Detection

Sensors help control the layout...

The Occupation detector

This schematic is designed to detect whether or not a train is present at a certain (piece of) track. It is connected to the common power mass and the "minus" of the track. In fact this is a current sensor, which is able to detect currents as low as 50 microampere.
For the circuit to work, the train engine and at least the last car should consume some power. This can be a simple 12 kilo-ohms resistor.

There is a 1200mV voltage drop over the entrance diodes when current passes through them. The transistor amplifies this and the condenser smoothes the signal. The 40106-schmitt-trigger converts this signal to digital levels, one signaling 'free', the other 'occupied'. As there are six triggers in one 40106, 3 detectors can be build with one IC. I constructed 9 detectors on one circuit board.
After mounting and connecting the first boards, the circuit proved very sensitive to interference. For that reason an extra 10k-resistor was mounted parallel to the diodes..

Modifying the wheelsets

This subject was published in Modelspoormagazine 50 (July 2006). More information can be found there

In order to have a reliable current detection, all wheelsets should have a resistor. This can be done using special resistance paint, but using a real resistor is far more reliable. Luckily, very small resistors (SMD types) exist.

Dismantling electric wire, we get some very thin copper wires. These are used to solder connection wires to a 12K SMD-resistor. Blokking the resistor on a piece of scrap wood using a needle pin, the wires are soldered to the resistor using soldering fluids..

After soldering, the excess wires can be clipped with a pair of scissors. Next, the wheelset can be dismantled gently

One wire against the axis, the other between the isolating tube and the wheel flange, the wheelset is gently re-assembled. Check the wheelset gauge.

Finaly, the resistor is gently pushed against the isolation and glued to it, in order to protect the resistor. Double-insulated wheelsets can be adapted using the same principle, although the wire will, in that case, pass along the wheelset axis.