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

PLC
Elevator control
Traffic light

Toggle on off

This circuit could be used to toggle on and off a NE555 output by a single momentary button. Similar circuits are often used instead of the mechanical bistable relay, especially when the circuit must back to the start condition by switching off the power supply. At the start condition the pins 2 and 6 are supplied with half voltage, then the output (pin 3) keeps the low level. By pressing the button the Q1 transistor turns on with a small delay, while the Q2 transistor keeps off due to the collector of the Q4 transistor. Once the capacitor is charged the Q1 collector leads the pin 2 and 6 to low voltage (about 3V) and prevents the Q2 transistor from switching on. By releasing the button the capacitor discharges.Since the output of the NE555 timer is now high the Q3 and Q4 transistors are off. So, by pressing the button again the Q2 collector switches on and leads the pin 4 to low voltage (about 0V), then the circuit backs to the start condition.

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