This simple device causes a lamp to light after a time that can be adjusted between a few seconds and over half an hour.
We can use it as a timer for fixing time for printed circuit boards, chess games, questionnaire response times in schools or companies, and many other similar applications.
The circuit is powered with a voltage of 12 V that can be obtained from source or battery. The time indication is made by a light bulb, but it can be replaced by a relay or a buzzer.
In figure 1 we have the complete diagram of the apparatus which is based on a unijunction transistor and an SCR. P1 adjusts the time.
To reset the circuit after the time delay, you must turn off the power for a moment.
In figure 2 we have the assembly of the circuit in a terminal strip.
Figure 2 - Bridging of terminals.
The SCR does not need a heat radiator and a time scale can be added to the potentiometer. The assembly can be mounted in a small plastic box.
Material list
Q1 - 2N2646 - unijunction transistor
SCR - TIC106 - silicon controlled diode - any voltage
X1 - 12 V - 100 to 500 mA bulb
C1- 1000 uF at 2 200 uF - electrolytic capacitor for 16 V or more
C2 - 100 uF x 16 V - electrolytic capacitor
R1 - 10k ohm x 1/8 W- resistor - brown, black, orange
R2 - 470 ohm x 1/8 W- resistor - yellow, violet, brown
R3 - 100 ohm x 1/8 W - resistor - brown, black, brown
Miscellaneous:
Power supply, terminal bridge, wires, solder, mounting box, etc.
