incandescent bulb 38 to edge light the plate 34 touch plate 30 is connected to the gate electrode of n channel fet transistor ft1 fet transistor ft1 and fet transistor ft2 are connected in series between a load resistor r1 connected to an operating voltage source v and a reference potential ground with the source electrode of transistor ft1 being connected to the drain electrode of transistor ft2 a resistor r2 of high value interconnects the gate and drain electrodes of transistor ft1 while the gate and drain electrodes of transistor ft1 are directly interconnected the source electrode of transistor ft2 is connected to the base electrode of npn transistor tr1 having a grounded emitter electrode and a collector electrode connected via a load resistor r3 to source v a filter capacitor fc connects the collector electrode to ground a difference amplifier da acting as a high input impedance threshold circuit has its positive input connected to the collector electrode of transistor tr1 and its negative input connected to a reference potential source vr the output of the difference amplifier da is connected to the toggle input t of binary divider bd3 whose q output is connected to an output terminal ot1 and the input of lamp driver.