alarm with countdown timer

This circuit is IC implementation schedule. No. 555 applied with IC number LM3905.By the IC to the number 555 are the circuits Stable multi-vibrator. How often do produce it.This frequency is then forwarded to the scheduling with the integrated circuit number LM3905.
When entering the power supply circuit IC1 No. 555. To be atable multivibrator circuit. Produce about 0.5 hertz frequency out.The R1, VR1, R2 and C1 determine the frequency.The output pin 3 will be out of a square wave LED1 will be lit.The signal is a negative split.and LED2 will be lit during a split positive signal.This will flash alternately throughout the duration of scheduled. IC2 is a timer IC.Includes VR2, R6 and C3.The setup time will be how long it can adjust the VR2.And while time is the output pin 7 of IC2 pin voltage will be 0 volt.And when it’s time to set the output pin 7 will have a positive voltage to Buzzer beeps out the rhythmic flashing of the LED.

Source : Countdown timer with alarm

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