PRESET_FREQUENCY_ALARM

Published:2009/7/6 1:35:00 Author:May | From:SeekIC

When selected frequency occurs in Nixie-driving counter, alarm circuit triggers and locks until reset manually. Requires one SN7441 Nixie decoder/driver, one decimal-type thumbwheel switch, and one 2N3905 transistor for each digit of display in counter. Four connections are made to each counter stage to get BCD inputs A, B, C, and D for 7441. Connections can be made to 7475 quad latch in typical counter. Circuits are for 8-digit display. When display reaches digit to which switch is set, switch output is grounded. When all switch outputs are grounded, all transistors are turned on and SOP fires to actuate alarm relay. If latching is unclesirable, use medium-power NPN transistor in place of SCR.-W. L. MacDowell, Frequency Detector for Your Counter, 73 Magazine, Oct.1976 p 50-51.



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