Simple 555 countdown timer circuit with alarm

This is simple IC-555 countdown timer circuit with alarm.The red and green LEDs will flash alternately and when time to setting the buzzer will loudly sound each time LED flashes. We use IC-555 timer as Stable multi-vibrator and next integrated circuit LM3905-Precision Timer. Parts used in this circuit are easily available in most of the local markets.

How this circuit works

Alarm Countdown Timer
simple IC-555 countdown timer circuit

When enter 9 volts power supply to the circuit. IC1- 555 timer act as atable multivibrator to generates oscillator in DC pulse about 0.5 hertz frequency out. In normally the output pin 3 will be out of a square wave. So LED1 will light up when the signal is a negative voltage or “low”, but in same time LED2 will go out. Then, the pin 3 is positive signal, LED1 is OFF and LED2 is ON. This will flash alternately throughout the duration of scheduled. Which the R1, VR1-potentiometer, R2 and C1 determine the frequency or the blinking LED rate.

The IC2-LM3905 is also precision timer IC. which it includes VR2, R6 and C3, to setup time will be long with adjusting the VR2. and change C3 capacitor to higher up cause long time. While the output voltage at pin 7 of IC2 is 0 volt, the piezo-buzzer not sound. But when it’s time to set, the output pin 7 has the positive voltage to BZ1-buzzer beeps out the rhythmic flashing of the LEDs.

Parts will you need
IC1: NE555-timer; Quantity=1
IC2: LM3905-Precision Timer; Quantity=1
C1: 33uF 25V-Electrolytic capacitor; Quantity=1
C2: 0.01uF 50V-Ceramic capacitor; Quantity=1
C3: 33uF 25V or more-Electrolytic capacitor; Quantity=1
R1,R2: 10K 0.25W Resistors; Quantity=2
R3,R5: 1K 0.25W Resistors; Quantity=2
R4: 680 ohms 0.25W Resistors; Quantity=1
R6: 1M 0.25W Resistors; Quantity=1
VR1: 50K Trimmer Potentiometer; Quantity=1
VR2: 1M Trimmer Potentiometer; Quantity=1
D1: 1N4001 50V 1A Diodes; Quantity=1
LED1,LED2: 3mm Red and Blue LEDs;Quantity=2
BZ1: 6V piezo-Buzzer;Quantity=1

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