Two boys in box forts with huge DIY cardboard telephones

DIY Telephone: Cardboard 2-way phones

Break out the cardboard and the duct tape– let’s wire up a DIY telephone!

This project and all Geek Pack Hack activities must be undertaken with a suitable adult completing their own risk assessment and supervising their children at all times.

Years ago we visited a friend who lived in a large apartment building. Because she spoke frequently with her downstairs neighbor, they decided to cut out the phone company and connect directly through their windows using a pair of kid toy phones.

Fast-forward to the present, and it is actually surprisingly difficult to find 2-way phones like this to purchase.

But with a cool little part from Adafruit and some cardboard, it’s a great project to build!

The Geek Pack Hack crew had a blast putting this project together, testing it out, and using the final product.

A basic wired telephone has just two main parts in its handset: a microphone that you talk into, and a speaker that you listen to.

(We have a separate post about how speakers work and how to build your own.)

If you wanted to build a phone to plug into a telephone jack, you’d need some way to dial a phone number.

Since we’re just connecting these two phones together, we don’t have to get into that circuitry.

Before getting started, if you want a crash course in electricity basics, check out our post on electrical current, resistance, and voltage.

If you aren’t already familiar with them, you might want to check out this post on how to use a breadboard.

We’ll list the parts we used and then give you step-by-step instructions for how to make a DIY telephone.

You can check out our video tutorial here and subscribe to our YouTube channel to keep up with new projects.

Everything is explained below as well.

Supplies to Make a DIY Telephone

2 Adafruit AGC Electret Microphone Amplifiers – MAX9814

2 Mini breadboards

2 electrolytic capacitors between 1 to 100uF

2 Pairs of earbuds

2 Audio jacks

Speaker wire

Insulated copper wire

2 Wire connectors

2 4-AA battery holders

8 AA batteries

Duct tape

Wire strippers



Steps to Make a DIY Telephone

Step 1: Solder the headers onto the Adafruit boards

Step 2: Attach the battery holders

Step 3: Wire up the breadboard circuits

Step 4: Connect the phones with speaker wire

Step 5: Attach the audio jack and plug in the earbuds

Step 1: Solder the headers onto the Adafruit boards

AdaFruit makes these cool little boards that include a microphone. This type of microphone is called an “electret” microphone. 

It also needs this amplifier chip there to boost the signal from the microphone. The amplifier chip is a Maxim MAX9814, which has auto gain control, meaning that it can automatically adjust to changes in how loud the input audio is from the electret microphone.

You can check out our post on how to solder, and also this post from SparkFun about through-hole soldering.

The headers for this little board come with an extra pin, so go ahead and break off that pin before soldering the header to the board.

As we learned, for this project, it doesn’t matter if you solder the header on upside down!

Step 2: Attach the battery holders to your DIY telephone

Use duct tape to affix the battery holders near the center of the handset.

Telephone handset

Step 3: Wire up the breadboard circuits

Here’s how we wired up the mini-breadboards.

Telephone breadboard circuit

The “Out” pin is connected to the capacitor, when then connects to the red speaker wire.

The “Gain” and “Vdd” pins connects to the positive voltage from the battery pack

The “Gnd” pin connects to the black ground wire from the battery pack and the black speaker wire.

Step 4: Connect the phones with speaker wire

Run the speaker wire from one handset’s mini-breadboard to the other cardboard handset. Affix the speaker wire to the handset with duct tape.

Do the same with the speaker wire coming from the other mini-breadboard, and make sure the speaker wires are cut to the same length.

Step 5: Attach the audio jack and plug in the earbuds

Here’s how we attached the audio jack and plugged in the earbuds:

Audio jack wiring

Pick up one end of speaker wire (the end that is not in a breadboard), and pull apart the red and black wires slightly. 

Strip off a bit of the insulation, and insert the red wire’s exposed copper strands into one side of a wire connector. Insert the black wire’s exposed strands into the adjacent opening. 

Prepare three short (~3 inch) lengths of insulated copper wire. Strip all of the wire ends to expose some of the copper.

Taking one of these wires, insert one end into the wire connector in line with the red speaker wire. 

With the other end of the insulated wire, split the stranded wire into 2 bunches and insert one of the bunches through the hole in the longer contact on the audio jack. Twist the two bunches back together tightly so that it makes a good connection with the audio jack contact.

Use the other two insulated wires in the same way. Insert one end of both wires into the wire connector in line with the black speaker wires.

Split and twist the other ends to form tight connections with the two shorter audio jack contacts.

Tape the audio jack to the handset and plug in the earbuds.

Repeat this process for the speaker wire coming from the other breadboard so that each handset’s microphone circuit is connected to the other’s audio jack.

Time to test them out– are they working?

Stay in touch!

If you enjoyed this project or have questions, please leave us a comment!

Check out all of our cool engineering projects.

Or, narrow in on our simplest basic electrical circuit projects or our slightly more advanced electrical engineering projects. 

We also have a helpful basic electronics page that teaches skills such as how to use a breadboard and multimeter.

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