This circuit was
designed as an aid to installers and maintainers of video systems. It is
basically a video sync separator (IC1) followed by a LED and buzzer driver (IC2, Q1 & Q2). In use, the device is connected to a video cable and if there is video present, the LED will flash at about 10Hz. If there is no video, the LED
flashes briefly every couple of seconds. A buzzer can also be switched
in to provide an audible indication. The buzzer is particularly useful
when tracing cabling faults or trying to find a correct cable amongst
many, where it is difficult to keep an eye on the LED.
Another use for the buzzer option is to provide a video fault
indication. For example, it could be inserted in bridging mode, with
switch S1 in high impedance mode (position 2) across a video line and
set to alarm when there is no video present. If someone pulls out a
cable or the video source is powered off, the alarm would sound. IC1 is a
standard LM1881 video sync separator circuit and 75Ω termination can be
switched in or out with switch S1a. The other pole of the switch, S1b,
turns on the power. The composite sync output at pin 1 is low with no
video input and it pulses high when composite sync is detected.
These pulses charge a 100nF capacitor via diode D1. When there is no
video at the input, oscillator IC2b is enabled and provides a short
pulse every couple of seconds to flash the LED.
The duty cycle is altered by including D2, so that the discharge time
for the 10μF capacitor is much shorter than the charge time. The short LED
pulse is used as a power-on indicator drawing minimal average current.
When video is present at the input, IC2b is disabled and IC2d is
enabled. The output of IC2d provides a 10Hz square wave signal to flash
the LED. The buzzer is controlled by switch
S2. In position 2 the buzzer will sound when there is video at the input
and in position 1 the buzzer will sound when there is no video at the
Author: Leon Williams – Copyright: Silicon Chip Electronics