Simple Switched Servo Driver

  29 Jan 05 - Corrected an error in the PCB design (spotted by Raymond Firth).

This is another design generated at the request of my father. It is designed to be a dimmer for a 12V reading lamp, but could also be used as a motor speed controller (for a drill). The circuit pulse the voltage seen by the load an the pulse width is variable across the entire range from 0 to 100% (i.e. from off to fully on). The circuit is extremely simple, using 2 inverter gates as an oscillator, and the remainder in parallel to drive the FET. The circuit schematic is:

Because the circuit is simple, and I only wanted one of them, I didn't think it was worth making an etched PCB, so a veroboard layout was produced. The layout below is intended to be mounted to a panel using the potentiometer thread and has screw terminals for the power wires. The FET will also be mounted on the panel so that some heatsink effect can be gained from the panel. To allow this, and easy access to the screw terminals, both the Potentiometer and FET are mounted on the track side of the board. The Potentiometer is mounted in contact with the board and also glue is applied to keep it in place. The leads of the FET are bent up as shown and it is soldered in place so it is flat on the back of the panel. As the leads of the 1N4148 diodes are so small, they share a single hole for the common connection. If you want to reverse the direction of operation of the potentiometer, just reverse both diodes from that shown.

I later decided that an etched PCB was worth the trouble and created the following layout, which is intended to use a connector block with a 4mm pitch. The PCB shown is at 600 dpi and shown looking from the track side.

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