Ham Spot Bits: Touch Dimmer Repair

In this video I repair a touch dimmer control.  This device is inserted between a lamp and the wall receptacle.  

 

A touch sensor is created by extending one plate of a very low value capacitor to a larger metal surface.  The capacitor is alternately charged and discharged several hundred times per second.  When a large conductive object, such as a person, touches the plate the discharge rate of the capacitor is slowed.  The touch sensor circuit measures the discharge rate comparing it to a stable timer.  If the discharge rate changes for more than a few dozen milliseconds the sensor triggers a mode change in the dimmer.  

In principal, this works similar to the Arduino circuit I built recently. The main difference is that the Arduino version of this circuit was done mostly in software.  The only external components required for the Arduino were the 1nF capacitor and the metal touch plate.