■ FIGURE 7. When you have made your holes, you can
super-glue the LED in place and attach the battery holder.
You can screw the 7805 down for the soldering process,
but it might be a good idea to unscrew it and have it lifted
away from the faceplate when you are using the heat gun
to collapse the heat shrink tubing over your solder joints.
The superheated air from the heat gun will deform the
faceplate quickly if you aren't careful.
start working on the other end. Basically, you need a
five volt power supply, since a nine volt battery is
enough to kill the Hall chips. You can use a simple
three-terminal 7805 voltage regulator in the TO-220
package. It's easy to work with and already has a hole
to allow you to attach it to the faceplate holding the
phone jack. When looking at the numbered face of
the 7805, the input voltage (nine volts) is connected
to the leftmost pin, the middle pin is connected to
ground, and the right pin is the five volt output that
will go to the proper colored phone terminal block
(red, in my case).
You need some kind of power switch to turn it on
and off; an ordinary light switch is hard to beat for
something low cost and easy to work with. The
positive side of the battery clip goes into one of the
switch's terminals and the other terminal is connected
to the left side of the 7805. Finally, we can add an
LED to make on and off more obvious.
The LED should be connected to a resistor of
somewhere between a few hundred and a thousand
ohms. The smaller the resistor, the brighter the LED. It
doesn't matter whether you connect the resistor to
the positive or negative leg of the LED. Assuming you
choose the positive leg, you then need to connect the
free end of the resistor to either the light switch
terminal (NOT the same one that the battery is
connected to or you'll never turn the LED off!) or the
five volt phone terminal. The negative end of the LED
needs to be connected to ground.
A few other things to be aware of include: 1) The
heat gun should not be held close to the phone jack,
light switch, face plate, etc., for long periods of time
or it will warp them; 2) The legs can break off of the
dual Hall chip if you aren't careful — they aren't made
for repeated bending back and forth at large angles
and will break reasonably soon; 3) You might find it
useful when testing your work to cut another test lead
cable in half and use one of the halves to bring the
ground connection outside of the electrical box.
Finally, it sounds hard to believe, but one glitch has
come up repeatedly over the years that I have helped
dozens of students build these: The phone wire will
look like it's properly plugged in, but you have to push
it in until it clicks to be sure.
■ FIGURE 8. To recap, your connections on the phone
terminal block are as follows: On the red terminal, we have
the 7805 output and the positive side of the LED-resistor
pair. On the black terminal, we have the 7805 ground, the
black wire from the nine volt battery clip, and the ground
side of the LED-resistor pair. On the other two terminals
(green and yellow), all you need to connect are two alligator
clip wires. These will ultimately be clamped to the probes of
your voltmeter. You should probably bunch the two together
using a long piece of heat shrink that spans the hole in the
box. That will give you a little strain relief on the cable and
make it less likely that the alligator cables will pull loose
during normal use.
September 2012 29