Retriggerable Status Indicator Circuit

Posted on Feb 5, 2014

It is sometimes necessary to monitor a device that generates a relatively short pulse to indicate a change of status. If the duration of the input signal is too short, the RFC-1/B may not have time to capture the pulse and respond appropriately. The RFC-1/B alarm system scans one alarm channel every ten seconds. Depending on the number of alarms i

Retriggerable Status Indicator Circuit
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n use, it may take several seconds for the alarm system to scan the channel and the input pulse may have expired. An effective solution is to extend the duration of the pulse to give the RFC-1/B time to sense the signal. The circuits below do just that. One detects a pulse on the rising edge and the other detects a pulse on the falling edge. These circuits use a readily available IC, the 74HC123A, to sense the input pulse and generate an output pulse that can last several seconds. If a second input pulse arrives before the first output pulse has completed, the output signal timer restarts. The 74HC123 is a dual timer IC. Only one timer is used for this pulse extender so two circuits can be built with one IC. The schematics show wiring for only one timer to simplify the drawing. Both circuits offer the option of either a high (+5 VDC) or low (0 VDC) output so these circuits can act as an inverter if necessary. Both outputs are shown in the schematics. The duration of the output signal depends on the choice of resistor R1 and capacitor C1. The values in the schematic produce an output pulse of about 30 seconds. Any regulated 5 VDC power supply should work. The parts list includes an inexpensive switching supply that will work. The optional 100 µF capacitor should be added if this switcher is used. Alternately there is a schematic for a regulated 5 VDC power supply shown below.

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