Motor speed regulation without encoder @evalspace

KRYZYS on the EvalSpace blog writes about how DC motor speed can be measured without using any external feedback device like an encoder.

During the entire cycle of the PWM modulation there are short pulses which are quite interesting, but we’ll get to them later. Now let’s focus on the “off” time. During the off time there is a small periodic waveform. Let’s call it an “off waveform”.

This waveform is created by rotor windings that are moving through magnetic field of stator magnets. I don’t want to get through details of a DC motor construction, but in general, this waveform period is the 1/6th of the of the full DC motor period (full revolution), so we can use this waveform to measure the real speed of the motor.

I built a small device using PIC16F1619 microcontroller with the Curiosity board to regulate the motor speed using this method as a feedback signal.

Above is measuring the voltage at V1 point noted below. The PWM period is 16.386 ms, and duty is set to about 50%.

The MPLAB project for this device is available in the github repository:

The most important advantage of this method is that there is no need for any other mechanical feedback components. But there are also some disadvantages. We can measure the speed of the motor only if the “off” time of the PWM signal is long enough. It has to be at least 2 times longer than the 1/6th of the period of full motor revolution. So if the PWM duty cycle is too high, or the motor speed is too low, we won’t have a good feedback signal.

See the full analysis in the post here.

As 2022 starts, let’s take some time to share our goals for CircuitPython in 2022. Just like past years (full summary 2019, 2020, and 2021), we’d like everyone in the CircuitPython community to contribute by posting their thoughts to some public place on the Internet. Here are a few ways to post: a video on YouTub, a post on the CircuitPython forum, a blog post on your site, a series of Tweets, a Gist on GitHub. We want to hear from you. When you post, please add #CircuitPython2022 and email to let us know about your post so we can blog it up here.

Stop breadboarding and soldering – start making immediately! Adafruit’s Circuit Playground is jam-packed with LEDs, sensors, buttons, alligator clip pads and more. Build projects with Circuit Playground in a few minutes with the drag-and-drop MakeCode programming site, learn computer science using the CS Discoveries class on, jump into CircuitPython to learn Python and hardware together, TinyGO, or even use the Arduino IDE. Circuit Playground Express is the newest and best Circuit Playground board, with support for CircuitPython, MakeCode, and Arduino. It has a powerful processor, 10 NeoPixels, mini speaker, InfraRed receive and transmit, two buttons, a switch, 14 alligator clip pads, and lots of sensors: capacitive touch, IR proximity, temperature, light, motion and sound. A whole wide world of electronics and coding is waiting for you, and it fits in the palm of your hand.

Join 32,000+ makers on Adafruit’s Discord channels and be part of the community!

Have an amazing project to share? The Electronics Show and Tell is every Wednesday at 7pm ET! To join, head over to YouTube and check out the show’s live chat – we’ll post the link there.

Join us every Wednesday night at 8pm ET for Ask an Engineer!

Follow Adafruit on Instagram for top secret new products, behinds the scenes and more

CircuitPython – The easiest way to program microcontrollers –

Maker Business — Pololu’s account of the chip shortage

Wearables — Stay frosty

Electronics — High voltage logic

Python for Microcontrollers — Python on Microcontrollers Newsletter: CircuitPython 2022 Survey, Python #1 in 2021 and more! #Python #CircuitPython @micropython @ThePSF

Adafruit IoT Monthly — 2021 in Recap!

Microsoft MakeCode — MakeCode Thank You!

EYE on NPI — Maxim’s Himalaya uSLIC Step-Down Power Module #EyeOnNPI @maximintegrated @digikey

New Products – Adafruit Industries – Makers, hackers, artists, designers and engineers! — New Products 1/14/2022 Featuring Adafruit QT Py ESP32-S2 WiFi Dev Board with uFL Antenna Port – STEMMA QT! (Video)

Get the only spam-free daily newsletter about wearables, running a "maker business", electronic tips and more! Subscribe at !

1 Comment

  1. I hope there’s some series impedance and clamp diodes before you feed that waveform into an ADC. most chips don’t like being driven more than 0.7V outside the rails.

Sorry, the comment form is closed at this time.