The circuit is simple and self explanatory. IC₁, a NE555 timer, is used as astable mode to generate clock pulses at a rate of about 20 Hz. The frequency can be varied with the help of potentiometer VR₁.

The clock pulses from the timer are fed to clock input of IC₂, a decade counter. The outputs of the counter are decoded by IC₃, (BCD to seven segment decoder/driver) which drives a common anode display (DIS₁) to show from figure 0 to 9.

On pressing switch SW₁, IC₁ starts working, with the display changing from 0 to 9. Capacitors C₁ is charged during this time.

Once SW₁ is released, the capacitor discharges through resistor R₃, VR₁, R₁ and R₂. Thus, the frequency of IC₁ decrease (and hence the rate of changing figures on the display) and ultimate becomes zero, once C₁ has totally discharged thereby stopping the displayed is enhanced by the time for which SW₁ is pressed. So, the random effect is natural outcome.

PARTS LIST

Resistors (all ¼-watt, ± 5% Carbon)

R₁ = 100 KΩ

R₃ = 1 KΩ

R₄ – R₁₀ = 330 Ω

VR₁ = 100 KΩ

Capacitors

C₁ = 47 µF/10V

C₂ = 0.47 µF

C₃ = 0.01 µF

Semiconductors

IC₁ = NE555

IC₂ = 7490

IC₃ = 74247

D₁ = 1N4001

DIS₁ = FND507 or LTS542

Miscellaneous

SW₁ = push to on switch