One second Audible Clock

Accurate, finger-operated portable unit

3 - 12V Battery supply

Circuit diagram:

One second Audible Clock


R1______________10K  1/4W Resistor
R2_______________4K7 1/4W Resistor
R3_____________100R  1/4W Resistor (Optional, see Notes)

C1_______________1nF  63V Polyester or ceramic Capacitor
C2______________10µF  25V Electrolytic Capacitor
C3_____________100nF  63V Polyester or ceramic Capacitor (Optional, see Notes)

D1,D2,D3_____1N4148   75V 150mA Diodes
D4______________LED   (Optional, any shape and color, see Notes)
D5___________1N4148   75V 150mA Diode (Optional, see Notes)

Q1____________BC337   45V 800mA NPN Transistor

IC1____________4024   7 stage ripple counter IC

BZ1___________Piezo sounder (incorporating 3KHz oscillator)

SPKR______________8 Ohm, 40 - 50mm diameter Loudspeaker (Optional, see Notes)

SW1____________SPST Toggle or Slide Switch (Optional, see Notes)

B1________________3 to 12V Battery (See Notes)


This accurate one-pulse-per-second clock is made with a few common parts and driven from a 50 or 60 Hertz mains supply but with no direct connection to it.
A beep or metronome-like click and/or a visible flash, will beat the one-second time and can be useful in many applications in which some sort of time-delay counting in seconds is desirable.
The circuit is formed by a CMos 4024 counter/divider chip and 3 diodes, arranged to divide the frequency of the input signal at pin #1 by 50 (or 60, see Notes).
The input impedance at pin #1 is very hight, so simply touching the pin (or a short track or piece of wire connected to it) is usually enough to provide the necessary input signal.
Another way to provide an input signal consists in a piece of wire wrapped several times around any convenient mains cable or transformer. No other connection is necessary.