Using a photo-interrupter as a train detector

The most common approach to detecting trains on model rail­way layouts is based on detecting the current drain of locomo­tives on sections of track (Twin-T detectors etc) or uses reed relays under the track with magnets attached to the rolling stock. A more convenient approach is to use light beams but this means you usually have to devise your own optical detectors.

This approach makes use of the photo interrupters used in many printers. The interrupter is a slotted module with an in­frared light emitting diode in one half and a photo transistor in the other. By cutting the interrupter in half and positioning the two halves on either side of a section of track, you have a good train detector.

The circuit feeds the voltage from the emitter of the photo-transistor to the input of a Schmitt trigger gate and its output drives a transistor and relay. The idea can be extended to suit a range of model railway applications, with the Schmitt trigger being used to trigger flipflops for signalling and au­tomatic reversing circuits and so on.

Suitable photo interrupters are available from Jaycar Elec­tronics (Cat. ZD-1901).

SILICON CHIP.

Monitor for 12V SLA batteries One of the disadvantages of sealed lead acid (SLA) batter­ies is that if they are too far discharged they become permanent­ly damaged and cannot be recharged. Therefore it is good practice to make sure that 12V SLA batteries are not discharged below 11V. This little circuit monitors the output of a 12V SLA battery and can be set to light a flashing LED when the voltage drops below 11.3V. IC1 is an LM336 2.5V reference and is connected to the non- inverting input (pin 3) of op amp IC2 which is connected as a comparator. The battery being monitored also powers the circuit and a portion of its output is fed to pin 2 via trimpot VR1. When pin 2 drops below pin 3, the output at pin 6 goes high to turn on transistor Q1 and the flashing LED. The base of Q1 is fed via a voltage divider consisting of the 15kΩ and 10kΩ resistors, to ensure that Q1 does not turn on when the output of IC2 is low. VR1 should be set so that the LED flashes when the circuit voltage drops to around 11.3V, to give adequate warning of exces­sive discharge. Laurie Marshall, Barrack Point, NSW. ($30)

How to cut clean holes in plastic front panels

This method for cutting round holes came about as a result of the article on making front panels in the February 1999 issue. The suggested method of cutting the holes using a sharp scalpel can be rather tedious, especially if quite a few holes are re­quired.

My method is to use a wad punch (a hollow punch), rubber mallet and a block of wood to make the holes. If the sizes of the holes in the artwork are made so as to be just visible around the circumference of the wad punch, a perfectly placed hole will be achiev­ed every time without the danger of slipping and ruining the whole panel.

A set of 12 cheap wad punches covering the range 3.2mm to 19mm can be obtained for around $15 and are adequate for this application. For smaller holes (eg, for LEDs and screws), a hand­held leather punch can be used.

I apply a self-adhesive laminate (no laminating machine required) to both sides of my artwork to make them stiffer and more durable. Small panels don’t need to be glued on and can be held in place by the hardware.

Barry Hubble,
Moulden, NT. ($25)

12V fan controller for lower noise If you wish to use a 12V computer fan for general cooling or you want to add it as an extra to your computer, you may find it worthwhile to cut the speed as it can make a big difference to the noise it produces without cutting the airflow too much. This circuit provides an extra benefit of temperature con­trol by incorporating a thermistor so the fan will be run at full speed as necessary. Op amp IC1 is connected as a comparator and the thermistor is connected to its inverting input, pin 2. The non-inverting input, pin 3, is connected to trimpot VR1 and this provides the means for setting the temperature. The thermistor has a negative temperature coefficient (NTC) and when the temperature is high its resistance will be low. This causes the voltage at pin 2 of IC1 to be low. If pin 2 is below the set-point of VR1, the output at pin 6 will be high and this will switch on transistor Q1 and the relay. As the thermistor is cooled by airflow from the fan, its resistance will rise, the voltage at pin 2 will also rise and the op amp’s output will switch low, causing Q1 and the relay to switch off. Since the 741 op amp cannot switch its output to 0V but only to about +2V or so, a 4.7V zener diode is connected in series with the base of Q1 to prevent it turning on when the output is low. The 1MΩ positive feedback resistor between pins 3 & 6 of op amp IC1 provides a degree of hysteresis so that the relay does not chatter at close to the switching point. The 30Ω 1W resistor may need to be varied to suit your 12V fan but should be selected to give around 8V across the fan motor when the relay is off. The thermistor should be mounted on the object to be cooled or in the airflow from the fan. Paul Walsh, Montmorency, Vic ($30)

Constant current load for power supply testing

While resistive loads can be used when testing power sup­plies and driver circuits, if the current is DC, it is better to use a semiconductor constant current load. This has the advantage that it can be set to provide any desired current and it will maintain it even in the supply voltage varies.

With this circuit, taken from a Maxim application note, you can select a current within a range up to 1A or 10A with switch S2 and then precisely set the current with potentiometer VR3 which ideally should be a 10-turn type for high setting accuracy.

IC1 is a 1.2V bandgap reference and it provides a very precise voltage reference for the circuit. Its output is fed via resistors to trimpots VR1 and VR2 and then to pin 3 of op amp IC2. These trimpots are adjusted to provide 1V for the 10A range and 100mV for the 1A range.

IC2 and Mosfet Q1 are connected so that the voltage across the 0.1Ω resistor is maintained at the same level as that set at pin 3; ie, it maintains a constant current. The Mosfet needs to be mounted on a large heatsink and the circuit is suitable for testing at supply voltages between about 2V and 50V. Note that it could not handle a 50V supply at 10A; the total dissipation of 500W would vaporise Q1!

Note that the op amp must be able to switch its output to 0V and its input common mode range must be able to go to 0V.

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