Hardware Help



This video operated relay is designed to key a small 12v relay when horizontal sync is sensed at its input. It can be used in a variety of applications, for example, to remotely activate a video recorder, in an ATV repeater system or as a receiver warning alarm. Should it be necessary to activate a heavy relay, say for transmitter power applications, the small relay may be used to switch in a large one. The VOR circuit first separates horizontal sync from the video and noise, this then passes to a NE567 tone decoder IC whose frequency is set to 15625Hz (line frequency). The decoder output signal changes state after about a tenth of a second and causes current to flow in the final transistor so energizing the relay. A switch is provided to enable the circuit to be de-activated. A drop out delay of around 0.5 second is provided to cater for quick fades or multipath dropouts. The attack and delay times are determined by the 10uF and the 47k resistors associated with pin 8.

The frequency of the decoder should be set by using a frequency counter connected to the test point and carefully adjusting the pot to within 100Hz of 15625Hz. This should be done with no video connected. The normal lock-in range is around plus and minus 800Hz to allow for odd computer horizontal frequencies or badly adjusted cameras.

This circuit appeared in A5 Amateur TV magazine for August 1984 and thanks are due to the author and publisher for permission to reproduce it here. The design is from P.C.Electronics of 2522 Paxson Lane, Arcadia, CA91006, USA who produce a printed board or assembled module.


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