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applications => Egg-timer
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A small homemade project - just for fun - but very practical
Who didn't already run into that: the n-th birthday celebration
of friend X.Y. is coming up, and you do not have any idee what
your friend's household is missing. Now, you can just go to a shop,
that sells any kind of presents, and buy something sense- and
useless, wrap it in nice paper, and just hope that no one else
will have the same (non-)idea. Otherwise the main question will
come up: exchange this piece to another piece, that no one needs?
If you are experienced in soldering some electronic parts together
and of programming an AVR, you have a clear avantage over all your
other friends: producing a very individual gift, that no one can
copy by chance, and put it on the birthday gift table. During the
next twelve monthes, all our friends, relatives and neighbours
receive an up-to-date, individually designed and personally
flavoured egg-timer. Of course, equipped with the latest, greatest
micro-professor, as available on the electronic device scene today.
Not just one of the old-fashioned and traditional flasks, filled
with acid-cleaned silica sand. Quietly rippling sand was yesterday,
today it is beeping and blinking.
The principle is very easy: a row of LEDs, e.g. placed like a
bar, is lit green and red (first variation: yellow and red),
whenever a minute is over, one more green light changes from
green to red. To save battery capacity and use the timer for more
than one egg to cook, only one LED is lit at a time in the
display cycle. The band is running fast, so that there is some
action in the kitchen and you can catch the timer's state in
just a short glimpse, looking up from your morning newspaper.
If you are too fascinated by all the news in your newspaper,
you do NOT even need to keep an eye on your timer: it just beeps
the number of elapsed minutes. That makes your egg-timer also
compatible with the needs of handicapped people, that arent
able to see the disco lights, which makes a major difference
to the sand-filled equipment. If ten minutes are over, a
yellow light and a continous beep tells you, that the egg yolk
has turned to green now, and that the egg can be thrown into the
I only describe the electronic and software design here, so you
must add your own creativity in the package design. For placing
the LEDs, consider the following opportunities:
From my own experience it has been proven top, to add an
individually designed instruction manual to that gift. Humorous
formulations, like english words combined with taiwanese
grammar, bring good vibes to the birthday celebration. Also,
you can write the instruction in english, translate this to
japanese language, retranslate it with Babelfish to english,
and then exchange single words by their dutch translation.
- A straight line of LEDs signals geometric strength, and is
more adapted to straight-thinking people like e.g. mathmaticians
or housekeepers, model "straight forward",
- A circle is possible, and more adapted to people with a
similiar belly, model "beercan",
- For the more playful people, the form of an egg is more
appropriate. This model is also good for the short-memoried,
because it reminds what the equipment is made for, model
- For the more chaotic, untidiness- and complexity-loving
neighbor, you can place the LEDs in a random manner on the
unregular-shaped casing, model "Hundertwasser" (see
- The design for the gamer: place the LEDs randomly, and
exchange the LEDs every minute randomly by software. He will
love to guess where the next LED will get red, model
"vote and surprise".
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The hardware scheme is pretty easy: the two batteries (2*AA, 2*AAA
is suitable, too) supply the hardware with 3 wonderful Volts. More
voltage destroys the LEDs very fast, so don't supply with
5 Volts. The row of dual-LEDs is cascated with resistors,
limiting the currents. Each LED can be driven in both ways by
the portpins. When soldering the LEDs, only ensure their
uniform direction, their colour can be easily changed by
All portpins that are not needed to drive a certain single LED,
are set to tristate (input) and therefore do not interfere. The
ATtiny2313V (without V is also fine) always drives only one LED,
the voltage drop, when sourcing from the pin under load, has
been calculated in the design of the resistor. The yellow
egg-throw LED has its own portpin. A small piezo-speaker is
directly pinned to the OC0A output. If you don't have a piezo,
you can use a small 32-Ohm-speaker, decoupled with a C of
Here a possible layout for a PCB for the eggtimer. The format
is 100 by 80 mm (3.15 by 3.94 inches).
Parts placement displayed here.
Building the eggtimer into a black case looks like this.
Somewhat more sophisticated, in plastic glas, with two layers: the battery and the
speaker are mounted on the lower layer.
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3.1 Processor programming
The processor hardware cannot be programmed in in-system-mode
(ISP), because the operating voltage should not exceed 3 V.
The processor has to be prgrammed on an extra programming
board and then moved to the egg-timer hardware.
First, the processor is changed to operate with the internal
4-Mcs/s-RC-oscillator, to reduce power consumption. The DIV8
fuse stays activated, so that the processor runs with a
500 kcs/s clock. Slow enough for the V-version.
Assembler source code for the basic version is for download
in asm format here and
can be viewed in HTML format here.
A newer version of the code provides a gamut and a melody at
the start-up, asm format here
and html format here.
In the source code, first the portpins are assigned. That means
that all LEDs can be mounted in any sequence and assigned here.
The drive row must be assigned by the correct output port,
portpin and direction port. The extensive program code to
drive the correct LED, at the end of the source code, is
attributed to that flexibility in placing the LEDs.
Time measurement and LED-switching is assigned task of the
16-bit-timer 1, that runs in CTC mode. When reaching the
value in the compare register A, every 0.2 sec, an
interrupt is triggered and the T flag in the status register
is set. The interrupt wakes up the processor from idle sleep.
The T flag is cleared, and the lowest bit of the LED counter
is checked. If this is one, the beep tone is switched off.
If it is zero, it is checked whether a beep tone has to be
generated. If yes, the speaker output is turned on (Timer
0 is activated) and the number of remaining beep tones is
After that, the last LED that was on, is switched off. The
LED counter in incremented by one and is checked, if it is
already higher than 10. If yes, the two-second-counter is
incremented and checked, if this has reached 30. If yes, a
minute is over and the minute register is incremented. If
that has exceeded 10, the yellow LED is activated and a
continous beep is initiated.
At the end, the next LED to be lit is calculated and, according
to the minute counter, is switched to red or green. For
calculation of the correct routine, the start address of
a jump instruction table (red or green) is written to the
register pair ZH:ZL and the LED number is added. By
executing IJMP a jump to the correct routine is performed.
After switching the correct LED, the routines all jump
back to the sleep instruction of the main program loop.
There is a broad palette of possible variations:
The number of possible variations makes it impossible to
offer the software for all these variations. The number of
variations should be big enough to design an individual
gift for each and everyone in your circle of friends, so
that no one gets a timer with exactly the same design.
But: keep good records on all the distributed variations.
Otherwise you might run into new trouble.
- LED band speed:
- hectic mode: for people that need high speed, we
increase the running speed of the LEDs to the double,
by reducing the value of 12500 down to 6250 and
changing the 30 for the minute recognition to 60.
- slow mode: for the one's that need more time to
move their eyes, we change the 12500 to 25000 and
change the 30 in the minute recognition to 15.
- Incorrect LED order? No problem: exchange the lines
ldi ZH,HIGH(TabRot) ; try with red, and
against the lines
ldi ZH,HIGH(TabGruen) ; no, it is green, and
and the line runs with opposite colors.
- Higher tone? No problem, in the line
ldi rmp,63 ; CTC-value
just exchange 63 by a smaller value for timer 0.
- Egg-timer for a musician? Possible. Just add a
table with the CTC values of the gamut to the flash,
and, after each minute, change the CTC value with the
next value in the table.
- If your customer prefers hard-boiled eggs of the type
"golfball": add four dual-LEDs to the four available
portpins and add them to the software.
And now: good luck with your handicrafts.
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