Introduction: Coffee Cup Telephone Pickup / Amp

 Herein are the instructions for building a Telephone (receiver) pickup (i.e. amplifier), using an electromagnetic loop to prevent distortion, and a coffee cup for directional amplification.

Step 1: Gathering the Parts

One needs for this project: 

1) paper coffee cup (rinsed out and allowed to dry, if it has been used)

3)  1 uF Electrolytic Capacitors (C1, C4, C5)

2)  100 mf Electrolytic Capacitor (C6, C7)

1)  .27  mf Mylar Capacitor (C2)

1)  .001 mf disc capacitor (normally marked  102 )  (C3)

1)  LM286 Audio Amplifier IC  (IC1)

1)  PM2222A transistor (or most any small signal NPN transistor will do (2N3606 for instance)   

1)  10 K resistor  (R1)

1) 7.5 K resistor  (R2)

Plus a pc board (the medium sized rectangular one from Radio Shack is about the right size,  go a little bigger if you are not very good with your soldering skills.

A 9v  snap connector, 8-16 ohm speaker, (a length of  "magnet" wire [chemically coated wire] for) a pickup coil, and a case or enclosure.  I also prefer using an IC socket rather then risk frying the LM386 while soldering it into the board.
And one  small switch to shut the mechanism off when through with it.

 The Picture of the pc board with the AA battery in it shows it's comparative size.

Step 2: Coil Construction....

 The Loop, SHOULD be no smaller nor larger then 2.5 inches in diameter. Although this is not overly critical, it could effect the circuits ability to pick up the signal. Originally, it was made to fit around the mouth piece of a cradle phone.

Find a sturdy round object (like a juice glass or bottom of some coffee cups) that is about 2 1/2 inches in diameter (about 6.4 cm or 63.5 mm) to use as a winding form.

The wire's gauge is not overly critical either,  but don't use such  a thick wire that you crush whatever you are using as a form to wind around, nor so thin it will not hold it's shape once it is wound.

Wind between 20-30 turns around the form you have chosen.  It may be helpful after removal from the form, at this point to wrap small pieces of electrical tape around the coil at 4-5 places to help it  keep from unwinding.

Something that IS pretty critical though:  once the coil is formed, make sure you have at LEAST 10 inches ( 25 cm or 254 mm) of lead on the ends, to keep the coil at least 10 inches from the amplifier board.  That is minimum.  I have added about 2 inches to this for my own prototype. 
I used a Cinnamon spice container for my form.

Take note also, for those not familiar with this type of wire,  it IS coated with a chemical lacquer and so before soldering, about 1/2 inch to an inch of this "lacquer" must be scraped or sanded off (or removed by  burning, although I have never had a LOT of success with this; especially on very small gauges of wire).


Step 3: Plan !

If you are using a standard pc board, and have not taken the schematic and converted it to a PC mask and created your own pc trace board,  I am assuming you will be "wiring" this together as best as you can using just the schematic. 

One caution here, if at ALL possible, try to keep the length of any wires connecting parts together on the board itself, at a minimum length.  This is especially true for anything connecting to any of the capacitors.  
Long wires act as antennas, and you may end up picking up signals you don't want to hear (static, whistles, etc,.).

There were a few modifications I made along the way.  One was to place a simple on/off switch between the positive leg of the battery snap and the board.  This way I could turn it off without having to remove the battery each time.

Once you have written down how you want to place your parts on the board and connect them (make a drawing of it so as to keep it clear in your mind),  I always place the IC  socket first and solder it in.  This  way, I can check closely, and test easily if I have bridge soldered any pins (soldered across two or more pins that I did not want to).  For those with fading eyesight, like I have, the magnifier glass on a set of "helping hands" can be both a blessing and a curse;  but they are very helpful most of the time (the curse comes in because of the need to practice how far to move your hands when they are being viewed through a magnifying glass). 
I also enclosed a copy of the foil pattern one would use to create an etched board, if one so desires; but it is not necessary.  

Step 4: Place the Rest of the Parts

Following the Schematic in step 3 (and foil pattern if you made one), and place the rest of the parts.

Take care to use "some" type of heat sink when soldering in components (this is crucial for IC's, LED's, and transistors, etc.). 

Heat sinks (temporary clip on types) used for soldering are not expensive.  I prefer using Hemostats since they can also be used for clamping parts for other purposes.

   The hemostat I have pictured is one of my older ones.  I have some chrome plated completely metal ones somewhere on my work table :-)  Notice on the handle, where the fingers go, the "tabs".  They are actually ridged, and will serve as adjustable pressure of clamping mechanisms.
BTW:  the coffee cup was modified a bit.  I pushed the bottom out a bit to make a rounded "cone" on the bottom, so I could attach it directly to the cone of the speaker.  This made for more amplified sound.

Step 5: The Case

Since I had emptied an old Walkie Talkie case of it's internal components (for an April Fools Joke, see The Talking Music Box  I used it. It was convenient and about the right size for my board. 

  It contained the board quite nicely  and even had it's own 9 v battery compartment already in place.
I used the "antenna hole" for the leads from the coil to the amplifier board. 

One of the things I tried was to electrical tape the leads together to help prevent over bending and subsequent breaking.  This didn't work so well, so I fed the whole thing though some heat shrink tubing.   This worked better, but I didn't like the LOOKS of it.  So I took some old plastic tubing from a replaced C-PAP machine (I get replacement components such as tubing and face mask every 6 months or so). and cut it to length.   Since I didn't want to unwrap the tape AND the heat shrink tube and THEN unsolder the loop, I slit the new tubing end to end in a slow spiral direction (very similar to cable organizing tubing (but much less expensive then cable organizing tubing) one sees in computer shops and such).  It worked like a charm, giving me both the proper amount of flexibility and the proper stiffness (IMHO).

I placed the board in the lower section of the box, and the speaker on the outside of the old speaker hole, as it was not going to fit inside.  My only means handy of attaching this one part was to use hot glue.  


Step 6: USE

Once the device is as nice looking as you would like it to be, it is time to see what it will do.

IF you still have an old "cradle style" phone (see picture) then the loop goes around the mouth piece.  This will pick up the electro-magnetic emissions from the voice coil in the phone.  
If you have a cordless phone however, the coil can be placed over the ear piece portion of the phone (once answered) and placed on the table. This allows for hands free operation of the phone (as long as the mouth piece is sufficiently close enough to be spoken into). 

This is mainly to show "concept";  that is, that one does not need to "amplify the sound" in order to amplify.  If the sound is being produced or transmitted electronically,  one can intercept the electromagnetic field and amplify the original signal, rather then add distortion with a sound amp. 


NamanS5 made it!(author)2015-10-27

What should i do if i am not able to get the .27mf mylar capacitor

huck+alexander made it!(author)2012-06-16

Wonderful design! While a couple of years old, I have not managed to run across this instructable before. Very glad I saw it!

Could something similar (in reverse) work for designing a directional mic? Would it be possible to isolate mic/earphone signals? (Perhaps by using a larger coil to change frequency, and then filtering out lower frequencies?)

Thanks for sharing. Must try this!

KentsOkay made it!(author)2010-05-13

Cool sir!

Goodhart made it!(author)2010-05-13

Thank you.  :-)

christine+malczanek made it!(author)2010-04-29

how about Java Phone :D

Goodhart made it!(author)2010-04-30
I have just now tested it on my Samsung cell phone, and it works extremely well on it (even better then on my land line cordless). 
christine+malczanek made it!(author)2010-04-30

t model samsung do you have??

Goodhart made it!(author)2010-04-30

It says  T-Mobile on it....that's about it.

NachoMahma made it!(author)2010-05-01

.  The model info may be under the battery. Yeah, great place to put it, eh?

Goodhart made it!(author)2010-05-11

I hadn't had a chance yet....I am not sure where it was put last, and I think my wife had it "off" I can't even call it....

Goodhart made it!(author)2010-05-02
If I feel better later, I will look....    :-(
Goodhart made it!(author)2010-04-29

If the "speaker" is a magnetically coil driven should.  I haven't tried it with a piezo or any other form of speaker yet

geekvig made it!(author)2010-05-06

Very nice.  Simple and effective.

Goodhart made it!(author)2010-05-07
Thank you :-)
christine+malczanek made it!(author)2010-04-29


~ Christine Malczanek

Goodhart made it!(author)2010-04-29

*blush*  why thank you.

Goodhart made it!(author)2010-04-28

Any opinions?   Suggestions? 

fungus+amungus made it!(author)2010-04-29

A new name, perhaps. Coffee Cup Phone Speaker?

Goodhart made it!(author)2010-04-29

Ok,  how about...

About This Instructable




Bio: I am, most definitely older than 00010101 and to put it simply, still curious about nearly everything :-) I then tend to read and/or experiment ... More »
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