The electronic project “Programmable Timer Circuit” published here can be programmed for more than 250 different time intervals. Programmable timer circuit is not new to electronics hobbyist, various timer circuits have been designed and published so far but the circuit of programmable timer posted here is most versatile, inexpensive and extremely compact timer with alarm.
Description of Programmable Timer Circuit
The main part or heart of the programmable timer circuit is a 14-stage binary counter CD4060 (IC1). Clock pulse is generated by connecting capacitor C1, resistors R4 and R3 to terminals 9, 10 and 11 respectively of IC1 as shown in circuit diagram. The frequency of clock pulse is adjusted by preset VR1 connected in series with resistor R4.
Power-on-reset is provided to reset the counter by giving a pulse from the supply line via capacitors C2, after which it starts counting the pulses generated by the oscillator.
Diodes D1 through D8 along with the resistor R2 are used to form an 8-input AND gate. The truth table is shown in table 1.
The timer circuit published here has eight programmable switches, each with fixed time interval. For example, with approximately 1.06 Hz frequency adjusted with the help of preset VR1, the output Q6 (pin 4) will go high after receiving 26 = 64 pulses, that is after about 60 seconds. In the same way, Q14 (pin 3) will go high after 214 = 16384 pulses, that is after about four-and-a-half-hours.
But if Q6 and Q14 are switched on simultaneously, their time interval get added because of the AND gate arrangement made by using the diode and the resistor. That is why 256 different time interval can be programmed. The circuit of programmable timer can be powered by a 9V battery.
PARTS LIST OF PROGRAMMABLE TIMER
Resistors (all ¼-watt, ± 5% Carbon)
R1 = 1 MΩ
R2 = 10 KΩ
R3 = 2.2 MΩ
R4 = 820 Ω
VR1 = 100 KΩ
C1 = 1 µF/25V
C2 = 0.001 µF
IC1 = CD4060
D1 – D8 = 1N4148
LED1 = Red LED
SW1 – SW8 = 8-Dip Switch