Toggle on off switch

This circuit toggles on and off a NE555 with a single momentary switch. Unlike a mechanical bistable relay the circuit backs to the start condition if you switch off the power supply. No relay contact is used. 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 mechanical bistable relays, especially when the circuit must back to the start condition by switching off the power supply. At the start condition, pins 2 and 6 are supplied with half voltage, then the output (pin 3) keeps the low level. By pressing the button Q1 turns on with a small delay, while Q2 keeps off due to the collector of Q4. Once the capacitor is charged Q1 leads pin 2 and 6 to low voltage (about 3V) and prevents Q2 from switching on. By releasing the button the capacitor discharges. Since the output of the NE555 is now high Q3 and Q4 are off. So by pressing the button again Q2 switches on and leads pin 4 to low voltage (about 0V), then the circuit backs to the start condition.

The described circuit utilizes a NE555 timer IC configured in a monostable mode to create a toggle function controlled by a single momentary switch. The operation begins with the NE555's pins 2 (trigger) and 6 (threshold) being connected to a voltage divider that supplies them with half of the supply voltage, ensuring that the output at pin 3 remains low initially.

Upon pressing the momentary switch, transistor Q1 is activated with a slight delay, allowing it to charge a connected capacitor. The charge on this capacitor influences the voltage at pins 2 and 6, dropping it to approximately 3V. This condition keeps Q2 off, as it is controlled by the collector of Q4, which remains inactive during this phase.

Once the capacitor reaches its charge threshold, Q1 will lead pins 2 and 6 to a low state, preventing Q2 from switching on. Releasing the momentary switch allows the capacitor to discharge, which in turn causes the output of the NE555 at pin 3 to transition to a high state. This transition turns off transistors Q3 and Q4, effectively resetting the circuit's state.

Subsequent presses of the momentary switch will activate Q2, which will pull pin 4 (reset) of the NE555 low (approximately 0V), returning the circuit to its initial state. This toggling behavior provides a reliable alternative to mechanical bistable relays, particularly in applications where a return to the start condition is necessary upon power loss. The absence of relay contacts enhances the circuit's reliability and reduces mechanical wear, making it suitable for various electronic applications.This circuit toggles on and off a NE555 with a single momentary switch. Unlike a mechanical bistable relay the circuit backs to the start condition if you switch off the power supply. No relay contact is used.

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  mechanical bistable relays, especially when the circuit must back to the start condition by switching off the power supply.
At the start condition   pins 2 and 6 are supplied with half voltage, then the output (pin 3)keeps the low level. By pressing the button Q1 turns on with a small delay, while Q2 keeps off due to the collector of Q4.

Once the capacitor is charged Q1 leads pin 2 and 6 to low voltage (about 3V) and prevents Q2 from switching on. By releasing the button the capacitor discharges. Since the output of the NE555 is now high Q3 and Q4 are off. So by pressing the button again Q2 switches on and leads pin 4 to low voltage (about 0V), then the circuit backs to the start condition.

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