Simple dimmer controlled by sensors

Again, I picked an unusual article out of the lot, which Rado sent me a long time ago: The dimmer is intended for controlling the brightness of a small light bulb powered by DC voltage 6V from a battery or mains power source. The light bulb is powered by an N-MOSFET type BUZ10, which with memory capacitor C1 forms a Miller integrator. With a finger placed on the sensor surfaces 1 a 2 a positive supply voltage or ground potential is applied to the control electrode G.
By touching the tiles 1 C1 is charging, the voltage at electrode G increases, collector voltage (D) drops and the light bulb lights up. Because the resistance of the finger is about 100k to 1M according to the pressure force, C1 charges relatively slowly and the brightness of the bulb changes smoothly (faster with stronger and slower with weaker pressure). When the desired brightness is reached, we move the finger away from the sensor and thanks theoretically “infinite” insulation resistance of the electrode G, the capacitor C1 remains charged with the achieved charge and the brightness of the lamp does not change further. On the contrary, when touching the tiles 2 the capacitor discharges, the collector voltage rises and the bulb goes out. By moving the finger away at the appropriate moment, we fix the selected brightness again.

Because the insulation resistance of electrode G is not actually infinite, the dimmer remembers the set brightness for only a few hours. To make this time as long as possible, we must use a high-quality foil capacitor with a capacity of approx 100 nF, there must be a high-quality insulator and a sufficient distance between the metal sensor plates. Electrode G must be connected “in the air”. The entire device must be placed in a dry environment. A transistor can handle a current of up to several amps. With a larger current, it heats up and therefore must have an adequate cooler. The dimmer could also work with 12V battery and bulb voltage, with a higher voltage, there is a risk of breakdown between electrodes G and S, between which the maximum voltage can be for ordinary transistors + – 20V.

It is also possible to experiment with the size of the capacity C1. The whole connection is an interesting toy thanks to which you can easily familiarize yourself with the function of MOSFET type transistors.

The original source: ELECTOR, 7-8/2000.