The circuit shown in the figure causes the lamp to light up after a certain time determined by the adjustment of the 2.2 M ohm potentiometer, and by the capacitor from 100 to 1000 µF in the emitter of the unijunction transistor. When the unijunction transistor trips, the pulse produced charges the 10 µF capacitor in the field effect transistor gate, keeping it in conduction. Since the input impedance of the field effect transistor is extremely high, the load and conduction can be maintained for long periods of time (more than 1 hour depending on the capacitor). To turn off the lamp and reset the circuit, simply discharge the 10 µF capacitor by pressing the pressure switch in parallel. Best results are obtained with the use of polyester capacitors from 1 to 10 µF. The delay time obtained with a 2.2 M ohm potentiometer and a 1000 µF capacitor can reach 1 hour.
