Here is an alarm circuits for clocks using IC 5387, 5316, LM8361 etc. (any IC giving an alarm and a 1Hz outputs). This circuit has the following advantages over most of other alarm circuits (especially alarm circuits based on IC 555):
Working Principle of Alarm Circuit for Clocks
- The quiescent current is only 0.1 mA which is much less than the 10-15 mA consumed by 555 circuits. Hence the life of 9V battery is increased by above three times.
- The operation current is also very small. IC 555 based circuitry consumes 30-50 mA to give the same amount of volumes that this circuits gives. The low currents consumption of this circuit is due to the high efficiency of the piezo-buzzer being used in the circuit.
- Through the operating current is small, the volume is at least equal to that of IC 555 based circuitry.
- With IC 555 based circuits, the continuous tone obtained is not very desirable. This circuit gives four beeps in the first half of every second and is silent for the next half of the second.
- This circuit is not costly. It costs only $0.5 – $0.6 which is only $0.25 more than the cost of IC555 based circuitry.
Circuit Description of Alarm Circuit for Clocks
Here IC CD4047 is configured as a true-gated astable multivibrator. When astable enable pin 5 is high, it oscillates at a frequency set by R4 and C1. C3 and R3 are used for power-on reset as the IC sometimes latches up on switching on power. D1, D2 and R5 from an AND gate so the IC oscillates only when both inputs, 1 and 2 are high. Then, during the high period of the output, transistor T1 is switched on and the piezo buzzer sounds emitting the characteristic alarm sound. Resistor R1 and R2 are used as pull-down resistors since IC 5387, etc. are PMOS ICs. R1 is the cause for the quiescent current.
If one gets less than four beeps, resistor R4 should be decreased ad vice-versa. The supply leads go to the DC supply of the clock (along with the supply of IC 5387).
PARTS LIST OF ALARM CIRCUIT FOR CLOCKS
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1, R2 = 47 KΩ
R3 = 100 KΩ
R4 = 330 KΩ
R5 = 220 KΩ
R6 = 33 KΩ
|C1, C2 = 0.1 µF
C3 = 1 µF/25V
C4 = 470pF
|IC1 = CD4047
T1 = BC147
D1, D2 = 1N4001
|PZ1 = Piezo Buzzer|