Tower Light Flasher Failure Alarm

This tower light flasher interface uses a 555 timer as a missing pulse detector. It was adapted from the Missing Pulse Detector The Forrest Mims Engineer's Notebook II by Forrest Mims.

The incoming dc pulse (3) is buffered by Q1, which provides the flash telemetry indication (1) and triggers Q2. When Q2 fails to discharge the C1/R1 circuit, U1 times out and provides a logic low to pin 3, illuminating D2.

Parts list
C1 47uF
C2, 3 0.1uF
D1, 2, 3 LEDs
D4, 5 1N4001
Q1 2N4401
Q2 2N4403
R1 200k
R2, 5 240
R3 5.6k
R4 2.7k
U1 555

The circuit can be built on a single PC board measuring about 2.5" square.

Diodes D4 and D5 were added by the designer to better interface with his station's MRC remote control. The diode polarity may need to be reversed depending on the relationship between the interface's voltage and the remote control's status voltage.

By changing the value of C1 and R1, the time delay can be adjusted. The current values provide about a 15 second delay until an alarm condition is indicated in the absence of any flash pulses.

The external connections noted are:
P . . . . power
0 . . . . flash failure alarm
1 . . . . flash status output
2 . . . . ground reference for point 3
3 . . . . dc pulse from tower lights

The image below can be used as a template to create the circuit board.

Information provided by John Harris, KUAC-FM, Fairbanks, AK.

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