30-second countdown timer

Photocoupler

The following is a circuit diagram of [30 seconds countdown timer circuit]

In this design, a ternary subtraction counter and two decimal subtraction counters are needed to achieve timing for 30s, and a 30-second countdown counting circuit to 0.1s is displayed. We can implement these three counters with three 74LS192s. The up counter pulse input terminal of the three counters in the counting module must be connected to a high level, and the borrowing signal of the low bit slice is added to the down counting pulse input end of the upper slice. The borrow signal of the high slice counter controls the alarm signal, and the borrow signal is always high when counting down.