Video Tracer For Trouble-Shooting


Posted on Feb 4, 2014

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 co


Video Tracer For Trouble-Shooting
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uple 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 75 © 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 10 F 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...




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