Meena – for Meditation

What is Meena?

Meena is a small wooden box containing a timer and a striker, which is intended to strike an instrument like a singing bowl or gong:

Why Build Meena?

I built Meena as a gift for someone who meditates regularly. There are items like this for sale:

I believe this is the most popular:
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Kind of can’t believe this exists:
not what I had in mind

I decided to build my own for this gift because:
– I wanted to enable someone to use their own singing bowl of any size
– I was not crazy about the 70’s aesthetics of the one for sale
– I wanted a simpler interface.
– I just wanted to build it myself, okay!?

Hardware Overview

  1. 24V 3A power supply – required for solenoid
  2. Adafruit 5v Pro Micro – brainz
  3. LM2596 Buck Converter – steps down power from 24V to 6V
  4. IRF520 MOS FET – lets arduino control Solenoid
  5. Rotary encoder & button – input: set & start timer
  6. OLED mini screen – display the time
  7. 24V 400mA solenoid – actuate the striker
  8. Wooden Box – to like, hold stuff.
  9. Veneer – to hide OLED electronics

Building Meena

I started with a plain wooden Box:
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Then there was a bunch of carving to do. Not one who usually works with wood, I ordered a carving set and got to work. Evidence that I never work with wood was apparent right away. Witness me trying to just make a circle at a specific place:
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Carving out room for the OLED screen. You can see where I slipped and cut too far. Wood is so unforgiving!
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Making the Striker

I needed a way to attach the solenoid shaft to the striker, and an old motor shaft hub did the trick:
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Making the Screen Cover

This involved getting my hands on some veneer, and cutting it down.

This took a few tries, veneer is fragile! The ordering of the cutting turned out to matter. Learned to cut against the grain first, while piece had most integrity, then cut with the grain last.

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Electronics

With the wood complete I started to play around with the electronics. The original intention was to use a potentiometer and an Arduino Trinket, however the potentiometer’s readouts were not steady, and the Trinket just did not have enough oomph. The GFX OLED library takes up a lot of EEPROM, and even though I found a smaller library, there was almost no room for application code. The final nail in the Trinket’s coffin was that when I switched from the potentiometer to the rotary encoder with pushbutton, I needed three external interrupts, and the Trinket only had one.

Picking the Right Solenoid, and Leg Room

To get a good “GOOOONNNGGG” sound, I needed a strong enough strike, and that meant a strong enough solenoid. I tried a few 6V and then 12V solenoids but they just didn’t do the job, so I ended up with a 24V 400mA solenoid. It took up most of the space in the box, and I knew at this point it was going to be a challenge to cram everything in:

24V to 6V: Buck Converter

The Solenoid needed 24v, but the Arduino needed only 6 so I used a buck converter. Tuning to the desired voltage is as simple as turning a screw:
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TTF to BMP Font Conversion

Since this project was going to be used for meditation, I wanted a font that felt soft. I landed on one called ‘Luna’, and from there I had to figure out how to get the digits from TTF format into BMP which actually involved cute software tool.
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Then it was a matter of making sure things looked right on the OLED:

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Final View of the Inside

As you can see, it was really crammed inside. On the bottom left of the photo you can see the jack from the 24v power supply. This power is split between a direct connection to the solenoid, and the buck converter, which in turn powers the Arduino:
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Code

The software running Meena is open source and found here. It was straightforward work, so there is not too much to say about it.

Retrospective

Retro On the Timeline

This project was technically simple, however it ended up taking quite a bit longer than anticipated. Primarily the extra time was due to figuring out the internal hardware configuration, then realizing I’d forgotten something and have to take a few steps back. Really, it made me wish I had a 3D printer, so that I could have figured everything out in software. I could have made use of a service like Shapeways, but I’m a bit impatient to wait for things in the mail, and they are a bit expensive.

The second major time suck was trying to fit everything onto the Arduino Trinket. It would have been cool, and allow easy access for code changes. After jumping up to the Adafruit Pro Micro, it became harder to program once the hardware was in the box.

Retro On the Functionality

The solenoid is pretty strong with 24v. The striker hits the singing bowl pretty hard, and its not a very calming sound. If I were to build it again, I’d prefer to have a second buck converter, one I could set programmatically. This way the user could “turn down” the power of the striker for smaller singing bowls. I’m probably going to add felt cushion to the end of the striker to dampen the impact, which I tested and works. It does take away from the crispness of the strike, we lose the high high sounds. Ideally the striker just hit more softly.

The striker has to be just the correct distance from the bowl, or the striker will not reach the bowl or the striker will knock the bowl way too hard and not sound good. I think some sort of base would be nice, that keeps the bowl the exact correct distance from the striker. Maybe I’ll build that one day!

Retro: In Closing

All in all I’m mostly happy with the outcome. The final result performs the intended function, and its reasonably pleasant to look at. Some fun lessons learned too. I’m glad I work with robots and not with wood. Wood is hard. When the carving knife slips and something gets ruined, there is no way to rollback your commit. Version control wont help. But it was fun, and I’m glad I did it 🙂

striker