Casio 3721 Operation Manual Download Page 1

Operation Guide 3721

1

MA0410-EA

Each press of the 

B

B

B

B

B

 button sounds a confirmation tone and cycles through available modes in the sequence shown below.

The watch will revert to the Timekeeping Mode automatically if you leave it in any other mode without performing any operation for about two
or three minutes.

(Mode Change)

(Light)

Minute Hand

Second Hand

Hour Hand

Date

Timekeeping Mode

If the time setting is not correct...
Normally, you should never need to adjust the time setting of the watch as long as it is 
able to receive a time calibration signal. If signal reception is impossible for some reason, 
use the procedures under “Adjusting the Home Positions” to check the home positions of 
the hands, and make adjustments as required. 

 Button

 Button

Receive 
Successful

OFF

ON

Receive Failed

Use this mode to adjust 
the home positions of 
the analog hands.
 
The home positions of the 
hands are correct when 
they point to 12 o’clock.

Use this mode to set the alarm time and to turn 
the alarm on or off. 
The second hand will move to either “ON” or “OFF”.

Use this mode to check if 
signal reception is being 
performed normally. 
The second hand will move 
to either “GET” or “NG”.

Receive Result Mode 

Alarm Mode

Home Position Adjustment Mode

Modes and Hand Movement

Charging Precautions

Avoid charging the watch in the following locations, and anywhere
else where the watch may become very hot.

On the dashboard of an automobile parked in the sun

Very close to an incandescent light source or other sources of
heat

In a location exposed to direct sunlight for long periods

Depending on the light source you are using, the
case of the watch may become quite hot during
charging. Take care to guard against burn injury
after charging.

To charge the battery

Point the solar cell (face) of the watch at a light source to charge
the battery.

Remember that even a partial blockage of the solar cell reduces
charging efficiency.

Example: Positioning the watch

Solar Cell

The illustration shows the resin band model.

Charging Guide

Starting from a full charge, the watch
should be able to continue operating
for about four months without further
charging under the conditions
described below.

Daily Use

Illumination: 1.5 second

Alarm: 10 seconds

Signal reception: 1

Making sure the watch is regularly
exposed to light ensures stable
operation.

Charge Times Required to Advance to a Higher Level

Exposure Level 

(Brightness)

Outdoor Sunlight 

(50,000 lux)

Sunlight Through a 

Window (10,000 lux)

Daylight Through a Window on 

an Overcast Day (5,000 lux)

Indoor Fluorescent 

Lighting (500 lux)

Approximate Exposure Time

 Timekeeping Restored 

Full Charge

– – –

– – –

1 hour

2 hours

3 hours

30 hours

12 hours

61 hours

Note that the above charging times are for reference only. Actual
charging time depends on a variety of environmental factors.

The power supply of this watch uses a solar cell to generate electrical power, which is stored by a rechargeable battery. Using or storing
the watch where it is not regularly exposed to light, or allowing it to be blocked from light by your sleeve for long periods as you are
wearing it can cause the power of the rechargeable battery to run down.

A low battery can cause timekeeping to stop, and can also cause time and alarm settings to be lost.

Face illumination or alarm operation can cause hand movement to stop due to the sudden temporary drop in battery power. This does not
indicate malfunction, and normal operation will resume when the watch is exposed to light. Though hand movement stops, timekeeping
continues internally, and the hands will be adjusted to the correct setting when normal operation returns.

Checking the Battery Level

Low battery power is indicated when the the second hand starts to jump at two-second intervals.

Jumps two seconds

1-second movement

Indicates date home position.

Normal

Low Power Alert

Timekeeping Disabled

Battery level 
drops due to 
insufficient 
charging.

Battery level 
drops further 
due to 
insufficient 
charging.

Hands 
stop at 12 
o’clock.

If 2-second hand movement is being performed when the current time reaches midnight, the date indicator will change to the home
position indicator (

). If battery power drops to the next lower level, and the hands will stop at 12 o’clock.

Even after the battery drops to the level that timekeeping becomes disabled, you will be able to resume normal operation by charging the
battery.

After timekeeping becomes disabled, the hands will move to the current time after the battery is recharged sufficiently.

Keep the watch exposed to light until normal operation returns.

Important!
The following
operations are disabled
during 2-second hand
movement.

Auto and manual
calibration signal
reception

Face illumination

Alarm

Power Supply

To ensure stable operation, be sure to allow the watch to be exposed to light as
much as possible.

Required Daily Charging Time

• 

The following is the daily 

amount of charging required 
each day to support the 
operations under “Daily Use”.

Exposure Level

(Brightness)

Outdoor Sunlight

(50,000 lux)

6 minutes

30 minutes

48 minutes

8 hours

Sunlight Through a 

Window (10,000 lux)

Daylight Through 

a Window on an 

Overcast Day 

(5,000 lux)

Indoor Fluorescent 

Lighting (500 lux)

Approximate 

Exposure 

Time

Summary of Contents for 3721

Page 1: ...n an Overcast Day 5 000 lux Indoor Fluorescent Lighting 500 lux Approximate Exposure Time Timekeeping Restored Full Charge 1 hour 2 hours 3 hours 30 hours 12 hours 61 hours Note that the above charging times are for reference only Actual charging time depends on a variety of environmental factors The power supply of this watch uses a solar cell to generate electrical power which is stored by a rec...

Page 2: ...only when it is dark The auto light switch does not illuminate the face when surrounding light is bright The auto light switch will illuminate the face for about 1 5 seconds in any mode To turn the auto light switch on and off In the Receive Result Mode hold down the A button for about three seconds to toggle the auto light switch on and off Face Illumination Location Reception is difficult and ma...

Page 3: ...keeping Mode hold down the B B B B B button for about two seconds The watch will beep and signal reception will start To interrupt reception Press any button When reception starts The second hand moves to READY The hour and minute hands continue to keep time normally During reception The second hand moves to WORK The hour and minute hands continue to keep time normally Do not move the watch until ...

Page 4: ...hour and minute hand home position is off 9 Use the following steps to move the hands counterclockwise Press the B button to align the hour and minute hands with 12 o clock Each press of the B button moves the hand setting 10 seconds Holding down the B button for about two seconds moves the hands at high speed High speed movement of the hands will continue until it completes a 12 hour cycle or unt...

Page 5: ...second movement of the second hand automatically After that you can try performing the manual setting procedure again from the beginning To manually reset the second hand to the top of the minute 1 In the Timekeeping Mode hold down the A A A A A button for about five seconds until the second hand stops at AM or PM If you do not perform any button operation for about two or three minutes the watch ...

Page 6: ...l cause the second hand to stop moving whenever the watch is left in the dark Note that hand movement may stop if the watch is blocked from light by your sleeve Power Saving How the sleep state works Sleep Level 1 The second hand stops whenever the watch is left in the dark for about one hour between the hours of 10 00 p m and 6 00 a m The hour and minute hand and the date continue to operate norm...

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