Build a Simple Bat Detector
Wiring the Phone Jack
The unconventional way in which the earphone jack is used to turn the
power off and on may seem confusing, so this page was written to help sort
it out. I hope it clears up any questions you may have about the earphone
The figure at the left compares a 2 circuit and 3 circuit phone plug. The
two circuit plug, also referred to as a monaural plug, has only a tip and
sleeve conductor. This was the prevalent plug style until portable stereo
sound equipment became common.
To accomodate stereo, the plug was modified by splitting the sleeve and
creating a ring segment for the second audio channel. This modified plug
is referred to as a 3 circuit, or stereo, plug.
In normal use, either plug type plug would have the sleeve segment used
as the ground, or common connection of the earphone set. A monaural, or
1 channel earphone would have it's signal connected to the tip segment,
while stereo earphones would have their left and right audio channel circuits
connected to the tip and ring segments.
Plugging a 3 circuit plug into a 3 circuit jack would connect the tip ring
and sleeve of the plug to the respective circuits on the jack, correctly
completing a three circuit connection.
However, what we are doing on the project is plugging a 2 circuit plug
into a 3 circuit jack. As you can see in the diagram to the left, without
the insulator that normally seperates the ring and sleeve segments of the
plug, the ring and sleeve segments of the jack are connected together by
the insertion of the plug.
So this brings us back to the project... The circuit you are building is
using a two circuit plug, the earphone, with a 3 circuit jack. The tip
circuit connects the earphone hot lead to the audio from the level control.
The sleeve circuit provides a ground connection for the earphone AND the
battery. The battery, being connected to the ring segment of the jack,
is connected to ground - the sleeve segment of the jack, by the long sleeve
of the 2 circuit plug. This provides the power switching for the project.
When wiring your project make sure to properly identify the tip, ring,
and sleeve connections of the jack.
If you choose to buy a jack not listed on the parts
list, please be aware that there also exist jacks that have additional
circuits for switching out speakers when earphones are plugged in. These
are usually referred to as closed circuit jacks, where we have been discussing
open circuit jacks. The contacts on these may look similar to the jack
used in the project. Be sure you have a stereo, or three circuit jack.
Acknowledgement: Thanks to Peter
Stimpson, in England, for suggesting this enhancement to the Simple
First published November, 1997 - Last update: November 12, 1997