Simple LocoNet^® Level Tester

Setting-up the LocoNet^® of a modular layout on a FREMO meeting, a tester is quiet useful. But it is usually sufficient to check the voltage levels. Such a simple tester is described below.

As the boosters are driven by the differential voltage between the two RailSync lines the voltage of these two lines should be measured against each other. A difference of 6 V should be sufficient. The LocoNet^® operates with a threshold of 4 V with a hysteresis of 1 V. For safe operation levels of 3 and 5 V should be crossed. Therefore LEDs for the following states have been implemented:

1. RailSync-A - RailSync-B > + 6 V
2. RailSync-A - RailSync-B < - 6 V
3. LocoNet^® > 5 V referenced to ground
4. LocoNet^® < 3 V referenced to ground

At error free operation the first three LEDs are on, the fourth blinks depending on the activity on the LocoNet^®. The following error conditions may be distinguished:

1. All LEDs off:
   Neither of the RailSync lines carries sufficient voltage.
2. LED a on, b off:
   RailSync-B is shorted to ground or the Chief not in 'run' mode.
   This state may be created by one broken RailSync line as well.
3. LED b on, a off:
   RailSync-A is shorted to ground or the Chief not in 'run' mode.
   This state may be created by one broken RailSync line as well
4. LEDs a and b off, but LED c and/or d is on:
   One RailSync line is broken or
   both lines carry a positive voltage.
5. LED c and d off:
   LocoNet^® it broken.
6. LED d blinks, but LED c stays off:
   The LocoNet^® is overloaded due to non conforming devices, possibly a short at a great distance.
7. LED d permanently on, LED c stays off:
   LocoNet^® is shorted to ground.

This covers the most frequent errors.

The circuitry is available as PDF file. The active parts are only a voltage regulator, a quad comparator and a FET. The voltage regulator may be a low voltage drop type. There are no critical parameters for the enhancement P-channel FET. Therefore any other type may be used, but the pinout must be checked.

The tester uses the same box as the LN-boxes, but just with a single LocoNet^® socket. The manufacturing data for the single sided PCB (drill drawing, place plan and layout) are available as PDF file. The hole positions marked 'A' and 'C' should be drilled with 0.7 to 0.8 mm (28 to 32 mil), those marked 'D' with 3,1 mm (1/8 inch). The holes marked 'B' are feed-throughs for the wires to the 4 mm banana jacks and may be drilled with a maximum diameter of 1.4 mm (55 mil).

The LEDs are soldered to the board with nearly unshortened leads and bent to the top to look through the 5 mm holes in the box. The 4 mm banana jacks are connected with the four signals at the RJ12 socket to allow further measurements. As the 4 mm banana jack are located on the other side of the board as the LEDs, the wires have to be feed from the solder side.

Initially a BSS92 was used instead of the BSS110, which is easier to get but has a different pinout. For the BSS92 version the manufacturing data is also available. Please note that in the drill drawing the letters 'B' and 'D' have swapped their meaning relative to the BSS110 version

I build my own tester without an etched board but on a simple hole matrix board. The three images show the construction. The arrangement of the parts follows roughly the placement plan.

Figure 1: LocoNet Level Tester from the inside

Figure 2: Top side of the board for the LocoNet Level Testers

Figure 3: Bottom side of the board for the LocoNet Level Testers