The circuit detects a change in video information and uses this to switch on an alarm in a closed-circuit television guard system or a video recorder.
The circuit is usable only where the same picture is shown on the TV monitor for some time. When the picture changes, the average brightness alters and this is detected by a window comparator. Capacitor Ci prevents any d.c. a component of the video signal reaching the circuit. Opamp ICia clamps the signal. On average, the potential at pin 3 of IC1a is always equal to half the supply voltage. The potential across C2 is buffered by ICib. The output signal of this opamp is applied to window comparator ICic-ICid via presets Pi and P2.
The reference voltage for the comparator is held at half the supply voltage (1/2 Ub) by R8-R9. Preset Pi is adjusted to make the potential at its wiper slightly higher than 1/2 Ub. and P2 to make its wiper voltage slightly lower than 1/2 Ub. The potentials at the wipers of Pi and P2 are applied to the comparator via fairly long (>10 s) time constants (Rf, C,-) and R7-C4). The smaller\ the set window, faster the reactor will ” change state.
When a level of the input signal rises, the potential across C2 decreases, whereupon the levels across P1 and P2 also drop. When the voltage at pin 9 of IC1, drops below 1/2 Ub, the output of this stage goes high. This change of state, indicated by the lighting of D2, is used to give an external apparatus a trigger or start pulse of up to 65 V via the open collector output of T1. Resistor R13 serves as a current limiter.
When the level of the input signal decreases, a similar action follows, but in this case, ICid changes state, which is indicated by the lighting of D3. When there is no change in the average input signal, T1 remains off.
The circuit draws a current of about 12 mA.