Again, I selected an unusual article from the lot that Rado sent me a long time ago: The dimmer is intended for controlling the brightness of a small light bulb powered by a DC voltage of 6V from an accumulator or mains supply. The light bulb is powered via N-MOSFET type BUZ10, which forms a Miller integrator with memory capacitor C1. With a finger placed on the sensor surfaces 1 and 2, a positive supply voltage or ground potential is applied to the control electrode G.
By touching the surfaces 1, C1 is charged, the voltage on the electrode G increases, the voltage on the collector (D) decreases and the bulb lights up. Because the resistance of the finger is about 100k to 1M according to the force of pressure, C1 charges relatively slowly and the brightness of the bulb changes smoothly (faster with stronger pressure and slower with weaker pressure). When the desired brightness is reached, we move the finger away from the sensor and thanks to the theoretically "infinite" insulation resistance of the electrode G, the capacitor C1 remains charged with the achieved charge and the brightness of the light bulb does not change further. On the contrary, when the surfaces 2 touch, the capacitor discharges, the voltage on the collector increases and the light bulb goes out. By moving the finger away at the appropriate moment, we fix the selected brightness again.
Because the insulation resistance of the G electrode 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 about 100 nF at C1, there must be a high-quality insulator and a sufficient distance between the metal sensor plates. Electrode G must be connected "in 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 a battery and lamp voltage of 12V, with a higher voltage there is a risk of a breakdown between the G and S electrodes, between which the maximum voltage of +-20V can be in common transistors.
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.
Original source: ELEKTOR, 7-8/2000.