TIME_OUT_CIRCUIT

Published:2009/7/11 1:11:00 Author:May | From:SeekIC

This circuit operatesin the astable mode and at the end of the first period (up to several minutes), itproduces a tone,When S1is placed h the time position,Q3 is cut off because pin 3 of U1 is high and D1 holds Q3in cutoff.Q2is off,and Q1is on,which grounds the negative end of D2 and C2,Therefore,C1and C2 are returned to ground.After a time of about 1.1 R6（C1 ＋ C2）,the timer cycle completes and pin 3 U1 goeslow,This turns on Q3 and Q2,cuts off Q1,and effectively disconnects C2.Now,the circuit oscillates with a period determined by,R7 and C1,because D2is forwardˉbiased. A toneis then generated and can be heard from SPKR1．Closing S2 resets the circuit．



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