Forming a thyristor pulse disappearance detecting circuit based on 555

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In an industrial control device, if a set of thyristor trigger circuits is faulty, it is difficult to accurately determine which part of the device or which group of thyristors has a problem. It is convenient to use the detection circuit as shown in the figure, simple and practical, and can automatically indicate.
When the thyristor trigger pulse is normal, VT1 is turned on, and the charging voltage on C is periodically discharged, so that the 555 pin 2 is always at a low level (<1/3VDD), the 555 is set, and the LED2 (green) light is on; When the silicon-controlled trigger pulse disappears or leaks more, VT1 turns off or turns on less than the original. The charging voltage on C is higher than 2/3VDD, 555 resets, 3 feet are low, LED2 is off, LED1 (red) is on. , indicating a fault.