Some users have a equipment that generates a relatively short pulse when indicating a change of status. When this signal is used as a telemetry input to the RFC-1, the RFC-1 may not scan the channels fast enough to recognize the pulse. This is especially true when the channel is used for an alarm since the alarm system scans one channel every ten seconds. Depending on the number of alarms in use, it may take several seconds for the alarm system to scan the channel and the status pulse may have expired.

The schematics shown below offer a possible solution to this issue. The circuit uses a readily available IC, the 74HC123A, to sense the input pulse and generate an output pulse that can last many seconds depending on the choice of resistor (R1) and capacitor (C1) in the timing section. The values shown in the schematic are suggested starting values that produce an output pulse of approximately 30 seconds. For reference, at C1=0.1µF and R1=10K the output pulse is approximately 1 minute and at C1=1.0µF and R1=10K the output pulse is approximately 10 minutes.

The two circuits are basically the same except that the top one senses a rising edge input signal and the lower one senses a falling edge input. Both circuits offer the choice of either a high (+5VDC) or low (0VDC) output so the output can be inverted if necessary.

The input is retriggerable. The output timer will restart at the set length if a second input pulse arrives before the ouput has completed its cycle from the first input pulse. The 74HC123 is a dual IC. The second portion of the IC can be used if required--this is not shown in the schematics.

Just about any regulated +5VDC power supply should work. The parts list includes an inexpensive 5VDC switching supply that will work. The optional 100µF capacitor should be added if this switcher is used. Alternately there is a schematic for a regulated 5VDC power supply shown below.

Do not apply an input signal larger than +5VDC to the 74HC123A or the IC will be damaged.