Wednesday, 4 January 2017

Children's countdown timer

Occasionally, our children need some help with the concept of time. According to them, some of those minutes are much shorter than others (think watching TV vs. driving to the Efteling). To help them with this, I have been promising to make something for the past half year. But now it is ready!

After spending about two days on it over a period of six months, here is the result: a children's countdown timer. It shows clearly how much time is left for a certain activity, and also plays a tune when time is up.

The children's timer. Unfortunately, the multiplexing of the leds combines unfavourably with the shutter of the camera. To the naked eye, all leds in the upper-right quadrant are lit.

By turning the knob clockwise, more and more of the leds along the ring are switched on. After not moving the knob for 5 seconds, the timer starts counting down1. One after another, the leds slowly decrease in intensity and switch off. Each led takes 100 seconds, so the full circle uses a full hour. When all leds are off, time is up and a song is played on a piezo buzzer while the leds show a flashy pattern. By turning the knob clockwise past "zero", it is possible to switch between silent mode and audible alarm (non-silent) mode. In non-silent mode, the buzzer also clicks when turning the knob, providing feedback on whether an audible alarm is enabled.

Under the hood

The main parts of the timer are 36 red leds, two 1.5V AA batteries, a 3.3V step-up converter from the parts bin (Sparkfun NCP1400-based), a rotary encoder (from conrad.nl), a piezo buzzer that came with an old computer mainboard, and an Atmel ATtiny2313 for a brain.
A look under the hood. The piezo buzzer, the step-up converter and the AVR are approximately on a straight line from the bottom-left to the centre. All leds are on, and the batteries deliver a current of 6.3mA.
The 36 leds are driven as 3 sets of 12 via charlieplexing, using 12 microcontroller pins. This arrangement uses more pins than multiplexing the leds in a single set, but it allows three leds to be lit at the same time, increasing the average brightness using less current. Without going into details, this has to do with the non-linear response of led vs. current.

Since debugging this kind of setup is very hard, I tried to keep the software as simple as possible. A single "master" timer fires an interrupt 2600 times per second. The main purpose of this is to refresh the "display". The correct leds are switched on or off according to what needs displaying. Four levels of intensity per led, 12 leds, and 2600 interrupts per second lead to a refresh frequency of 55 Hz, which shows no flicker (to the naked eye at least ;-)).


A look at the back side of the board. The other side can be so clean because this one is a bit of a mess.
After updating the display, the routine samples the two input pins connected to the rotary encoder. By filtering over 16 samples, the signals are debounced to get a reliable input. And finally, a software counter divides the master clock to generate a timer "tick" at 5 Hz to be used by the rest of the software for time-keeping.

Events for timer "ticks" and changes of the rotary encoder are communicated to the rest of the software using bit flags. After every interrupt, the main software loop wakes up, and if there is an event, this is handled in the loop.

To play music, a separate timer is connected to the piezo speaker, that generates the notes. The music is stored encoded in the flash image and played at a pace determined by the "tick" events ultimately derived from the master timer.

When the countdown timer is switched off, the oscillators of the microcontroller are disabled and the ATtiny enters a deep sleep mode. Just before this happens, one of the inputs connected to the rotary encoder is set-up such that it will wake up the microcontroller when the signal changes. If the rotary encoder is in a position where one of its inputs bounces between on and off, this could prevent the microcontroller from entering deep sleep. To counter this, the input that wakes the micro is alternated between the two inputs every time sleep mode is enabled. So if the AVR wakes up due to a bouncing pin, it will time out since there is no actual input after the debouncing filter. It then switches to the non-bouncing pin, and enters sleep mode again.

The software is written in C using avr-libc, and compiled by avr-gcc. The current version takes 1748 bytes of flash memory, leaving exactly 300 bytes for possible extensions. The circuit has a header that is compatible with an USBasp to allow in-circuit updating of the software, something that has already proved very useful. Since the circuit normally runs at 3.3V, but the USBasp needs 5V, it is necessary to disconnect the battery before programming, and power the circuit from the programmer itself. In hindsight, I should have added a jumper to make this more convenient and less error-prone, but so far I remembered to remove the batteries when needed.

I put the software on https://github.com/bart-h/avr-timer for those that are curious.

Power usage

Because the contraption runs of a battery, power usage is an important consideration. When switched off, the system itself draws an idle current of 0.5μA after the step-up converter. Assuming a battery capacity of 1Ah, this would correspond to a battery-life of 228 years! Or actually, it means that other things will dominate the battery drain. Before the step-up converter, about 12μA is used, mainly by the converter itself. Initially I was irked to fix this 4% efficiency problem, but then I realised that the battery life would still be 9.5 years. Close enough to infinity for my purposes.  And when the system is fully active, the efficiency increases to about 90%.

When the countdown timer is running and all leds are switched on, the system draws 6.3mA at the battery. Running a countdown of a full hour (starting with all leds on, and ending with all leds off) then uses 3.15mAh. Again assuming a 1Ah battery capacity, this would allow 317 runs, more or less one per day for a full year.

The largest power drain is by the buzzer and its series resistor. When it is driven high, it draws ~30mA. Fortunately, when playing notes, the buzzer is switched on and off constantly, as sound is made by voltage changes, not stable levels. This already halves the average current. In addition, the music contains pauses that set the buzzer to 0V. Because I do not have an integrating power meter, I do not know how much power is actually used when playing music. But because the tune takes no more than 20 seconds, I assume the power usage can be neglected in the overall scheme of things. While turning the knob, the buzzer also toggles to make clicking sounds. Here I took care to switch the buzzer level back to 0V as soon as possible, to conserve power.

Because an real Alkaline AA battery actually has a capacity closer to 2Ah, all calculated lifetime numbers should be doubled as well in reality. But the final proof will be when I have to replace the batteries...


The timer running when it is dark. Because the camera uses a much larger exposure time, all leds that are lit also appear on in the picture.

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