This circuit is intended for alerting purposes after a certain time is elapsed. It is suitable for table games requiring a fixed time to answer a question, or to move a piece etc. In this view it is a modern substitute for the old sandglass. Useful also for time control when children are brushing teeth (at least two minutes!), or in the kitchen, and so on.
Circuit diagram: Parts:- R1 = 220R
- R2 = 10M
- R3 = 1M
- R4 = 10K
- R5 = 47K
- C1 = 100nF-63V
- C2 = 22µF-25V
- D1 = 1N4148
- D2 = 3mm. Red LED
- Q1 = BC337
- P1 = SPST Pushbutton (Start)
- P2 = SPST Pushbutton (Reset)
- PS = Piezo sounder (incorporating 3KHz oscillator)
- B1 = 3V Battery (2 AA 1.5V Cells in series)
- IC1 = CD4081 Quad 2 input AND Gate IC
- IC2 = CD4060 14 stage ripple counter and oscillator IC
- SW1 = 4 ways Switch (See notes)
Pushing on P1 resets IC2 that start oscillating at a frequency fixed by R3 & C1. With values shown, this frequency is around 4Hz. LED D2, driven by IC1A & B, flashing at the same oscillator frequency, will signal proper circuit operation. SW1 selects the appropriate pin of IC2 to adjust timing duration:
- Position 1 = 15 seconds
- Position 2 = 30 seconds
- Position 3 = 1 minute
- Position 4 = 2 minutes
When the selected pin of IC2 goes high, IC1C drives Q1 and the piezo sounder beeps intermittently at the same frequency of the LED. After around 7.5 seconds pin 4 of IC2 goes high and IC1D stops the oscillator through D1. If you want to stop counting in advance, push on P2.
Notes: