TRANSISTOR_RADIO_TURNOFF

Published:2009/7/12 21:15:00 Author:May | From:SeekIC

Circuit switches radio off after delay of about 30 min, if ganged switch is set to other position and radio switch itself is left on. Current drain of timer circuit is negligible in both positions. Article describes timing action of transistors in detail. Tr1 should have current gain above 25 with collectorcurrents of 10 to 100 mA; 2N3706 can be used here and for Tr5. Other transistors should have current gain above 50, as in 2N3702. For operation from 4.5 to 9 V, omit diode and cut values of R4-R7 in half.-S. Lamb, Delayed Switch Off for Transistor Radios, Wireless World, Aug. 1973, p 373.



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