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'[PIC:] First Project Finished! But... it used up a'
2004\06\04@144806 by Lindy Mayfield

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Hi Guys.

I finished my first project!  Finally.  I got the idea from Mike Predeko's book for the LED clock.  But I couldn't find the right parts to put together the multiplexing for all the LED's, so I decided to work with what I have available and made a binary clock:  20 small 3mm LED's that show the hours, minutes, and seconds in bcd binary.  
The design so far is simply a 9v battery, a 5v voltage regulator, a PIC 16f877, a 32.768 kHz crystal + 2 22pf caps, resistors and LED's, and one resistor tied to MCLR.

The programming was fun.  Building and soldering the rat's nest out of wires on the back of the board wasn't.  (-:  (Took 3 weeks.)

I left it running for a test last night and it sucked all the juice out of my 9 volt battery in just a few hours.  
Would someone give me hints as to how to figure out what the problem is why it is using so much power?

Thanks in advance.
Lindy


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2004\06\04@150437 by Fred Hillhouse

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Let's see: LEDs can draw quite a bit of current. That would be a good start.
What is your current limit for each LED?

Second, 9V batteries are not known for high capacity either.

Best regards,
Fred


{Original Message removed}

2004\06\04@151058 by Fred Hillhouse

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Although many LED clocks have 9V batteries in them, usually the LEDs are off
if the AC power fails for some reason. The 9V battery only powers the 'time
keeping' circuit, not the 'time displaying' circuit.

You may want to consider a 9V wall wart instead.



{Original Message removed}

2004\06\04@151514 by Lindy Mayfield

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I connected my multimeter from +5 on the battery to the circuit and it reads about 75 mAmps.  I'm quite new to this, but that seems like a lot.

Since all the LED's are connected to a common ground in my circuit (already soldered), can I add a resistor between the cathode of the LED's to the battery ground?

{Original Message removed}

2004\06\04@153137 by Robert Ussery

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----- Original Message -----
From: "Lindy Mayfield" <.....Lindy.MayfieldKILLspamspam@spam@EUR.SAS.COM>

>I connected my multimeter from +5 on the battery to the circuit and it
reads about 75 mAmps.  I'm quite new to this, but that seems >like a lot.

It's a lot for a 9V battery, but not really a lot, considering that you're
powering 20 LEDs, presumably at pretty high current. I don't think you can
easily do much to reduce current consumption enough to use a 9V, besides
dimming the LEDs. As someone else mentioned, 9V batteries are rarely or
never used in always-on LED clocks, because of the high current consumption
of LEDs. What you *could* do is to have the LED's turned off most of the
time, with a button to display the time when you want to see it.

Like someone else mentioned, just buy a 9V wall-wart, and use it instead of
the battery.


- Robert

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2004\06\04@153553 by Fred Hillhouse

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You shouldn't connect every LED through one resistor.

What are your LEDs rated at? What is the forward voltage?

If you have 5ma LEDs and the forward voltage is 1.2v, then a resistor of 760
ohms [(5V-Vforward)/.005=resistance] should be in series with the LED. If
you are using a transistor to turn them on, then you will need to also
account for the voltage drop across the transistor.

The purposes of multiplexing the LEDs are less parts and less current.

Something else to consider: if you are using 20ma LEDs and they are all
connected to you PIC, then if they all turn on, you can exceed to overall
current rating of the PIC. Even though each pin can sink/source 25ma, all of
them can't at the same time.




{Original Message removed}

2004\06\04@153800 by Lindy Mayfield

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Oh, I think I understand (I'm a bit slow).  It is the battery that is the problem.  If I used instead 2 AA's then it should last a long time?  (I wanted to put it on my desk at work. (-: )

{Original Message removed}

2004\06\04@153801 by Paul James E.

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Lindy,

You may want to consider multiplexing the LED's so that only 1 is on
at a time.  But it will appear as though they are all on at once because
of our 'persistence of vision'.  Most commercial clocks do this.  If you
have a LED ( 7 segment) digital clock around, shake it in front of you
and watch the digits.  They will all appear to spread apart because they
are multiplexed.   This allow the numbers to appear to us as though they
are all on at the same time, but in reality only one is on at a time, and
therefore, you have only one LED current to be concerned about.
You could set your LED current to about 10-15 ma, and save a lot of
battery power.   If you don't have a need for reading this clock across
the room, you could set the current even lower.

                                               Regards,

                                                 Jim


> I connected my multimeter from +5 on the battery to the circuit and it
> reads about 75 mAmps.  I'm quite new to this, but that seems like a
> lot.
>
> Since all the LED's are connected to a common ground in my circuit
> (already soldered), can I add a resistor between the cathode of the
> LED's to the battery ground?
>
> {Original Message removed}

2004\06\04@154424 by Fred Hillhouse

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The battery and the number of LEDs is the problem. Paul's suggestion of
multiplexing the LEDs is a good one.

3AA's would be good too.


{Original Message removed}

2004\06\04@165726 by Harold Hallikainen

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Running LEDs off a 9V battery is gonna give you pretty short battery life.
Looking at
www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_Alkaline_Hdbk_03-04_v1.pdf
a 9V battery is rated at 570mAH. Note also that the cut off voltage is
4.8V.  Your cutoff voltage is probably higher when considering the
regulator drop out voltage, etc.

If you have 20 LEDs and run 5mA each, that's 100mA when all are lit,
which, at best, would give you 5.7 hours operation. Looking at the
discharge curves, a 100mA load would be equivalent to a 90 ohm resistor.
With a 5.4V cutoff, you can expect about 8 hours of operation. Increase
the time a bit since not all LEDs are on all the time, but decrease the
time as your average LED current increases (peak current times duty cycle
if multiplexed).

This, of course, is why watches use LCD instead of LED.

Harold


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2004\06\04@171637 by Lindy Mayfield

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Kiitoksia paljon.  I didn't consider those factors in my design. (-:  Whoops.  Anyway, time for a new project.

Thanks also to all who took their valuable time to help me. I'm really learning a lot.

{Original Message removed}

2004\06\04@172015 by Fred Hillhouse

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At least the ones you buy today! :^D

       "This, of course, is why watches use LCD instead of LED."

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2004\06\04@172639 by Andrew Warren

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Harold Hallikainen <@spam@PICLISTKILLspamspammitvma.mit.edu> wrote:

> This, of course, is why watches use LCD instead of LED.

   Back in the old days when watches did use LEDs, the display only
   turned on while you held a button on the watch.  If
   battery-powered operation is important to you, it should be
   pretty simple to modify your clock to work the same way.

   -Andy

=== Andrew Warren -- KILLspamaiwKILLspamspamcypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

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2004\06\04@174937 by Jan-Erik Soderholm

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And, there are "low current" LEDs that gives aprox the same
amount of light @ 1mA as a "standard" LED @ 10mA. But note
that the *max* current in these are sometimes as low as 2 mA with
7mA as a more common value.

Cost a little more, of corse, if they didn't there would not be any
"standard" LEDs probably :-) :-)

Still, powering a LED based clock (even using low current LEDs)
with the LEDs constanly "on", is probably not a clever idea...

Jan-Erik.

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2004\06\04@175558 by Matthew Brush

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Like other people mentionned, persistence of vision thingy will save some
juice.  You could probably use the PWM ports on the PIC to save some power.
Or the multiplexing scheme mentionned previously.

Personally, I use at least 1K ohm for the LEDs to save power.  They always
look bright enough for me.

Good luck.  Peace

MJ Brush

{Original Message removed}

2004\06\04@180220 by Bob Ammerman

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Since your circuit obviously lets you control what LEDs are on at what time,
you could probably just reduce the on-time percentage to proportionally
reduce power draw.

Bob Ammerman
RAm Systems

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To: <RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Friday, June 04, 2004 3:36 PM
Subject: Re: [PIC:] First Project Finished! But... it used up all my bat
tree


> You shouldn't connect every LED through one resistor.
>
> What are your LEDs rated at? What is the forward voltage?
>
> If you have 5ma LEDs and the forward voltage is 1.2v, then a resistor of
760
> ohms [(5V-Vforward)/.005=resistance] should be in series with the LED. If
> you are using a transistor to turn them on, then you will need to also
> account for the voltage drop across the transistor.
>
> The purposes of multiplexing the LEDs are less parts and less current.
>
> Something else to consider: if you are using 20ma LEDs and they are all
> connected to you PIC, then if they all turn on, you can exceed to overall
> current rating of the PIC. Even though each pin can sink/source 25ma, all
of
> them can't at the same time.
>
>
>
>
> {Original Message removed}

2004\06\04@190000 by Jinx

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> Would someone give me hints as to how to figure out what the
> problem is why it is using so much power?

Congratulations on Step 1 - get it going

Now Step 2 - make it pretty. This is where you optimise components

As you've found out, LEDs and batteries aren't the best of mates. I
blame Hollywood. If there weren't so many bombs with timers and
big fat red displays for lead actors to spot and audiences to fret
over........

Ditto for other's comments about battery life and LCDs. You might
want to look at adding an LDR for automatic dimming, LDO regulator,
higher efficiency LEDs etc

If you switch to a mains supply, maybe try some parts of this circuit

http://home.clear.net.nz/pages/joecolquitt/solenoid-clock.html

50Hz goes to INT0 for accurate timing and the output of the transformer
is monitored for mains presence/absence. An internal timer count is
reset on external interrupt. Vsense, TMR0 and INT0 are used to detect
missing mains cycles (ie mains off/power cut or glitch) and either insert
one tick or switch over to full battery mode without missing a beat. The
quite long tick period gives s/w plenty of time for decisions

Bear in mind when testing that mains frequency is not the same at
all times of the day. It will slow down around peak periods and be a
little faster overnight, but the number of cycles/day is constant.

Comparing a mains clock to the hourly radio pips (a rough guide for
example) will show some slight difference through the day, but noon-
to-noon will be accurate (on average)

Mains also pulls in a small relay. Note that the normally-open contacts
are used for mains so that the battery can connect through the normally-
closed contacts

The LM317 is used here as a current limiter. The solenoids (~4 ohms)
take a lump of current from the 470uF reservoir cap, the recharging of
which would collapse the 12V. Controlling the charge rate means a
smaller transformer can be used. It does put a limit on the Fast Forward
or Step rate, but being a ratchet and pawl mechanism the clocks can't
go too fast anyway

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2004\06\04@204328 by Jinx

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> Kiitoksia paljon

Te aari mieluinen (?)

> Anyway, time for a new project

Well, while your brain's in clock mode........

http://home.clear.net.nz/pages/joecolquitt/0sp505.html

An old design, I'd revamp this if making one up now to use
more recent devices

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2004\06\05@033627 by Peter L. Peres

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Your problem is the LEDs and the 7805. You try to run 20 leds showing time
which means 10 leds on the average on all the time. So you draw 10*20mA =
200mA from the po' little 9V battery. It should last only about three
hours like that. Go and buy a $5 wall wart rated same (9V 500mA) and it
will be better. You can still use the battery to provide backup when the
power goes out. The list will likely help you with this (ask).

Peter

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2004\06\05@034703 by Peter L. Peres

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>Still, powering a LED based clock (even using low current LEDs)
>with the LEDs constanly "on", is probably not a clever idea...

LEDs mounted in a suitable enclosure (dark background) are visible when
powered with 300-500uA. High efficiency ones even more so.

Peter

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2004\06\05@035529 by Peter L. Peres

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Jinx, may I ask how you keep the SLA charged in this circuit ? A small
typo ? (1k comes from unreg. ?):

http://home.clear.net.nz/pages/joecolquitt/solenoid-clock.html

Peter

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2004\06\05@051154 by Jinx

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> Jinx, may I ask how you keep the SLA charged in this circuit ?

Yes. Yes you may

> A small typo ? (1k comes from unreg. ?)

The jacked-up regulator - diodes + resistor provides a float charge for
a battery that has been fully charged before installation. I found there
were conflicting opinions from "experts" about the requirements of a
float charge. That float charging is not desirable for long periods, or
that float charging is acceptable if less than 1A, or that the lower the
better. I opted for the last and monitored the battery for a couple of
years at the bi-annual daylight saving adjustments. So far they seem
healthy enough, and are capable of discharging well into a 1A load.
As far as I know only one has been called upon to do any actual work,
which it did, and that was for just a few hours last year

However, I'm always open to any suggestions for improvement

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2004\06\05@094024 by michael brown

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Peter wrote:
> Jinx, may I ask how you keep the SLA charged in this circuit ? A small
> typo ? (1k comes from unreg. ?):
>
> http://home.clear.net.nz/pages/joecolquitt/solenoid-clock.html

Did you see the 9V1 zener between the 5V regulator and ground?  ;-)

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2004\06\05@105153 by Lindy Mayfield

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You're all correct on this.  It is mind boggling all the things that have to be considered when making cool stuff. (-:

{Original Message removed}

2004\06\06@153414 by Peter L. Peres

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>Peter wrote:
>> Jinx, may I ask how you keep the SLA charged in this circuit ? A small
>> typo ? (1k comes from unreg. ?):
>>
>> http://home.clear.net.nz/pages/joecolquitt/solenoid-clock.html
>
> Did you see the 9V1 zener between the 5V regulator and ground?  ;-)

Nope ;-) But trickle charge through 1Kohm is a little low imho. If Jinx
says it works, then it works.

Peter

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