Easily Make A Simple Programmable Timer
This is one of the simple circuits programmable timer in the series of timer. The time range of this programmable timer is of few second to 90 minutes can be selected by single circuit.
Circuit Description of programmable timer
The whole circuit of programmable timer is build from two ICs i.e. IC 555 (IC1) and IC 7442 (IC2). IC1 is used in monostable mode where output is low when trigger pin 2 is high and vice-versa. The output is high only for preset time determined by the value of capacitor C1 with its corresponding preset with the help of switch SW5.
IC2 is used for selecting the required time duration. The 10 different outputs from programmable timer are connected to its corresponding 3 Volts relay where preset VR1 through VR10 is connected to RL1 through RL10 respectively. Table in below shows the appropriate binary codes.
NOTE: If the relay does not work then a relay driver circuit may be used to operate.
PARTS LIST
Resistors (all ¼-watt, ± 5% Carbon)
R1, R2, R3, R4, R6, R7 = 10 KΩ
R5 = 1 KΩ
R8 = 220 Ω
R9 = 520 Ω
VR1 = VR10 = 2.2 MΩ
Capacitors
C1 = 2200 µF/25V
C2 = 0.01 µF
Semiconductors
IC1 = NE555 (Timer IC)
IC2 = 7442
T1 = BC108
D1 – D20 = 1N4007
TR1 = ST44
Miscellaneous
SW1 – SW4 = ON/OFF switch
S5 = push to on switch
RL1 – RL10 = 3V, 100 Ω relay
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