When our son was about three or four years old we started expecting him to do things in a timely manner. I would say things like “We need to go in ten minutes!” or “Play with that truck for five more minutes, and then give someone else a turn”. The problem was (a) he could not read a clock, either digital or analog, and (b) he had no sense of how long a minute is. I realized that he needed a clear way to see what five or ten minutes looks like, and be able to track the time remaining. So, I set out to design and build a visual timer. I had seen many variations, including the classic egg timer, but I felt like the rotation metaphor was not intuitive to very young children — it is all about looking at angles and fractions of a circle. Once I settled on a more linear design, the idea of an electronic hourglass seemed obvious: it provides the right visual metaphor, and I could use an accelerometer to implement the ergonomics of turning it over to start and stop the timer.
My final design, which I’ll show you here, is relatively simple. The visual part consists of a strip of individually addressable RGB LEDs (e.g., WS2812). Programming is accomplished with a single button (a capacitive touch button, because it is flat). An Arduino-compatible microcontroller (an Adafruit Metro) runs the program and reads orientation information from a 3-axis analog accelerometer. Power is provided by a rechargeable battery pulled from a phone power bank (a standard 18650 lithium ion). The nice thing about the power bank is that it includes the charging circuity, a micro-USB port, and it outputs 5V, which is what the microcontroller and the LED strip expect.
Depending on your level of experience you can probably make this timer in a weekend or two. You will need basic soldering skills and the associated tools and supplies. The biggest variable is the enclosure: I have woodworking power tools that make this job easier, but I’ll also discuss alternative ways to build the enclosure without lots of tools.”