How to Design Schematic Circuit of Timer

Small home appliances and circuits can be turned on atter a time delay ranging from seconds to minutes using this simple timer. School and hobbyist

experiments can be time-controlled byusing this circuit.

The unit has a constant-current source to charge cl. Cl and Pl adjusthe

time range.You can alter C1 within a range of 100 and 1000 uF to change the

time range.

Using a 1 ampere relay you can control AC loads up to 100 watts wired to the

1 17 VAC power line.

To operate the unit, connect the load to the relay contacts (points A and B,

used as a switch) and adjust P1 to the desired time delay. Press 52 do discharge Cl completely . Then close Sl to start. After the adjusted time delay,

the relay will close, and the load will be powered ON.

Conversely, by using the NC (Normally Closed) contacts of the relay you can

turn a device OFF after the adjusted time.

To use the unit again you must press 52 to discharge C1 before starting.

Figure I shows the schematic diagram of the tlmer. Observe the ditferent

components used in this project: a unijunction transistor (UJT) and an SCR

(Silicon Controlled Rectifier).

An mechanical view is shown in Figure! using a terminal strip as chassis.

All the components and power supply can be housed in a small plastic box.

Take care with'positions of critical components such as the UJI, SCR, diodes and dilectrolytic capacitors.

The relay depends upon the power supply voltage and voltage drop through

the SCR. This drop is about 2 volts, so you need to use 6 to 7.S-volt relays

with a 9-volt power supply.

The SCR needn't be mounted on a heatsink, as the current drain is low.

Parts List -Timer

Q1 - 8C548 NPN general-purpose silicon transistor

Q2 - 2N2646 unijunction transistor

SCR - TlC106 silicon controlled rectifier

D1,2 - 1N4002 silicon diodes

R1 - 47,OOO ohm, 1/4W,5% resistor

R2 - Lz}ohm, 1/4W,5% resistor

R3 - 100 ohm, 1/4W, 5% resistor

P1 - 100,000 ohm potentiometer

C1,2 - 100 uF,12WVDC electrolytic capacitors

K1 - Relay - see text

51 - SPST slide or toggle switch

S2 - Normally Open momentary switch

81 - 9V - six AA cells