The project “programmable digital timer circuit” presented here can be used for setting time intervals in the range of 1 to 99 minutes. The circuit of programmable digital timer does not use large resistors or capacitors to obtain the timing interval. Instead, it uses the 50 Hz mains frequency as a reference. The mains frequency is fairly accurate for this application.
The main frequency is shaped up by Schmitt-trigger gate N1 to obtain a square wave. This 50 Hz square wave is divided by IC1, IC2, IC3 and IC4. As a result a single pulse per minute is obtained at pin 12 of IC4.
The heart of the circuit programmable digital timer consists of two presettable synchronous up-down BCD counters (IC5 and IC6) in cascade. Two seven-segment LED displays are included to indicate the number of minutes left in the timing interval.
To preset the timer, SW1, a DPDT switch, it thrown into Preset position after a momentary push of the Reset switch SW2. The display now shows 00. Press SW3 or SW4 until the display shows the number of minutes you want to time. Throw SW1 back to Operate position. Pressing SW5 once sets the NAND latch formed around N5 and N6. The output of gate N5 goes high and saturates transistor T1 and the relay is energized.
IC5 and IC6 start counting in the down counting mode as soon as the output of gate N5 goes high. The borrow pin (13) of IC5 is used to reset the latch N5 and N6 after completion of time interval. The borrow output of IC5 goes low when IC5 and IC6 are on their terminal count. The output goes low on the high to low transition of the clock and the output of gate N5 goes low de-energized the relay.
During the timing operation, the display stays on for about 30 seconds at the beginning of each minute being timed. The display shows the number of minutes left in the timing operation.
PARTS LIST OF PROGRAMMABLE DIGITAL TIMER
Resistor (all ¼-watt, ± 5% Carbon)
R1 = 680 Ω
R2 = 470 Ω
R3, R4 = 1 KΩ
R5 = 220 Ω
R6 – R19 = 330 Ω
C1 = 1000 µF/16V
C2 = 0.1 µF
IC1, IC3, IC4 = 7490
IC2 = 7492
IC5, IC6 = 74192
IC7, IC8 = 7447
IC9 = 74132
IC10 = 7410
IC11 = 7805
D1 – D3 = 1N4001
D4, D5 = 1N4148
D6 = 1N4007
T1 = 2N2222
SW1 = Double Pole Double Throw switch (DPDT)
SW2 – SW5 = PUSH – TO – ON Switch
SW6 = ONN OFF switch
X1 = 230 V AC primary to 9V-0V-9V secondary Transformer
RL1 = 9V, 100 Ω relay