Helios

Helios

[blocked]

A real-time lamp controller for my room. Keeps the time (even when unplugged) and flips a relay once the clock and alarm values match.

My room doesn't have windows so I created this device to automatically turn on my bedside lamp and put myself into fight or flight every morning to help me wake up in time for work/school.

🎥 Demo Video 🎥

Design

Some notes on why the board is built the way it is.

A DS3231 instead of the MCU's own timer

The RTC has its own coin-cell backup, so the clock survives a power outage or being moved to another room. Keeping time on the MCU would mean re-setting the clock every time the device lost power, which is exactly the kind of small annoyance that would make me stop using it.

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Discrete multiplexing over a dedicated driver

A MAX7219 or TM1637 would have done this with fewer parts and less code. I used two 74HC595s for the segments and a 74HC138 plus PNP high-side drivers per display because I to practice building the multiplexing myself instead of buying it in a chip.

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A polled encoder, not an interrupt

Every input has a 10k resistor and a 100nF capacitor on it to filter out switch bounce. The tradeoff to this is that signals now fade between HIGH and LOW over about a millisecond instead of snapping. During that fade, the Arduino can't really tell which one it's looking at, so a little noise makes it read HIGH-LOW-HIGH-LOW a few times before settling. An interrupt on ENC_CLK (D2) would count every one of those HIGH-LOW flips, so one click of the encoder knob would add 3 or 4 counts. However, reading the pin in the loop() function only ever sees the value after it settles, which solves the issue.

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Two board versions

The 2-layer version is meant to be manually etched. But I know most people dont have an etching machine, so the 4-layer version is for anyone who doesn't have an etching setup and would rather send it to a manufacturer (I'm also experimenting with curved traces on the 4-layer version).

Features

Hardware

Full parts list: helios_bom.csv

PCB Design

Schematic

Images