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In the kit Voltage Doublers Using 555 Amplifier produces a voltage that is twice its voltage supply. In the kit Voltage Doubler first block is power supply in the input is 230V AC which is step down using the transformer (12-0-12) .The 12V ac input is fed to the bridge diode to gives 12V pulsating DC. This DC voltage is filtered through the capacitor to remove the ripples. The filtered DC is fed to 7806 regulator to fetch +6v regulated output.
In the kit Voltage Doublers IC NE555 is wired as an astable mutivibrator. The base of the two transistors is shorted and output of the astable multivibrator (pin 3) is connected to it. When the output of astable multivibrator is low, BD139 will be OFF and BC548 will be ON. The negative terminal of the capacitor will be shorted to ground and it will be charged to the input supply voltage. When the output of the astable multi vibrator is high, transistor BD139 will be ON and transistor BC548 will be OFF. The capacitor will be charged to the voltage across capacitor plus the input supply voltage (that is double the input voltage). This voltage Doublers circuit can deliver only up to 50mA output current and above that current limit the output voltage will be dramatically reduced. The actual output voltage will be around 19V for a 12V DC input and also the output voltage will be a bit unstable. Anyway, for low current applications this circuit is well enough.