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This is a simple low-power circuit that uses push buttons and transistors to control the activation status of a high-voltage circuit.
In the example, the AC power source is configured as a common power socket (in Brazil, power is provided with a 60 Hz frequency and a 127 V amplitude), and the 9 V DC power source can be a standard charger connected to the same power plug.
When the upper push button is hit, it puts the circuit in a status that activates both the transistors and keeps the relay coil powered, even when the button is released. The same principle works with the bottom push button to deactivate the circuit. If both the push buttons are pressed simultaneously, the bottom button will have the dominant effect and the circuit will be kept off.
The red LED is used only to indicate if the whole system is powered.
This system can also be used to control low-power circuits. In this case, the main circuit power supply will take the place of the relay coil, and the values of the DC power source and of the resistors shall be checked and adjusted to fit the voltage and current demands of the new circuit.
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