An electronic circuit is a combination of electronic devices like resistors, capacitors and transistors. The function of a circuit can be amplifying, oscillating, switching, controlling etc. These basic circuits or stages can be combined to an electronic equipment.
The electronic circuit runs with low voltage DC but can control high voltage AC applications. A power supply which transforms the AC high voltage to to the needed DC low voltage is always needed.
To understand an electronic equipment it is important to indicate the basic circuits and know their functions.