The Off-Grid Cup Phone — a Simple Phone With No Electricity

Sometimes, the simplest ideas are the ones we forget.

For this project, I made a very simple telephone using two drinking cups and a piece of string. No batteries, no electricity, no Wi-Fi, no water, and no gas are needed. Just two cups and a string.

This is not a new invention — many people probably remember making these when they were younger. But some people may have never tried it, while others may have simply forgotten how it works.

I wanted to bring this simple experiment back and show how you can communicate using nothing more than vibration traveling through a string.

It can also be a fun little project to make with children or grandchildren.

What You Need

1. 2 drinking cups
2. A piece of string
3. A small sharp object for making holes
4. Something to secure the string inside the cups

That’s it!

Take two drinking cups and make a small hole in the center of the bottom of each cup.

The hole should be just large enough for the string to pass through.

You can use plastic or paper cups. For this project, I used simple drinking cups that I already had.

Pass one end of the string through the hole in the first cup.

Secure the string on the inside of the cup so it cannot easily pull back through the hole.

Do the same thing with the other end of the string and the second cup.

Now you should have something that looks like a very simple telephone:

Cup → String → Cup

Give one cup to each person and move far enough apart to make the string completely straight.

This part is important!

The string should be tight, not hanging loosely between the two cups.

One person places the cup against their ear while the other person speaks into their cup.

Try not to let the string touch objects or the ground while you’re testing it.

Now comes the fun part.

Speak normally into one cup while the other person listens with the second cup.

When you speak, your voice makes the bottom of the cup vibrate. Those vibrations travel through the stretched string and make the bottom of the other cup vibrate.

The second cup then creates small vibrations in the air, allowing the other person to hear your voice.

It’s basically a tiny mechanical communication system with no electricity at all.

A Quick Test

I made a short video to test the cup phone and see if the sound could actually be heard through the string.

However, I decided not to upload the video for now because there were other people talking in the area, and I don’t want to accidentally violate anyone’s privacy.

I could have moved to another location, but the other available places were either too far away or too small for the experiment.

So, for now, you’ll just have to take my word for it although making your own cup phone is probably a much better way to test it anyway! 😄