555_ASTABLE_FREQUENCY_METER

Published:2009/7/16 3:16:00 Author:Jessie | From:SeekIC

The circuit shown is a simple digital frequency meter, which indicates the frequency in hertz of an astable 555 timer. It could, perhaps, be adapted for other uses. An NPN transistor, TR2, is connected to the 555 astable to be measured, as shown in the in-set, and the output signal obtained is hooked to the main circuit via transistor TR1 collector (c). Two CMOS 7555 timers, IC3 and IC4,are wired as monostables, and these will trigger via R6 and C1 when the DISPLAY swith S1 is closed. The period of IC3 is adjustable using VR1.while IC4 times for a fraction of a second. When S1 is closed,IC4 sends a RESET signal to IC1 and IC2.which are two decade counters. each driving a seven-segment common-cathode LED display directly. IC3,the first decade counter. Monostable IC3 needs to be trimmed so that it triggers for approximately 100ms.Pressing switch S1 then allows 100ms'worth of pulses to clock up on the display, and simply multiplying the result by 10 will yield the value in hertz. It would be feasible to cascade several counters to show a higher range of frequencies.



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