Unlock smarter workouts with a programmable fitness timer that guides your reps, rest periods, and routines with intuitive tones and an OLED interface. Built around a compact D1 mini Pro and fully customizable in software, this DIY timer keeps your exercise sessions on track.
Unlock smarter workouts with a programmable fitness timer that guides your reps, rest periods, and routines with intuitive tones and an OLED interface. Built around a compact D1 mini Pro and fully customizable in software, this DIY fitness timer keeps your exercise sessions on track.
The Circuit
The fitness timer is built around a LOLIN/WEMOS D1 mini Pro microcontroller paired with an OLED shield that includes two built-in push buttons and a small display. A rotary encoder with a push-button function is added for selecting and assigning workout programs, while two external push buttons (wired in parallel with the shield’s buttons due to limited GPIO pins) let you quickly start routines.
Fitness timer circuit diagram.
A piezo speaker generates the workout tones, and the entire device runs from a lithium battery supported directly by the D1 mini Pro, eliminating the need for a separate power supply. The components are wired as shown in the circuit diagram and housed in a custom 3D-printed FreeCAD enclosure.
Key Features
The fitness timer acts as a personal workout assistant by guiding you through timed exercise routines using audible tones and simple button controls. It plays a low tone to signal the start of a rep, a higher tone when the rep is complete, and a triple-tone at the end of the full exercise set, with programmable work and rest intervals. Users can choose between five predefined programs, assign two of them to dedicated buttons, and view selections on a small OLED display, making the device an easy, hands-free way to keep workouts consistent and properly timed.
The enclosure was designed using FreeCAD.
The Fitness Timer Project
The original article, “Fitness Timer,” appeared in Elektor Circuit Special 2023 Bonus Edition. You can read the article for free during the two-week period following the publication of this post. Enjoy!
Editor's Note: This article first appeared in a 2023 edition of Elektor. Given the project’s age, some components might not be readily available. Still, we think the design will inspire you to start a project of your own.
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