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 Posted: Jun 17, 2007 - 09:44 PM |
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Joined: Jun 15, 2007
Posts: 110
Location: Gdańsk, Poland
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hello everyone,
It's a shame for me to ask such a dumb question.
according to my calculations, a pack of two 2400 mAh AA batteries should be able to operate for one hour supplying 2400 mA current on 3V. This means that to let device work for a month, it should consume not more than 2400/(24*30)=3,(3) mA.
this result seems to be very strange to me, because there are a lot of devices, such as pagers, mp3 players, watches, which can operate for even longer period with lower-capacity batteries... is it because these devices are so power-saving or have i made something wrong with my calculations?
I want to build a watch, which obviously needs to work constantly, and this issue makes me doubt if that's even possible. i've already abandoned the idea to use 14 LED diodes to visualize time in binary code, but this idea still is welcomed to be supported against faggy lcd decimal presentation.
anyone has a clue how to make it work for at least one month?
thank you! |
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Not entirely familiar with AVR thingies... yet.
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Posted: Jun 17, 2007 - 09:50 PM |
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Joined: Nov 22, 2002
Posts: 12046
Location: Tangent, OR, USA
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For long operating times, think of micro-amps, not milli-amps!
Most of these use some sort of idle or sleep mode with very low current. The device wakes up when a button is pushed or something else happens. Then, after there is no use for some specified time interval, it goes back to sleep.
LED watch simply won't do what you are asking, with the LEDs on all the time. Maybe, with a little button to push and it shows time for 5 seconds, or such.
Jim |
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Posted: Jun 17, 2007 - 11:37 PM |
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Joined: Jun 15, 2007
Posts: 110
Location: Gdańsk, Poland
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thanks for your advice! do you think it is possible to put ATTiny into sleep between counting another seconds? will the external timer be essential to wake it up? what accuracy can i get with a device operating in such way?
my wish was to make it show time constantly, but even with the latest 1mA PicoLED(R) it is not possible. I'm still experimenting with the balance of brightness and resistance put on 3mm LEDs. if this fails, and the search for low-powered LCDs will also fail, most probably it'll end up with such a button you mentioned. |
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Not entirely familiar with AVR thingies... yet.
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Posted: Jun 17, 2007 - 11:51 PM |
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Joined: Dec 17, 2005
Posts: 1498
Location: Europe- Estonia- Tallinn
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maybe you can get a hold of a "cereal box" wrist watch. those 1$ costing things sometimes have an LCD on them. You could use that to get ultra low power. LEDs aren't made to be low power sadly. I know. I used 2 7segment LED displays on my last project and it almost eats through the AA batteries.  If low power is needed, then an LCD is a must (for any display). Also you have to use all sorts of sleep modes. Every time you wait something, you have to sleep. You won't use _delay_ms(x) command but rather _sleep_ms(x), what you have to make your own probably. |
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Posted: Jun 18, 2007 - 12:30 AM |
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Joined: Jun 15, 2007
Posts: 110
Location: Gdańsk, Poland
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ok, i'll try to learn from datasheet how much time does it spend on waking up and falling asleep to find proper sleep time. thanks.
my other idea is to aid battery by solar cells like this one:
http://www.allegro.pl/item204954024_ogniwo_sloneczne_70ma_bateria_sloneczna_.html
(just scroll down to see technical spec.)
i could afford buying few of those. do you think this could work in cobination with small condenser? if it would, how could i make it? do i need at least six of those to be helpful or less than 3V is not useless too?
how about using that cells to power the lcd seperately? please comment those ideas, thanks a lot!
EDIT: i came into an idea to fully supply lcd by those solar things, and in case of darkness manually power the lcd from battery for the fixed duration. what do you think? |
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Not entirely familiar with AVR thingies... yet.
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Posted: Jun 19, 2007 - 10:19 PM |
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Joined: Jun 15, 2007
Posts: 110
Location: Gdańsk, Poland
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| how about using external 32kHz oscillator to make attiny work less? is it going to save up significat amount of precious uAmps? please comment! |
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Not entirely familiar with AVR thingies... yet.
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Posted: Jun 19, 2007 - 10:37 PM |
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Joined: Dec 17, 2005
Posts: 1498
Location: Europe- Estonia- Tallinn
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better use highest clock possible. Takes more amps when running, but gets faster back to sleep, in witch clock is turned off anyway  |
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Posted: Jun 19, 2007 - 11:41 PM |
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Joined: Apr 20, 2007
Posts: 6066
Location: Long Island New York
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Keep in mind also the battery chemistry, is it ni-cad or Ni-MH, or Li-ion. Nicads do self dischrge amazingly fast and have a memory effect.
The 240mah is a figure in which the rate of draw/time to a certain capacity is given. A 1.5v aa cell is considered dead at 1.2v. You are correct in your thought that 2400mah is 2.4amps draw should last an hour, but i like to hedge the better side of caution and say 70% of the rating.
Rinning the tiny at 32khz is also a good idea. it won't use as much juice, and you won't need to divide the clock to much to get your timing references.
Jim |
_________________
Jim
I have decided that I am no longer going to plan anything in advance. In a court of law this is called Pre-Meditated, and does not look good for the defense.....
Timer function not working properly? Check CLKDIV8 Fuse first
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Posted: Jun 20, 2007 - 02:36 AM |
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Joined: Jun 15, 2007
Posts: 110
Location: Gdańsk, Poland
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thank you both for advice. i guess i'll need to run some tests (when i finally have my devices shipped) with fast sleep reaction and dead slow rating, but honestly i guess the crystal will probably consume much enough to be useless in power saving. or is there a way to run tiny at 32kHz without using external oscillator?
for now i consider GP T-154 2,4V 300mAh battery pack, or GP T-113M 3,6V 320mAh (or such, those are Ni-MH), and put a 4,5V power input onboard to charge it, let's say, once a week, if only it's possible.
taking into consideration Jim's advice 320*0.7/24*7=4/3[mA]. now that's the quest.
on the other hand, i doubt if 2,4V or 3,6V are enough to power the lcd. do you know if there are graphic LCDs running less than 5V?
thanks again for all your good will. |
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Not entirely familiar with AVR thingies... yet.
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Posted: Jun 20, 2007 - 02:40 AM |
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Joined: Jun 15, 2007
Posts: 110
Location: Gdańsk, Poland
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| um... another thing. is there a way to make tiny wake up itself after putting it into sleep? how could i make it asleep and still measuring a second for me? is it possible without external crystal? thanks! |
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Posted: Jun 20, 2007 - 04:34 AM |
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Joined: Mar 28, 2001
Posts: 20363
Location: Sydney, Australia (Gum trees, Koalas and Kangaroos, No Edelweiss)
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| The Ampere/Hour rating is usually given for a 20 hour period, so a 2.4A battery would give you 120mA per hour for 20 hours. |
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John Samperi
Ampertronics Pty. Ltd.
www.ampertronics.com.au
* Electronic Design * Custom Products * Contract Assembly
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Posted: Jun 20, 2007 - 05:55 AM |
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Joined: Aug 29, 2006
Posts: 229
Location: Kingston, Jamaica
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One way I think you could do this is with a low power RTC chip. Have it wake the uC every second, the uC outputs the time to the LCD then goes to sleep waiting for the next wakeup call.
Edward |
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Posted: Jun 20, 2007 - 06:57 AM |
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Joined: Apr 01, 2004
Posts: 3812
Location: New Mexico
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futrtrubl wrote:
One way I think you could do this is with a low power RTC chip. Have it wake the uC every second, the uC outputs the time to the LCD then goes to sleep waiting for the next wakeup call.
I done this with good results with a DS1306 outputting a 1Hz signal. Another advantage of low power RTC chip like the DS1306 is they usually accept battery backup so you can power down your MCU and associated circuitry without losing the current time. |
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Kevin Rosenberg
http://b9.com
http://kevin.hypershots.com
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Posted: Jun 20, 2007 - 11:18 AM |
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Joined: Dec 17, 2005
Posts: 1498
Location: Europe- Estonia- Tallinn
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For clock I use DS32C35 from Maxim/Dallas.
Integrated 32.768kHz Crystal
♦ Fast (400kHz) I2C Interface
♦ RTC Counts Seconds, Minutes, Hours, Day, Date,
Month, and Year with Leap Year Compensation
Valid Up to 2100
♦ RTC Accuracy ±2ppm from 0°C to +40°C
♦ RTC Accuracy ±3.5ppm from -40°C to 0°C and
+40°C to +85°C
♦ Nonvolatile Memory with 10 Years of Guaranteed
Backup Time and Write Protection
♦ Two Available Densities of Nonvolatile Memory
2048 Bytes (DS32B35)
8192 Bytes (DS32C35)
♦ No Cycle Limitations on Memory
♦ Power-Switching Circuit Selects Between Main
Power and Battery Backup for the RTC
♦ Programmable Square Wave with Frequency of
32.768kHz, 8.192kHz, 4.096kHz, or 1Hz
♦ Two Time-of-Day Alarms
♦ Reset Output/Pushbutton Reset (Debounced)
Input
♦ Programmable Output Provides Interrupt or
Square Wave
♦ Calibrated 32.768kHz Open-Drain Output
♦ Temp Sensor with ±3°C Accuracy
♦ 3.3V Operating Voltage
♦ Backup Battery input with charging capabilities (when power is back on the Vcc lines)
♦ 20 pins, 9 of them are N/C
I like it. It's still running on the Lithium cell I charged for it a month ago. Can't say the time though. Don't have a display on the board just the chip with the battery and pins for TWI for prototyping use.
Also the cost isn't very high. I got that as a sample (a few of those actually).
Also you can get all sorts of LCD's that work on 1.5V also. you should really go shopping and find yourself a small wristwatch for kids. you know those with mickey mouse images etc. cost is about 1$. those run on low voltage probably. |
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Posted: Jun 20, 2007 - 01:50 PM |
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Joined: Jun 15, 2007
Posts: 110
Location: Gdańsk, Poland
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1Hz clock sounds better than the 32 kHz I suggested. i googled DS1306 and it costs enough to take a special care of it, so i'll refer back to this forum about using such stuff when i get things ready to build it.
i also googled both DS32C35 and DS32B35 but there were no results in polish, probably meaning i'll have problems to get it here. but i'll doublecheck in the store to make myself sure.
about the lcd, i'd rather have one manually switched on than a faggy kid wristwatch display  thanks anyway for the clue. i'm still thinking about solar cells to power lcd exclusively, and installing a button letting me temporarily turn on lcd in dark rooms. gosh, seems like this watch will be big enough to cover my forearm 
alright, thanks for all your help, i think i know what to do now. but any other comments are still welcomed! |
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Posted: Jun 20, 2007 - 03:39 PM |
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Joined: Dec 17, 2005
Posts: 1498
Location: Europe- Estonia- Tallinn
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| sry if I have missed it, but what are you building? what is the maximum size of your project? (just to make better suggestions) |
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Posted: Jun 20, 2007 - 05:33 PM |
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Joined: Jun 15, 2007
Posts: 110
Location: Gdańsk, Poland
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a wristwatch, but showing time in binary code. but as folks agreed that diodes consume too much power it's going to be equipped with lcd (graphic, most probably), and in that case i'm going to add some cool functions.
i haven't been thinking about size before, but i guess the only limit is 4,5cm wide, well, maybe not longer than 10cm as well. i'm not going to minimize it too much because i'd like to make it work first.
the choice of avr device is not fixed yet, but i guess one of the smallest attinys should be enough (i performed the calculations for attiny11L). perhaps ill develop in the further future a memory bank to keep graphics for lcd and easily update them.
btw where did you get samples of DS32C35? i'd be interested too, because it might be tough to find it here... |
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Posted: Jun 20, 2007 - 06:08 PM |
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Joined: Dec 17, 2005
Posts: 1498
Location: Europe- Estonia- Tallinn
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| examples? no examples available. This is a VERY new chip. I used the datasheet of both DS32C35 and the AVR. If you need, i can show you my codes to write data to it using TWI. Written in GCC. Just let me know and I'll add them here. |
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Posted: Jun 21, 2007 - 03:06 AM |
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Joined: Jun 15, 2007
Posts: 110
Location: Gdańsk, Poland
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um, no, thanks, don't bother yourself, i think i won't need it while not having any idea about TWI or GCC things
perhaps i'll just focus on DS1306, check out its power consumption and make a right decision. i wish myself this thing working as i need, otherwise i'll bug you folks again |
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Posted: Jun 21, 2007 - 03:17 AM |
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Joined: Apr 20, 2007
Posts: 6066
Location: Long Island New York
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If you are buil;ding a wristwatch with these parts I hope you have a tree trunk for a wrist!! Also, putting your micro to sleep means you will need to have an RTC to keep time. and do not use the 1hz to run the micro, this means every instruction is going to take at least a second to perform.
JS was 100% correct in his amp/hour description. Mine formula is close, ut a tad flawed. What i said about the chemistry is correct though. Also, if this is going to be a wristwatch, then the coin cells are the way to go, but do not try to recharge them!! they will explode
Jim |
_________________
Jim
I have decided that I am no longer going to plan anything in advance. In a court of law this is called Pre-Meditated, and does not look good for the defense.....
Timer function not working properly? Check CLKDIV8 Fuse first
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