Tum on a game, computer, videodisc player, or whatever, and its output takes priority over the antenna-derived signal-the antenna is disconnected and the alternate source is fed to the monitor. When the alternate video source is no longer detected, the switch automatically reconnects the antenna. When the rf carrier is detected, a high logic output appears at pin 15 of U1-squelch status. The signal is then buffered to VMOS FET Ql. This FET can drive CMOS inverter/buffer U2, can be an open-drain output to drive a relay, or can convert to a 5 V logic level.
Automatic-video-switch - schematic

When pin 15 of U1 becomes high, Q1 turns on, pulling pin 11 of U2e low; in tum, pin 12 becomes high. The olitput of U2e is fed to U2f, forcing its output, at pin 15, low. When the signal at U2 pin12 is high, in is biased on, allowing the signal at]3 to flow through C14, D1, and out to ]2 through C13. When the signal at ]3 is removed, U1 pin 15 decreases. That decrease causes U2 pin 12 to decrease and U2 pin 15 to increase. When the signal at U2 pin 15 is high, it biases D2 on, allowing the signals at Jl to flow through Cl2, D2, and out to ]2 through Cl3.

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