DIY optical rotary encoder for geared motors

One of the most important things in robotics is to accurately sense and control the rotation of a motor. One way to do this is to use stepper motors but they’re bulky, have very less torque and speed. Another is to buy a motor with quad encoders which work on the basis of hall effect, they give a very high resolution of about 10000-20000 counts/rotation but they’re very expensive. In this tutorial I’m going to show you how to build a very simple attachment to any geared motor to measure the angular rotation. This is my first video tutorial, so feedback is appreciated.

Things you’ll need:
1. Geared motor.
2. IR LED and detector
3. Aluminum strips
4. Super glue
5. Drilling machine with 5.5mm drill bit
6. Electronic components(refer circuit)

Its very difficult to make anything stick on the back end of the shaft since its only a few mm long, thats why I decided to extend it using solder to give the disc a better grip. The reason I took out the armature is because I couldn’t file it properly and it almost melted the plastic casing when I was soldering but if you’re careful you might not need to remove it. The encoder disc is just a piece of paper with alternate quadrants of a circle printed with white and black. You can use any image editing software to make the design and take a print out.

Circuit –

You can try to build the circuit without the transistor, I’d built two encoders, one of them worked without it but the other did not, so test it and see if it works for you.

I’ve built encoders before and I usually mounted them on the bot itself and i put the encoder disc on the wheel itself so I used to get a very low resolution, and another problem with that was they would misalign within a few hours, so I decided to build a sturdier one, hence the aluminum strips. After you superglue everything, measure the voltage difference between black and white and bend the aluminum strips till you get a difference of atleast 2V and if you have a glue gun, glue the two strips together so that they can’t move. This way you’ll have a very sturdy encoder and you’ll never have to calibrate it again. Set the potentiometer to a voltage mid-way between the lowest and highest voltage measured.

Calculation of angular resolution –
On the CRO I got a frequency of 430Hz. In one time period there are two transitions, so that’s two counts every cycle. That means it gives out 860 pulses every second, and if you have a geared motor of 120rpm, that’s two rotations per second. So that gives you a count of 430 per rotation which is way more than a stepper motor and you can easily double it by dividing the circle into 8 parts.
So every count gives you an angle of 360/430 = 0.8372 degrees.

You can now give the output to a microcontroller and control the rotation of the motor very accurately.

Hope you enjoyed the tutorial. If you have any questions or comments, please leave them below.