LM556_FREQUENCY_DETECTOR

Published:2009/6/18 3:51:00 Author:May | From:SeekIC

The circuit (see the ftgure), is built around an LM556 dual-timer IC. The 556's first timer is wired as a one-shot and is used to stretch the incoming pulses into fixed-length pulses. The second timer, which is similar to an astable multivibrator (pin 13 remains disconnected), functions as follows:The one-shot's fixed-length pulses, which are output on pin 5, turn on Q1 and discharge C2 through R5. If the frequency of the pulses is high enough, the voltage on C2 will fall below1/3 VCC and the second timer's output, pin 9, will go to a logic 1. Conversely, if the frequency is low enough or is zero, the voltage on C2 will charge through R6 to a level above2/3 VCC, and the pin 9 output will go to a logic 0. The idea is to keep the upper and lower peak voltage on C2 below 2/3 VCC and1/3 VCC, respectively for a logic 1, and above2/3 VCC and1/3 VCC, respectively, for a logic 0.To find the one-shot values, R3 and C1, select a pulse width (1.1 × R3 × C1) that's greater than the largest input pulse width and less than twice the inverse of the highest input frequency. To find R5, R6, and C2, first determine the duty cycle (ton/toff) of the input signal. Next, choose a standard value for C2 and calculate R6:Also, R5 = R6 (ton/toff). A tweak of resistors R5 and R6 might be needed to get the preferred response. Input signals with low duty cycles work the best. Finally, notice that capacitors C3 and C4 can be any value between 0.01 and 0.1 μF.



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