Electronic device

1. An electronic device capable of switching power on and power off by a switch outside the electronic device, the electronic device being connected to an AC power source via a power source supply line, the switch being connected in the power source supply line, the electronic device comprising:

a determination unit configured to determine whether power is supplied from the AC power source or not;

a second power source different from the AC power source;

an output unit;

a power source line connected to the power source supply line;

a relay provided in the power source line and opened when the determination unit determines that no power is supplied from the AC power source;

a potential setting unit configured to set the power source line between the relay and the switch to a first potential;

a potential detecting unit configured to detect a conductive state of the switch based on the first potential set by the potential setting unit; and

a control unit configured to perform at least one of cutting off power to be supplied to the output unit, supplying power from the AC power source to the output unit, and supplying power from the second power source to the output unit based on a determination by the determination unit and the conductive state of the switch detected by the potential detecting unit.

2. The electronic device of claim 1 , wherein the potential setting unit is supplied with power from the second power source.

3. The electronic device of claim 1 , wherein the determination unit comprises a voltage variation detecting unit configured to detect voltage variation of the power source line, and determine power supply and no power supply from the AC power source by detecting the voltage variation in a predetermined period.

4. The electronic device of claim 3 , wherein the potential detecting unit and the voltage variation detecting unit are configured to share:

a resistor connected to the power source line; and

a photo-coupler configured to detect a current flowing through the resistor and output a detection signal to the control unit.

5. The electronic device of claim 4 , wherein the control unit is configured to cause a resistance of the resistor to be smaller when no power is supplied from the AC power source, as compared with a state in which the power is supplied from the AC power source.

6. The electronic device of claim 1 , wherein

the power source supply line comprises a hot wire and a ground wire, and the switch causes at least the hot wire conductive or cuts off at least the hot wire,

the power source line includes a first line connected to the determination unit and the potential detecting unit, and a second line different from the first line, and

the electronic device further comprises a polarity switching unit configured to switches the power source line to connect the first line to the hot wire and connect the second line to the ground wire.

7. The electronic device of claim 6 , wherein the control unit is configured to automatically switch the polarity switching unit in accordance with a level of the voltage applied to the power source line.

8. The electronic device of claim 1 , wherein the output unit comprises a light emitting unit.

9. The electronic device of claim 8 , wherein the control unit is configured to control the light emitting unit to emit light in a first emission mode when the power is supplied from the AC power source and to emit light in a second emission mode when the power is supplied from the second power source.

10. The electronic device of claim 9 , wherein the light emitting unit is caused to flash at least one time in the second emission mode.