Posted on Apr 28, 2012 9911
This circuit operates in the astable mode and at the end of the first period (up to several minutes), it produces a tone.
When SI is placed in the time position, Q3 is cut off because pin 3 of Ul is high and Dl holds Q3 in cutoff. Q2 is off, and Ql is on, which grounds the negative end of D2 and C2. Therefore, CI and C2 are returned to ground. After a time of about 1.1 R6 (Ci + C2), the timer cycle completes and pin 3 Ul goes low. This turns en Q3 and Q2, cuts off Ql, and effectively disconnects C2. Now, the circuit oscillates with a period determined by R7 and CI, because D2 is forward-biased. A tone is then generated and can be heard from SPKR1. Closing S2 resets the circuit.