics modules and below 10v will indicate a fault somewhere in the system.
The LEDs are arranged in 2v increments and two LEDs will flash when the voltage is between two LEDs:

TRACK TESTER CIRCUIT

THE CIRCUIT
The circuit has two red LEDs to indicate the positive track and a voltage detector to convert the voltage to one of the LEDs in the BAR GRAPH.  This means you can put the tester around either way and will produce the correct results.
Our project is quicker and more convenient to measure the voltage with the spring-contacts. 

CONSTRUCTION
The project is

The other Loud Mobile Ring Circuit

USING THE TRACK TESTER
Place the tester on the rails and the springy terminals will make contact. One of the red LEDs will illuminate and one or more of the LEDs in the BAR GRAPH will indicate the voltage.
The piezo diaphragm will buzz or hum if the track is DCC.
It will also buzz if the track is DC but has a lot of "ripple." This means the smoothing electrolytics in the power supply are not working to produce SMOOTH DC. This may be the reason why DC motors are "humming"

PROGRAMMING
This project is part of a PIC microcontroller course to teach programming.
The kit comes with a pre-programmed microcontroller and you just have to solder the kit and it is ready to work.
However there are a number of constructors who will be using this project to help them understand how to program a microcontroller and this section is part of the PIC micro course.  

Before choosing a PIC we tried zener diodes and resistors to turn ON the LEDs at different voltages, but the result was very poor. It was very difficult to get them to turn ON at the desired voltage and turn OFF at a higher voltage.
We could have used a LED BAR GRAPH chip but it does not have the flashing feature to show when the voltage is midway between each range.
A micro is small, cheap and is a lot more versatile and anything else.
All you have to do is write an A-to-D program to detect the voltage.
This requires just a single input/line, a 100n capacitor and a high resistance resistor.
The micro discharges the capacitor at the beginning of the cycle then turns the line into an INPUT.
It then calls a very short delay and looks to see if the capacitor has charged to about 3v. This voltage is detected by the micro as a HIGH. If the input line is not HIGH, the program calls the short delay again and looks for a HIGH.
A file is incremented every time the delay routine is called.
The lowest voltage we need to detect is 9.9v and this resulted in about 40 delay loops. As the voltage is increased, the number of delay loops decreases, so that 18.9v resulted in 19 loops.
The whole secret to setting up this "test-routine" is to get a delay routine that is short-enough to result in about 4 loops for every volt increase.
We used an adjustable voltage bench power supply and the delay routine was shortened until we got about 4 loops for each volt. This allowed us to detect each 250mV increments.
Once we have a table of loops for 9.9v to 18.9v we had values from 19 to 40.
All we have to do is put these values into a sub-routine so that each value produces a result on the "bar graph."
You must remember this: You do not know what value will be produced by the micro so all values must be handled and a result on the display must be produced. Otherwise the micro will jump to the wrong part of the program.
The value of loops we get is called RAW DATA and we must "clean it up" by removing the values 1 -19. The first instruction subtracts 19 to get a result of ONE to TWENTY.
This means we now have to allocate 20 values to the display in which the the value of ONE represents 18.9v.
Every value must go to a LED or a pair of flashing LEDs.
We now decrement "loops" and if it zero, we know the result is 18.9v. This means we flash the top LED.
If the answer is not zero, the micro goes down the sub-routine and decrement the value again. If it is zero, the top LED is turned ON.
The sub-routine keeps decrementing "loops" and turning on one of the LED or flashing two LEDs, until "loops" is decremented to zero.