Introduction: The Cheapest (20 Cents) Flex Sensor

Picture of The Cheapest (20 Cents) Flex Sensor

Hello everybody, this is a project made in order to build a robotic hand for an open day in university, to give visibility to a student association of which I belong, that engaged in promoting free and open source software.

I was searching for cheap flex sensor, I'm a student and I wouldn't spend too much to build the hand. I found the solution realizing flex sensor with a photoresistor and a led, adding the resistor that the led and the photoresistor need directly in the sensor, in order to have a plug-and-play sensor with 3 wires to be connected: one for power supply, one for ground and the last for the data.

I hope that you'll appreciate my work, enjoy it and make your own flex sensor almost without money! :-)

...and, if you like it, vote it in some contest, thanks! :-D

Step 1: How It Works?

It's a simple "trick"; we will use the photoresistor as the provider of the flexion of our device: there will be a straw between our led and the photoresistor, so, when the led will produce light, the intensity of it will be read by the photoresistor; modifying the position of the straw, it will arrive more or less light to the photoresistor, depending on the position of the straw. The photoresistor will read this value and convert it into an analog value, so we could use it as we prefer to analize the bending of our device, and all we possibly want to associate with it.

Let's see what we need to build it!

Step 2: Ingredients

Picture of Ingredients

The cost for one sensor is made by (prices found on

  • 1 led (I used a white one): <0,10$;
  • 1 470 ohm resistor: <0,01$;
  • 1 10 Kohm resistor: <0,02$;
  • 1 photoresistor: <0,04$;
  • 30cm of copper wire, approximately, for excess: <0,01$;
  • 20cm of insulating tape, approximately: <0,01$;
  • 1 plastic straw: <0,02$;

total: <0,21$;

I think that is an affordable price also for a poor university student! :-)

The other tools required to make you own sensor are:

  • a tin welder, with some tin;
  • a glue gun with glue (optional).

Here we are, go make it!

Step 3: Where to Start?

Picture of Where to Start?

We have to build the connections indicated in the scheme picture.

We can start connecting the resistor and making longer connection, adding pieces of copper wires.

On the other side of the copper wire, we can add the led, and then connect the "extended led" to the 470 ohm resistor on the anode (the longer part), and the other will go to the ground.

We can add the photoresistor, unifying one side with the led ground (it's indifferent which side to take for the photoresistor), and then splitting the other side between the power supply wire and another wire that will be the data that the photoresistor will provide to our microcontroller.

You could see that I use female attachment on the connection side of the copper wire, but it's only a comfort choise, based on the fact that the wires have been taken from an old computer case, so... there where included in the price! ;-)

Bend the led as shown in the photo, and put the light into one end of the straw; on the other side we will insert the head of the photoresistor, in the best way that allow receive light from the led.

Step 4: Fixing All the Connections

Picture of Fixing All the Connections

It's a good idea to fix every our connection first with tin welder, and then assure them with glue gun. Alternatively, we can isolate connections with some insulating tape. You could use what you have, but insulating tape will be stronger to block any connection changes.

Step 5: Insulating Tape, Insulating Tape Everywhere!

Picture of Insulating Tape, Insulating Tape Everywhere!

At the end of our work, we can cover our home-made sensor with the insulating tape, always our friend when there are home-made hardware project to make! :-)

Step 6: Home-made, and Proud of It!

Picture of Home-made, and Proud of It!

At the end, we will have something like what shown in the photo.

Hoping that you'll appreciate it, remember to vote me in some contest, if you want! :-)


tiredjazzman (author)2017-09-21

I need 4 sensors where to buy ?

destone28 (author)tiredjazzman2017-09-25

Hi, you can find all you need to build this sensor on Ebay or Aliexpress, or in some common electronics store.

vennev (author)2017-08-12

I really like the concept! I have a question though, plastic straws don't typically bend uniformly. When I try to bend a straw it flexes until it hits a pressure point, then folds at that point. I'm thinking Any further bending I'd do from there wouldn't be seen because of that folded point. Has this been an issue? If so, how did you counteract it? Great job, I'd love your feedback!

destone28 (author)vennev2017-09-01

Sorry for late answer, but I didn't receive notification about your question.

As you can see in photos, I used straws that bend gradually in the part of the straw where you can regulate the direction when you use it to drink something! :-)
If you use this type of straw, you will be able to have a better measuring range comparing with rigid straws, because, when you bend it, it will have a gradual response, instead of changing instantly the value of the light that passes in the hole of the straw, as in a rigid one.

I hope that I answer to your question, if not, let me know! ;-)
Thank you for your interest!

jeanniel1 (author)2017-08-06

Neat concept. You got my vote

destone28 (author)jeanniel12017-08-07

Thank you :-)

SilkeC2 (author)2017-08-01

Very simple yet effective. Quite ingenius!

destone28 (author)SilkeC22017-08-03

Happy that you liked it! ;-)

Patel Darshil (author)2017-08-01

Hey Nice Idea. You got my Vote!!!

destone28 (author)Patel Darshil2017-08-01

Thank you Patel! :-D

About This Instructable




Bio: I am a computer science engineering student at the Polytechnic of Milan, I love life and I follow a sort of polymath lifestyle being curious ... More »
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