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Resistors: 1/4 Watt
R1 = 220K
R2 = 10K Pot ( Volume Control )
R3 = 10 Ohm
Capacitors: - Disc Or Electrolytic ( Electrolytic Shown with
+/- ) All Low Voltage
C1 = 1 uF ( Non Polarized - 100V )
C2 = .1
C3 = 100 uF
C4 = 47 uF
This is a Ne-2 Neon Bulb
9 V.D.C. - Use a standard SPST switch for the on/off switch. The Positive Side Of The on/off
switch connects to the circuit, the negative side of the battery to ground as shown on the
Clip 1 and Clip 2 - Standard Alligator Clips.
CP1:LM386-1 Audio Chip - Digi-Key Electronics NOTE: Attach a jumper
wire between pins 2 and 4 or when you build a board add a trace between
these two pins.
Any 8 Ohm Speaker
Any 8 Ohm earphone or headset ( Mono )
S1: Any SPST Switch
Any DPDT Switch
Operation Of This Unit:
The active telephone wires into any home or office are the green and red wires ( Actually there
are 4 wires - Red, Green, Black and yellow ) In 99.9% of all situations, the black and yellow
are no longer used and have been disconnected. - To utilize this unit, install two alligator clips
- as shown on the schematic, of course with a couple of lead wires leading from the unit of
about 20 or some odd inches to the alligator clips themselves.
You will find the red and green wires in the telephone wire that leads from the main connection
into an office, home - or the panel into an office building. You can find these wire in the main
telephone wire in your basement, etc and it is Beige in color, of about 3/16" in diameter.
Carefully, strip the outer covering off and locate the red and green wire. Expose the copper in
these two wires with a pocket knife - or whatever - but do not cut the wires themselves.
Clip the two alligator clips into these two wires ( RED and GREEN ) - Dosen't matter which
clip connects into which wire - as long as you clip one or the other into the red and green!
When the phone is picked up, you will hear everything from both sides. You can monitor the
conversation two ways. Switch SW2 over to earphone - or speaker, your choice. The Neon
Bulb ( Ne-2 ) in this circuit serves two purposes. First, it drops the 20 cycle 90 volt ringing
signal to a comfortable level that will not blow the front end of the circuit, and 2nd - it allows
you to physically see that a call is coming in - when it starts blinking. The unit also features
two other optional attachments (1) a probe ( Optional ) if you want to use the unit as a signal
tracer and (2) a female telephone cord jack (J1) ( Optional ) RJ-11W for single line phones, or
other setups for other wall jacks that can be plugged into the wall connection to check for any
hook switch devices that may be present on the line that may be active, or for use when
listening to any other phones in operation on the same pair.
You can Modify This Circuit to include a port for recording what you are
listening to by following the instructions shown on the right hand side of this page.
All Electronic Components - Digi-Key Electronics
Add A Recorder Port To This Unit: To add a recorder port to
this unit, simply tap off of C3 in the main schematic (shown at the red
line in the image below ) and use a patch cord between the jack and
the "Mic" input of any portable tape recorder. This allows you to
record what you are hearing. Modification Shown Below.