The timer we present has its time regulation made through a 1 M ohm potentiometer. When S1 is closed by sourcing on the power, the capacitor starts charging through the potentiometer and R1 until the SCR trigger voltage is reached.
When this occurs the SCR triggers causing the lamp to light on that thus remains indefinitely. To deactivate the circuit, simply press S2 momentarily.
In the potentiometer can be adapted a timescale that will be obtained with the help of a common clock or stopwatch.
Instead of the lamp a 6 V relay can be used which can then be used to control a high power external circuit. The power supply, in this case, must be made with 9 V to compensate for the voltage drop in the SCR. The range obtained varies from a few seconds to a few minutes, depending on the value of the capacitor.
The assembly, based on a terminal strip, is shown in figure 2. The SCR can be the TIC106 and the power will be made with 4 common batteries.
The resistors are 1/8 W and the capacitor must have a working voltage of at least 6 V. The assembly can be installed in a small plastic housing.
Material list
SCR-TIC106 or equivalent - silicon controlled diode
L1 - 6 V x 50 mA - common lamp
P1- 1 M ohm- potentiometer
S1 - Single on/off switch
S2 - Pressure switch
B1 - 4 small batteries
R1, R2 - 10 k ohm - resistors (brown, black, orange)
C1-470 uF or more x 6 V-electrolytic capacitor
Miscellaneous:
Battery holder, terminal bridge, mounting box, potentiometer knob, wires, solder, etc.
