Summary

A three-digit initial state, determined by three 4-position dual inline (DIL) switches, is set by pressing Switch 1 (SW1). Pressing Switch 2 (SW2) starts a count down to zero. When zero is reached, the light emitting diode (LED) lights up. The progress of the count is shown by a three-digit seven-segment liquid crystal display (LCD).

This improves on Countdown Mk1 by using different multivibrators, so that the count down is close to 1 Hz and the duty cycle of the signal driving the LCD is close to 50%.

Circuit

 

Schematic

As for Countdown Mk1, but each LM555 integrated circuit (IC) used as an astable multivibrator is replaced by a HCF4047B IC, which guarantees a 50% duty cycle. One timing circuit is at about 1 Hz for the count down and the other at about 69 Hz to drive the LCD.

The frequency is given by:

f = 1 / [4.40 x RC]

Stripboard

The stripboard size is 36 tracks of 50 holes. Apart from the timing circuits the stripboard layout is the same as for Countdown Mk1.