AngelSounds JPD-100S Manual Del Usuario página 6

Turn the wheel (1) on the device, the LED lights up to turn on the device.
Gradually turning the wheel adds volume, turning to the opposite side
gradually reduces the sound until the device is completely turned off.
The device offers two connectors. Headphone jack and audio jack. You
can use this by connecting the recording cable to record the sound of the
fetal heartbeat.
If you don't need to record audio, just connect the supplied headphones
to listen.
Battery
How to find the heart of the fetus
Apply some gel (not supplied) to the signal transmission area (4) and place
the device on your stomach. Move it slowly, trying to find the heart of the
fetus. After finding the sound, continue to move the probe slowly, looking
for the place where the sound signal will be clearest.
Battery included. It's located directly on
the device, but not connected. Before
first use, the battery must be connected
as shown in the following picture.