LOGIC_STATE_CHANGE_INDICATOR

Published:2009/6/18 3:07:00 Author:May | From:SeekIC

A circuit consisting of a one-shot multivibrator IC, a pair of diodes, and some resistors and capacitors delivers an output pulse when the logic state at its input terminal changes-either from high to low or from low to high. Thus, this circuit can serve as a state-change indicator or as a frequency doubler for a square-wave input.Any monostable can be used; the arrangement in the figure achieves low power dissipation(80 milliwatts)by using half of a Fairchild 96L02 transistor-transistor-logic dual multivibrator. The 96L02 is triggered when pins 3 and 5 are high and pin 4 changes state from low to high. It also triggers if pin 3 is high, pin 4 is low.The circuit shown here allows these conditions to be satisfied with a single input terminal, plus the fixed bias on pin 3; the arrangement of resistors, capacitors, and diodes automatically biases pin 5 high when transmitting a rising transition to pin 4 and biases pin 4 low when applying a falling tran-sition to pin 5.For example, if the input terminal has been low and then goes high, C1 charges through a for-ward-biased diode that shunts its 100-Ω resistor; therefore pin 5 goes high immediately. C2 charges through 100 Q; however, because its diode is back-biased, the rising level is not applied to pin 4 un-tilSis already high. Therefore, the conditions for triggering an output pulse are satisfied.The output pulse duration, f, is set by the value of time constant RXCX.



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