by pebe » Sat Aug 23, 2014 10:30 am
OK. Here is the modified circuit. It uses an LM555 as a bistable switch whose output can drive the buzzer directly. With a 12V supply, when pin 6 goes above 8V the output goes low. When pin 2 goes below 4V the output goes high.
When first switched on, C1 is discharged so pin 6 is at 12V. That ensures that the output is low and the buzzer is off. As long as SW1 is closed, it will stay that way.
If SW1 opens and SW2 closes, C1 will start to charge via R1. After about 10secs the cap will have charged so that pin2 will have gone down to its 4V threshold. Pin 3 will then go high turning on the buzzer. The charging time equals C1 x R1 = 10 x 1 = 10secs. If it is not what you want, alter R1. Use a bead tantalum for C1 for consistent timing.
When SW1 closes, C1 discharges and the buzzer is turned off. When SW2 opens, the buzzer is turned off, but this takes several seconds because C1 has to discharge via R1 and R2 until pin 6 gets up to 8V. If you want a faster recovery time, fit a 1N4148 diode across R1, with its cathode to pin 2.
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- 555-delay.GIF (3.45 KiB) Viewed 5665 times