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T bridge rectifier with 7806 as regulator for the power supply and charger. hese are the days of competitive basic oscillator frequency is divided by Pressing the reset button momen- examinations. Nowadays, s tu- 16,384. Here the oscillator frequency tarily pulls all the internal counters dents are required to answer a set may be set using potmeter VR1 (in of CD4060 to zero and the timer starts of objective type of questions in a given series with resistor R1) and capacitor again. The timer is reliable, accurate, time which may extend to 15 minutes, C1. Potmeter VR1 is adjusted for a time and consumes about 10 mA of current 30 minutes, or one hour. Speed and ac- period of 15 minutes at output Q12 (pin when the buzzer is active. curacy win the race. Practice helps in 2). The output Q13 (pin 3) will go ‘high’ Piezo-electric buzzers are reliable, achieving it. after 30 minutes. Use a multi-turn pot- rugged, and serve as cheap aud io Here is a timer that gives an alarm meter for better accuracy. alarms. They are available for operation after a preset time of 30 minutes. It also When Q13 goes high, the 3-terminal in continuous and intermittent modes, gives out a few beeps at half the selected piezo-buzzer becomes active and gives and also with musical notes. However, time period. The timer could be pro- out a tone. This buzzer also beeps at if you ever require louder sound, say for grammed for other periods as well. The half the time for a few seconds. Transis- a classroom, then you have an optional alarm can be reset for a fresh start. tor T1 prevents further oscillations after 555 circuit wired up as an astable mul- The heart of the circuit is a CMOS the lapsed time. Red LED1 is used as tivibrator. This can be brought into IC CD 4060. It has a built-in oscillator activity indicator. operation by shorting points x-x in the and a 14-stage divider, wherein the For simplicity, 6-volt supply is cho- circuit