Tinkering 2020-10-15 »

Tinkering: 2020-11-18: An Alarm Clock


A while back, I built the 'About O'Clock' wall clock. Because it came out quite nice, I wanted an alarm clock with this design. After learning how to do SMD PCBs with Kicad and manufacturing them in China, I was able to achieve the level of integration necessary for fitting everything into a small enclosure. The first prototype is finished and functional, and here are the first pictures. I plan on building two more alarm clocks like this, with slightly improved specs based on what I learned here.

completed word alarm clock This is the finished alarm clock. The clock face uses a nice font that took me a long time to find: Triplicate by Matthew Butterick. The font is really good for this because it is monospaced and beautiful. I can use natural letter spacing for the grid. This is so much nicer than using Helvetica all upper case letters with huge empty spaces between the letters, like many word clocks out there do.

The size of the alarm clock is 67mm x 67mm x 26mm (excluding the knob and alarm switch), so it is pretty small, but not the smallest alarm clock in the world. It weighs 146g.

completed word alarm clock The crystal is glass. Not acrylic, but real glass.

The enclosure is wood, because my wife had joked that it would be nice. She is right, so there we are.

completed word alarm clock The clock uses 176 LEDs for the display in two colours: red and yellow, i.e., two LEDs for each of the 88 letters. Red is used for night display (6pm..6am) and yellow is used during the day (6am..6pm).

The two colour display distinguishes this alarm clock from my wall clock. For setting the alarm, I wanted to be able to use a full night and day 24h range instead of just 12h.

My wife and I had long sessions of comparing the colours of LEDs to select the most beautiful ones. Unfortunately, the camera does not really do a good job at capturing the colours.

completed word alarm clock The alarm time display is indicated by using the preposition 'um', meaning 'at'.

completed word alarm clock For the normal time display, the clock does not use a preposition.

completed word alarm clock This is the back side of the alarm clock with one hole for the buzzer and another hole for the coffee machine connector. Yes, that small 2.5mm audio jack can switch on a coffee machine. OK, that coffee machine is not built yet, but it's on my list of projects...

The coffee machine connector is an open drain that can switch quite a bit of power, limited mainly by the small wires that I use inside the clock. The MOSFET can do 20V at 3A or so (it's an IRLML2502), but I would not feel very comfortable with a current above 250mA.

enclosure of word alarm clock The clock uses a lithium polymer battery (3,7V) with 1000mAh, and it has a USB connector (currently micro USB-B) for charging. That's what this oblong hole is for. The two holes next to it are for status LEDs of the charger circuit ('charging' and 'error').

I use an LTC4053 and LTC4410 combo for doing the USB stuff -- the main micro-controller does not care at all about this. It does have a USB power sense, though, so that the display will stay on when there's wall power.

enclosure of word alarm clock This is the back side of the wooden enclosure where the plies are visible: I cut two plies of around 1mm from a piece of beech hardwood and glued them back together with the grain direction at a right angle to stabilise it. The difference in grain direction causes this nice colour change, framing the enclosure.

I suppose one would usually use three plies or any other odd number for stabilisation, but the wood is glued to an inner 3D printed plastic skeleton that also acts as another layer of stabilisation, so 2-ply should be fine.

The wood is finished with hardwax oil.

enclosure of word alarm clock The buzzer and coffee machine holes again, and the countersunk screw hole for bolting the enclosure to the electronics inside.

enclosure of word alarm clock The top of the clock has holes for the ambient light sensor, for the main control knob, and for the alarm on/off switch, where there's also another red LED for indicating whether the alarm is switched on.

enclosure of word alarm clock The clock face and crystal.

enclosure of word alarm clock The face and the crystal are glued together with epoxy, and it came out OK. This corner looks perfect, but I need a few more tries to get it really right. This is a prototype, and I am still experimenting. There are tiny bubbles in the epoxy (I will purchase a pressure pot to deal with that) and the glue layer is unevenly thick -- a dumb mistake. The mistakes are not visible in these selected pictures.

The glass was cut from a 2mm sheet of glass as precisely as possible, with a goal of 0,2mm larger dimensions, and it took only two attempts to get there. Then I wet sanded it by hand with water-proof sand paper with grit sizes 120, 220, 400, 800, 1500, and 3000. The front side of the glass has a small bevel. The corners are rounded just like the clock face and the enclosure. The finished edges and corners came out really nice.

enclosure of word alarm clock This is the back side of the clock face, showing that it is just another PCB that I made with Kicad and Perl. I converted the Triplicate font files to an SVG font with fontforge, so that I could easily read the outlines with Perl. Then I processed the outlines with OpenSCAD to make them a bit larger. From this data, back in Perl, I computed the convex hulls for the back side, and generated the Kicad copper layer and solder mask polygons.

knob of word alarm clock This is the main knob that I made from brass. I tried to find a nice knob, but there just was none that I liked. I then got myself a brass rod from eBay and used very crude techniques involving a drill stand, hand saws and files, and battery powered drills to turn the knob. I have no lathe, so this was the only way for me. The knurling was done with a file and some violence. The knob has an M2 thread for a grub screw -- I also cut the grub screw myself from a slotted, countersunk M2 brass screw because there was nothing with exactly the right length that I could find.

Oh and I do not want this shiny. It was shiny, but the knurl will quickly look used anyway, and I like the patina.

enclosure of word alarm clock These are the screws for bolting the enclosure to the inner parts of the clock. I tried to find M2 brass screws with torx drive (DIN965), because torx screws just look more elegant than slotted or philips screws. But I couldn't find any. Stainless steel was no problem, though. So I got steel screws and heat treated them with a blowtorch (well, with a Searzall, really) to get a brass like colour. It worked perfectly: the screw on the left is the same steel screw as on the right, only heat treated. The nut it is screwed to is brass, which is almost the same colour.

enclosure of word alarm clock This is a view of the interior of the wooden enclosure. The black skeleton is a 3D printed plastic part made for this to hold the rotary encoder the knob is mounted to, and the alarm switch. Oh, and the coffee machine connector.

overview of parts of word alarm clock This is an overview of the parts. Starting at 11 o'clock and moving clockwise: the lipo battery, the CR2032 real-time clock backup battery, the enclosure and face, the main electronics board, the switches and connectors, and the knob.

electronics of word alarm clock This is the smallest DCF77 radio clock receiver that I could find: from Pollin. Despite the size of its antenna, it has remarkably good reception. OK, I'm really close to the DCF station, but compared to other receivers I had used before, this one really was a positive surprise.

electronics of word alarm clock The black plastic part is the 3D printed inner base to which all the inner stuff is mounted. Yes, I could have disassembled it more for more detailed photos, but there needs to be something I can show when the two final alarm clocks are finished.

On the right, the buzzer is visible, and there is a metal mesh on it. This is a piece of tea sieve that I epoxied to it for protection against someone poking the coffee machine connector into the wrong hole.

electronics of word alarm clock This is another overview of the electronics. Antenna on the bottom, and the controls will end up at the top side.

electronics of word alarm clock These are the controls: the alarm switch with a satisfying 'click' (yes, I also did a switch selection session to find one with the right click (that is small enough to fit into the clock)). Then the rotary encoder with 30 positions and a push button, and the audio jack for the coffee machine control.

The alarm switch is mounted to another small PCB. This was the easiest way I found to put an LED in the right spot and to have a mounting hole for securing the switch.

electronics of word alarm clock This is the empty PCB of the control board of the clock. I had this manufactured by JLCPCB. The quality and precision are really good.

electronics of word alarm clock This is the front side with 3D printed LED mask. It came out so ugly! But it works like a charm and no-one can see it when the face is attached to it.

electronics of word alarm clock Finally, this is the controller I used: an Atmel atmega324pb 8-bit micro-controller. As usual, to have more fun, I programmed it completely in assembler. Due to the additional logic for a low-power functions, and for alarms, etc., this is 7kB of code (plus 1kB for tables for the display). The wall clock uses only 4kB.

This battery powered circuit was really fun to do because it was a totally different kind of optimisation compared to what I had built before. The whole design is centered around low-power. I got it down to 15µA (measured right at the battery) when the clock is sleeping, i.e., including the sleeping µC, quiescent currents from the USB charge controller, the LDO, and all the other tiny things that all draw power.

The silent DCF reception draws around 600µA. This is needed once a day during the night for typically 80s if all goes well, and for up to 600s if there's no reception.

When the display is switched on, the clock draws between 5mA and 10mA, and the buzzer draws an additional 15mA.

With these numbers, I reckon the 1000mAh battery should last at least half a year, but actually more like a year. We'll see.

The CR2032 is the backup power for the real-time clock that I use (a DS3234 with an SPI bus because SPI draws less power than I2C). Using another battery may be overkill, but why not. This battery should last around 5 to 10 years, hopefully. The µC can monitor both batteries and will warn the user if one runs low.


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November 19th, 2020
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