Running LED
Two button control
Relay clock
Toggle on-off
Electronic key
Bistable relay

Elevator control
Traffic light

Two button control

Two button control systems are used on dangerous industrial machines in order to protect the operator’s hands. Just for hobby I made a circuit that turns on a relay by pressing two buttons simultaneously. By pressing one button the collector of the second transistor keeps almost off, so the input voltage of the integrated circuit (pin 2 and 6) is more than 8 Volts. The second button must be pressed within 0,5 seconds, otherwise Q2 is disabled by Q1 and the operation has to be repeated. After few seconds the relay automatically switches off, then the buttons must be released and pressed again. The button switches in series with the coil prevent the auto-activation of the relay in case of component failure. Another rilevant risk is the fact that relays may get stuck on. In order to improve the safety we may use two relays an a light signal which alerts the operator if that anomaly occurs.

Similar circuit

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