Radio-controlled wristwatch

1. A radio-controlled wristwatch, comprising:

a solar cell;

a control circuit which stops operation under a predetermined condition;

an illuminance detection circuit which outputs a signal indicating whether or not illuminance of light irradiating the solar cell is higher than a given threshold value;

threshold value switching means for switching the given threshold value between a first illuminance threshold value and a second illuminance threshold value that is larger than the first illuminance threshold value;

control circuit starting means for starting the control circuit in a stop state when the illuminance detection circuit outputs a signal indicating that the illuminance is higher than the first illuminance threshold value;

satellite signal receiving means for receiving a satellite signal containing time information from a satellite when the illuminance detection circuit outputs a signal indicating that the illuminance is higher than the second illuminance threshold value; and

time displaying means for displaying time corresponding to the time information contained in the received satellite signal.

2. The radio-controlled wristwatch according to claim 1 , wherein:

the illuminance detection circuit comprises:

a first circuit element, which is connectable in parallel to the solar cell, and has a first resistance value;

a second circuit element, which is connectable in parallel to the solar cell, and has a resistance value that is smaller than the first resistance value; and

a comparator circuit which outputs a signal indicating whether or not an output voltage of the solar cell is higher than a predetermined threshold voltage; and

the threshold value switching means switches a circuit element to be connected in parallel to the solar cell between the first circuit element and the second circuit element so as to switch between the first illuminance threshold value and the second illuminance threshold value.

3. The radio-controlled wristwatch according to claim 2 , wherein:

the first circuit element comprises a first resistor connected normally in parallel to the solar cell;

the second circuit element comprises the first resistor and a second resistor that is connected in parallel to the solar cell and the first resistor via a switch; and

the threshold value switching means turns the switch on and off so as to switch the circuit element to be connected in parallel to the solar cell between the first circuit element and the second circuit element.

4. The radio-controlled wristwatch according to claim 3 , wherein:

the threshold value switching means switches the given threshold value among the first illuminance threshold value, the second illuminance threshold value, and a third illuminance threshold value that is larger than the first illuminance threshold value and is smaller than the second illuminance threshold value; and

the radio-controlled wristwatch further comprises:

means for operating in a power saving state under a predetermined condition; and

means for finishing operation in the power saving state when the illuminance detection circuit outputs a signal indicating that the illuminance is higher than the third illuminance threshold value.

5. The radio-controlled wristwatch according to claim 2 , wherein:

the first circuit element is connected to the solar cell via a first switch;

the second circuit element is connected to the solar cell via a second switch;

the first switch comprises a normally closed switch which is turned on when the operation of the control circuit is stopped; and

the second switch comprises a normally open switch which is turned off when the operation of the control circuit is stopped.

6. The radio-controlled wristwatch according to claim 5 , wherein:

the threshold value switching means switches the given threshold value among the first illuminance threshold value, the second illuminance threshold value, and a third illuminance threshold value that is larger than the first illuminance threshold value and is smaller than the second illuminance threshold value; and

the radio-controlled wristwatch further comprises:

means for operating in a power saving state under a predetermined condition; and

means for finishing operation in the power saving state when the illuminance detection circuit outputs a signal indicating that the illuminance is higher than the third illuminance threshold value.

7. The radio-controlled wristwatch according to claim 2 , wherein:

the threshold value switching means switches the given threshold value among the first illuminance threshold value, the second illuminance threshold value, and a third illuminance threshold value that is larger than the first illuminance threshold value and is smaller than the second illuminance threshold value; and

the radio-controlled wristwatch further comprises:

means for operating in a power saving state under a predetermined condition; and

means for finishing operation in the power saving state when the illuminance detection circuit outputs a signal indicating that the illuminance is higher than the third illuminance threshold value.

8. The radio-controlled wristwatch according to claim 1 , wherein:

the illuminance detection circuit comprises a comparator circuit which outputs a signal indicating whether or not an output voltage of the solar cell is higher than a given threshold voltage; and

the threshold value switching means switches a threshold voltage to be supplied to the comparator circuit between a first threshold voltage and a second threshold voltage that is higher than the first threshold voltage, so as to switch between the first illuminance threshold value and the second illuminance threshold value.

9. The radio-controlled wristwatch according to claim 8 , wherein:

the threshold value switching means switches the given threshold value among the first illuminance threshold value, the second illuminance threshold value, and a third illuminance threshold value that is larger than the first illuminance threshold value and is smaller than the second illuminance threshold value; and

the radio-controlled wristwatch further comprises:

means for operating in a power saving state under a predetermined condition; and

means for finishing operation in the power saving state when the illuminance detection circuit outputs a signal indicating that the illuminance is higher than the third illuminance threshold value.

10. The radio-controlled wristwatch according to claim 8 , wherein:

the illuminance detection circuit further comprises:

a first constant voltage output circuit capable of supplying the comparator circuit with the first threshold voltage as the given threshold voltage; and

a second constant voltage output circuit capable of supplying the comparator circuit with the second threshold voltage as the given threshold voltage; and

the threshold value switching means switches a constant voltage output circuit to supply the comparator circuit with the given threshold voltage between the first constant voltage output circuit and the second constant voltage output circuit, so as to switch between the first illuminance threshold value and the second illuminance threshold value.

11. The radio-controlled wristwatch according to claim 10 , wherein:

the first constant voltage output circuit is connected to the comparator circuit via a third switch;

the second constant voltage output circuit is connected to the comparator circuit via a fourth switch;

the third switch comprises a normally closed switch which is turned on when the operation of the control circuit is stopped; and

the fourth switch comprises a normally open switch which is turned off when the operation of the control circuit is stopped.

12. The radio-controlled wristwatch according to claim 11 , wherein:

the threshold value switching means switches the given threshold value among the first illuminance threshold value, the second illuminance threshold value, and a third illuminance threshold value that is larger than the first illuminance threshold value and is smaller than the second illuminance threshold value; and

the radio-controlled wristwatch further comprises:

means for operating in a power saving state under a predetermined condition; and

means for finishing operation in the power saving state when the illuminance detection circuit outputs a signal indicating that the illuminance is higher than the third illuminance threshold value.

13. The radio-controlled wristwatch according to claim 10 , wherein:

the threshold value switching means switches the given threshold value among the first illuminance threshold value, the second illuminance threshold value, and a third illuminance threshold value that is larger than the first illuminance threshold value and is smaller than the second illuminance threshold value; and

the radio-controlled wristwatch further comprises:

means for operating in a power saving state under a predetermined condition; and

means for finishing operation in the power saving state when the illuminance detection circuit outputs a signal indicating that the illuminance is higher than the third illuminance threshold value.

14. The radio-controlled wristwatch according to claim 1 , wherein:

the threshold value switching means switches the given threshold value among the first illuminance threshold value, the second illuminance threshold value, and a third illuminance threshold value that is larger than the first illuminance threshold value and is smaller than the second illuminance threshold value; and

the radio-controlled wristwatch further comprises:

means for operating in a power saving state under a predetermined condition; and

means for finishing operation in the power saving state when the illuminance detection circuit outputs a signal indicating that the illuminance is higher than the third illuminance threshold value.