Powering an ATMega8 and a LCD char. display?

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I'm developing a camera controller using the ATMega8 as my platform. This is my first foray into the whole microcontroller thing and, now i think about it, my first real electronics project.

I've drawn up the schematic, coded the C and tested it all in Proteus. Of course, in the simulation it just has a +5V rail and a ground rail...

The device needs to be portable so this, to my mind, means either using a 9V battery - searching this forum and wikipedia would imply this is bad (low energy density) or 3xAA batteries.

The LCD is a bog standard, JHD162A based on the HD47780 controller. According to the datasheet, it needs a minimum of 4.5V to work (though i imagine slightly less would be ok). The IC is, as far as i can tell, rated up to 6V and requires between 2.2 and 5.5 when running between 0-10Mhz. I'm using an external 8MHz crystal so that's applicable. The only other output will be a couple of LEDs (the device controls the camera via an IR signal).

The battery life needs to last for a while, shooting could take hours so i guess 30 hours is a good lifetime on one charge. I think it's reasonable to assume that the power consumption of the entire device is never likely to exceed 75-100mA. Assuming an AA battery has 2500mAh then that should give a good 30 hours allowing for idle periods. I'm going to put in a switch to turn off the LCD backlight when not in use - say for time lapse shooting where the device is unattended and simply needs to send a short burst of IR light to the camera every so often.

As i'm not going to be running off anything higher than the maximum voltages, would i be able to wire up the AVR and the LCD in parallel directly to the battery pack or do i need to regulate it?

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I suggest using a switched mode power supply.
when the batteries run empty the voltage of a cell will decrease so when the batteries are half way through the only deliver (lets say) 1,2V(rechargable cell) then you end up having only 3,6V. I have a switched mode power supply on all my boards and run batteries down to really empty (1V cell voltage) making sure I got the money out of the batterie. You probably have to change batteries when they are over 60 or 70% emptied zo will never make the full expected usage time.

If you use 3 x aa cell and a SMPS you always have your defined 5V right until the batteries are empty. You can use the adc to measure the individual cells and then let the user know if the batteries are to be replaced or not.

regards

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http://www.analog.com/en/power-m...

Something like that would do the job then?

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yep,

note that it needs a large coil to do the job, but it can do it.

There are an almost enless number of switcher IC's in de the market today so you can make an endless list of devices you can use....

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http://www.rapidonline.com/Elect...

Thinking about using this one. Just trying to work out what inductor to use with their datasheet, i think 470µH for a 4.5-5V boost with 100mA.

The formula they give is:

L = Vin (Vo − Vin)/(∆IL fosc Vo)

Vin = 4.5V
Vo = 5V
DeltaIL = 2 • Io (minimum) = 2* say.. 30mA? = 60mA
fosc = 20kHz

so L = 4.5(1.5)/(60*20*5) = 375µH

But i guess i need to round up to the next standard value hence 470µH.

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Look for higher frequency SMPS controllers like the LT3471 which will allow for tiny inductors. The data sheet has examples for boosting from 3VDC to 5VDC, etc.
IF you use that part, note the ground pad is under the part and needs to be soldered.

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At the moment i'm not looking to go down the PCB route, it's a lot more effort (for me anyway) and i don't have the means to get a board fabbed cheaply in a decent time frame. i.e. i'm just going to put it on stripboard for the moment.

Are there any equivalents that come in a DIL package?

Thanks!

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Well you have some problems. High Freq. SMPS data sheets tell you that layout is important. You do not want to use a PCB. Perhaps you may have to opt for a controller that has say 5 legs out of a package that looks like a TO-220 style. Then you would have to rig up the inductor and resistors on your strip board.

Otherwise you are looking at a Linear LDO regulator and in that instance you would want the input voltage just a little higher than the output to keep heat down and battery current drain down.

If you do want to stay with switcher then look at National Semi web site as they offer a lot of package choices for their parts. Also Linear Tech offers some that are packaged in those 5 pin or less style packages.

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Is there any reason why i can't use the lower frequency (in tens of kHz) models?

I'm guessing that the best option, if i was to go with an LDO would be simply 4xAA batteries to keep, as you say, slightly above the output voltage?

I'm happy to rig up the inductor/resistors but by the looks of it i'd have to do it anyway with the surface mount option? And it also looks like i'd need a lot more components with the LT3471

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Quote:

As i'm not going to be running off anything higher than the maximum voltages, would i be able to wire up the AVR and the LCD in parallel directly to the battery pack or do i need to regulate it?

I'd say go direct. For a 1-off, any LCD that works is fine; for a design pick a model that likes 3V-5V.

Just like your cell phone or digital camera, fairly quickly turn off the backlight and other power suckers when not active. I'd reverse the logic and have a button/command to force the backlight >>on<<.

Are you an IR transmitter or receiver? How often do you need to send a command, and how long is it on? Would this be similar to a TV IR remote control? If so, it (the AVR) could be sleeping 90+% of the time drawing minimal current.

If you need to keep the display on, there isn't much info for that model but an equivalent model indicates about 2mA.

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

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Whiternoise wrote:
Is there any reason why i can't use the lower frequency (in tens of kHz) models?

I'm guessing that the best option, if i was to go with an LDO would be simply 4xAA batteries to keep, as you say, slightly above the output voltage?

I'm happy to rig up the inductor/resistors but by the looks of it i'd have to do it anyway with the surface mount option? And it also looks like i'd need a lot more components with the LT3471

You can use the lower Freq. parts. You will need a larger inductor value is all.

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Whiternoise wrote:
Is there any reason why i can't use the lower frequency (in tens of kHz) models?

I'm guessing that the best option, if i was to go with an LDO would be simply 4xAA batteries to keep, as you say, slightly above the output voltage?

I'm happy to rig up the inductor/resistors but by the looks of it i'd have to do it anyway with the surface mount option? And it also looks like i'd need a lot more components with the LT3471

Efficiency goes up with switching frequency, and part size (especially the inductor) goes down. Generally quiescent current goes up too, but the above two usually compensate for that.

The simplest option is AA/AAA cells with an LDO. I would suggest a low quiescent LDO, such as the MCP1700/1702. Gate everything on your board (such as the LCD) with a P-channel FET, so it can be hard-powered off with a simple logic signal from the controller. Also utilize the sleep modes of the processor whenever possible.

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It's an IR transmitter on for something like 100ms at a time. It can be turned on when the remote is in manual mode, at periodic intervals or on an ADC condition.

The MCP1700 looks fine, though there are loads to pick from it seems.

It needs to be on pretty much all the time. For things like triggering, the µC takes an ADC input and needs to compare it constantly to a user defined condition. So yeah, sadly sleeping isn't much of an option. Obviously i'm doing what i can in software, like making sure the ADC is turned off when the controller isn't in triggered mode.

Thanks for all your help guys!

I think i'll get one of these:

http://www.rapidonline.com/Elect...

which looks like it'll do the job.

I'm afraid i'm not entirely sure what "gating" is? And surely i could just put a switch in front of the battery instead?

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"gating" just means an "electrical switch". That is - power to parts of the circuit can be turned on/off by triggering the "gate" voltage on a FET that switches their power rail.

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Ah yeah, seems obvious in hindsight, basic transistor switch then?

But why would that be preferable over a manual switch in this instance?

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For a similar project I used a Sipex SP6641A which comes in a SOT23-5 and only uses 10uA quiescent current. I used a ATtiny84 with a tiny 2x16 LCD module, some buttons and an IR led to do timelapse for my Nikon DSLR.

It runs from one AA cell and I used sleepmodes extensively, also turning of the LCD. The LCD does not (yet) have a backlight though.

You don't need a real PCB to use this little converter, but you need to be very good at soldering small components ;) (The total converter is approx just as big as the body of a TO220)

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It's tempting to get a solution like that, but the trouble is - for what i'd like mine to do - it'd be hard to sleep it (displaying a countdown timer during the timelapse for instance). I suppose the obvious solution is to see how long it lasts on some AA's with a long timelapse and then work it from there.

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Mine does show countdown, but only for a minute or two then shutsdowns the LCD. The idea being that you are not going to stare at a little LCD day after day ;) A press of the any button is enough to briefly turn on the display again.

Though a LCD does not consume that much power, around 0.5 to 2mA. I don't know how many digits you need, but you could consider using a simple (direct drive) 7 segment display and run the AVR nice and slow. I have a bicycle computer here that has been running on a single CR2032 for over 15 years ;)

You can reduce power consumption of the MCU by trying to sleep as much as possible, turning off all unnecessary periphals; some AVRs have a PPR register allowing you to selectively turn off stuff.

For example, my widget's MCU goes to powerdown after a few seconds when it's not running and not being setup. No time wasting power waiting for a button press, I use pinchange for that. I use the watchdog timer in this particular mode that wakes up the MCU only to turn off the LCD and MCU again. In other modes, I put the MCU to idle mode between 50Hz timer ticks and it runs at only 250KHz.

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Whiternoise wrote:
It's tempting to get a solution like that, but the trouble is - for what i'd like mine to do - it'd be hard to sleep it (displaying a countdown timer during the timelapse for instance). I suppose the obvious solution is to see how long it lasts on some AA's with a long timelapse and then work it from there.

You absolutely want to put the micro to sleep. Use the timer interrupts to nap the processor whenever you can. Even sleeping for 100ms can lead to dramatic power savings.

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Right, i'll shoehorn it in somewhere :P

I was thinking of doing:

-user starts the interval timer
-timer starts ticking to keep track of time generally
-interrupts enabled
-every second an interrupt is called (the interval is calculated in seconds as per the user input) simply to update the shots fired, time remaining. If the time remaining is zero, then shoot and reset the clock
-in tandem to that, run an interrupt to start a loop if any button is pressed
-the loop will run for 5 or 10 seconds updating the lcd screen with the countdown timer (whatever it happens to be at). It's also run initially when the interval starts.
-after the loop finishes, the lcd is powered down and the cpu is put into sleep mode (allowing for external interrupts and

The only issue i can see with using the ATMega8 is that it doesn't have a register to detect when any pin has been pushed, so i'd have to wire all of them (in parallel) to INT0 or INT1. Though perhaps this would work anyway, it'd be good to have one button to cancel the time lapse in progress and all the other buttons to just display progress.

Similarly with manual mode, it can just sleep until the shoot button is pressed (i.e. take a shot). With the ADC function, i guess it'd be roughly the same as the interval timer with regards to power saving.

So. In theory the CPU would only need to run for bursts of 10 seconds or a few milliseconds while variables are updated and so on.

Thoughts?

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Why not change to an mega88 which does have pinchange interrupt?

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I've already got an ATMega168 here so i figured i'd use that instead :)

Hah, and yet another question now.

I've started writing up the interrupt scripts but i'm a bit confused as to what to use as what timers.

Initially i thought i'd just use a 4MHz external crystal in combination with the Accurate Delay library to get good delays for the IR pulses. I'd like to use a 1Hz interrupt (the longest i can feasibly leave it for power saving) but that seems to be more difficult than i planned.

If i use Timer1 with a compare value, then i could use this: ?

// Run when timer 1 overflows (got 2^16 to play with so... every 40000 should = 10ms)
ISR(TIMER1_COMPA_vect)
{
//10's of milliseconds
millisecond_counter++;

   if(millsecond_counter == 100)
   {
   millisecond_counter = 0;
   //time remaining (seconds) decrement by one
   timeremain--;
      if(timeremain == 0)
      {
         shoot();
         shotcount++;
         timeremain = interval;
      }
   }
}

Argh! Too many timers to choose from!

And then to check for input, i'm going to wire up one switch to a different port (the registers are 0-7, 8-14, 16-23) so that a pin change of any of the general buttons just shows a 10 second status display on the LCD and a pin change on the trigger button stops the timelapse.

So i'm thinking the code for that would just be

Initialise variables, calculate interval, enable interrupts, start the timer

while(continue == 1)
{
  Sleep the CPU

  //Interrupts run in the background to control shooting

  IF trigger pressed (pin change interrupt)
      continue = 0
  IF any other button pressed (pin change interrupt)
      display_status();
}

----
//this runs once when shooting starts and then when buttons are pressed

display_status()
{
turn LCD on (transistor controlled perhaps?)

   clear screen, home cursor
   FOR 0 - 10
      display shot count, seconds remaining
      delay 1s

turn LCD off (again, transistor controlled)
}

Sorry for the ninja edit... :P

Last Edited: Tue. Sep 15, 2009 - 01:00 AM
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I use two timers to control the IR LED. One generates the 38KHz carrier using CTC mode and toggle output mode; while the other, 16 bit timer turns on and off the other timer using compare match interrupts to schedule the next event in the pulse stream. Takes little code and very little CPU time.

I use the same 16 bit timer as general housekeeping timer, again using a second compare match interrupt, incrementing the compare value on each interrupt with a certain value.

What kind of IR pulses do you need to send? Mine sends in the Nikon format.

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It's the Nikon pulse, my code as it stands is:

void shoot(void)
{
//We need to blink the LED on for 13us and off for 13 us to satisfy the 38.4kHz modulation required by the ML3
//Remeber to call this TWICE wherever a picture is needed!
	//77*2*13 = 2000 = first pulse
	for(count=0; count <=77; count++)
	{
	OUTPUT_PORT ^= (1 << IROUT_BIT); //turn on
	Delay_us(13);
	OUTPUT_PORT ^= (1 << IROUT_BIT); //turn off
	Delay_us(13);
	}
	
	Delay_us(27830); //wait pulse
	
	//15*2*13 = 390 = second on pulse
	for(count=0; count <=15; count++)
	{
	OUTPUT_PORT ^= (1 << IROUT_BIT); //turn on
	Delay_us(13);
	OUTPUT_PORT ^= (1 << IROUT_BIT); //turn off
	Delay_us(13);
	}
	
	Delay_us(1580); //wait pulse
	
	//15*2*16 = 403 = third on pulse (need 410 so we put another on cycle in at the end and add the off time to the next delay)
	for(count=0; count <=15; count++)
	{
	OUTPUT_PORT ^= (1 << IROUT_BIT); //turn on
	Delay_us(13);
	OUTPUT_PORT ^= (1 << IROUT_BIT); //turn off
	Delay_us(13);
	}
	
	OUTPUT_PORT ^= (1 << IROUT_BIT); //turn on
	Delay_us(13);
	OUTPUT_PORT ^= (1 << IROUT_BIT); //turn off
	
	//wait pulse (plus last 7us from previous wait)
	Delay_us(3587); 
	
	//15*2*16 = 403 = third on pulse (should be 400, but three microseconds won't matter much!)
	for(count=0; count <=15; count++)
	{
	OUTPUT_PORT ^= (1 << IROUT_BIT); //turn on
	Delay_us(13);
	OUTPUT_PORT ^= (1 << IROUT_BIT); //turn off
	Delay_us(13);
	}
}

It uses the accurate delay library provided on this website which runs based on clock cycles (given f_cpu) so in theory works.

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You must send the code twice BTW ;)

This is part of my solution:

static volatile unsigned char ir_state,ir_count;

void release_shutter(void)
{
 if (ir_state!=0) return;
  
 PRR&=~(1<<PRTIM0); // Turn on Timer 0
 TIFR1|=(1<<OCF1A); // Clear int flag
 
 ir_state=0; 
 ir_count=0;

 OCR1A=TCNT1+100;
 TIMSK1|=(1<<OCIE1A);
}

//
// Macro magic
// Convert from microseconds to timer 1 counts
//
#define TIMER1_PRESCALER    1
#define F_TIMER1            (F_CPU/TIMER1_PRESCALER)
#define TO_TICKS(x)         ((unsigned int)(F_TIMER1*((unsigned long)(x)/1000000.0)))

//
// Compare match A ISR
//
// Times the IR pulse train
//
#pragma interrupt_handler timer1_compa_isr:iv_TIMER1_COMPA
void timer1_compa_isr(void)
{
 // Table of periods, alternating between on and off times
 // Specified in microseconds but with 250KHz the resolution is 4us which is good enough
 // Timings as found on the internet vary from source to source.
 static unsigned int delays[]={TO_TICKS(2000),TO_TICKS(27830),
                               TO_TICKS(390) ,TO_TICKS(1580),
                               TO_TICKS(410),TO_TICKS(3580),
                               TO_TICKS(400),TO_TICKS(63000)}; 
 // Proceed to next state
 OCR1A+=delays[ir_state];
 
 if (ir_state & 1)
  {
  LED_OFF
  }
 else
  {
  LED_ON
  }

 ir_state+=1;
 
 // End of impulse train?
 // Must be repeated twice
 if (ir_state==8)
  {
  if (ir_count==0) 
   {
   ir_state=0;
   ir_count=1;
   }
  else
   {
   ir_state=0;
   TIMSK1&=~(1<<OCIE1A); // We're done, disable ourselves
   PRR|=(1<<PRTIM0);    // Power down timer 0
   }
  } 
}

Some macros:

#define F_CPU               250000UL
#define LED_ON              TCCR0A|=(1<<COM0A0);
#define LED_OFF             TCCR0A&=~(1<<COM0A0);

Setup code for timer 0

DDRB|=(1<<2);  // IR LED output
 PORTB=0;
 
 // Set system clock to 8MHz/32=250KHz
 CLKPR=(1<<CLKPCE);
 CLKPR=(1<<CLKPS2)|(1<<CLKPS0); 
 
 // Setup timer 0 to 38.4KHz square wave output
 TCCR0A=(1<<WGM01);     // CTC mode
 TCCR0B=(1<<CS00);      // Prescaler is 1
 OCR0A=(F_CPU/2)/38000; // ~38KHz 
 //OCR0A=0; // For use with 77.5KHz crystal
  
 // Setup timer 1
 TCCR1A=0;
 TCCR1B=(1<<CS10); // Timer 1 prescaler is 1
 
 OCR1A=0;   // Compare A unit is used to time IR stream
 OCR1B=0;   // B unit is used to get 50Hz master tick interrupt  
 TIMSK1|=(1<<OCIE1B);   // Enable compare match interrupt B, A is enabled when IR stream is needed
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Twice with a 63ms delay i believe :)

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Yes, that's the last entry in the periods table in my code, 63000 us ;)

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My point-and-click Olympus digital camera uses 450 milliamps at +3.75V when running from a wall-wart power convertor. It has two AA batteries also. I use a pair of rechargable batteries, but they only last for a few images before the camera says that they're out of power. They are supposed to be rated at 2500 mA/hrs so perhaps they are experiencing 'memory effect'.

In any event, switching mode power supplies can be quite complex. If you want easy, then I suggest a wall-wart using standard 7805 voltage regulator with a big (4 square inches) metal plate bolted onto it to get rid of the heat. The wall-wart should be at least 8V/800 milliAmp.

Sometimes easier is better if the focus (bad pun) is more on the photos than the electronic design of the camera's power supply.

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I think you've misunderstood, i'm not changing the camera's power supply, i'm building a standalone remote!

For what it's worth, i've got a Nikon D90 and the battery life is ridiculously good so i doubt i'd want to change it :)

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Back to the power supply question, for more runtime and efficiency I'd plan on four or more AAs in series and a cheap buck converter to regulate the output voltage. Then you could also run it with a power tool battery or car battery.

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I have a D80 and battery life is at least 600 shots.

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Provided i don't use the flash it's pretty good.

My "test to destruction" was the manual time lapse in Poland. Filled 15GB of photos - by the time the battery went flat we had around 1000. All 8 second exposures out in the cold. Awesome stuff :)

A thousand curses on Digikey! What's the use in having every imaginable component in your catalogue, only to say "oh sorry, you can only but these in 2500 lot orders". I really like the look of some of the Maxim converters but they're impossible to get hold of it seems - i thought i might save a lot of hassle and get some inspiration from the Mintyboost circuit.

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I get 600 shots with regular flash use and regular chimping.

You can buy from Maxim directly ;) Linear Technology also has a webshop, as do some others. I tend to prefer LT chips though.

You could also look at the SimpleSwitcher series from National Semiconductor. There are thousands of switchers you can choose from, there is at least one fitting part for almost every imaginable situation :)

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Ahh genius!

Yeah there are thousands indeed, but the shop i use in the UK - RapidOnline - sells a good deal of other stuff that i use (at reasonable prices) so i'd like to get it in one order to reduce shipping costs.

Trouble is they don't have a vast catalogue like Digikey and stock the majority of common components and they stock things that will do the job, but they don't always have specific things.

Methinks i'll order a few of these: http://www.maxim-ic.com/quick_vi...

MAX856 500kHz switching, 5V or 3.3V preset, 100mA max, 25uA quiescent. And best of all is easy to set up!

EDIT: Ok, they quoted me 5 units at 1.77 each which is pretty good. Only downside is that they're non-RoHS so i'd have to switch to a surface mount option for that. Eh, maybe in the future!

Last Edited: Wed. Sep 16, 2009 - 01:03 AM
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Oh, you are from the UK ;) Then ordering from Maxim or LT directly is quite prohibitive due to the insane shipping costs.

IC looks nice :) Comparable to the one I used. Can you get it locally?

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No, sadly - though i'm a bit wary of it because of the lead-free bit. Especially when they said they might ask me to buy 2500 if i order because of some new legislation. i.e. That's out of the window!

Ordering from LT, i found this one: http://www.linear.com/pc/product...

Basically identical specs and it's RoHS. Only catch is that they want $30 on shipping. So i'd pay ten bucks for a few chips and then 3 times that to ship it which seems laughable given that these things would probably weigh the same as a postcard...

Where do you get your stuff from? If you're in the Netherlands is there an EU dealer you could recommend?

EDIT: Ok, i could get one from Farnell here for £4.63 per unit. Ouch, but doable.

Last Edited: Wed. Sep 16, 2009 - 01:33 AM
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Farnell and RS Components.

RoHS is not really important, unless you are designing a commercial product. Almost all components now are RoHS compliant anyway, apart from some old stock.

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Quote:

Farnell

For buying components in the UK I'd highly recommend Farnell as there is free delivery which is great on small orders for just a handful of bits (Digikey charge an arm and a leg so you need to make a big order to amortise it)

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Ah thanks or the heads up, i didn't notice that!

I ended up buying a couple of these: http://uk.farnell.com/maxim-inte...

They're reasonably cheap (if i did ever decide to kit, they're RoHS and £2 for 10+) and though they only give out 50mA, i doubt i'll even need that.

Efficiency wise it's around 80% with 3V input which is not so bad.

Limited to 2 AA's, or a max input of 3.6V, minimum of 2V. But it covers all the other bases and only needs four caps to operate.

I've decided that the easiest way to power the LED is with a transistor switch anyway, so i don't need to pull much current through the regulator, i'll just wire LED straight to the battery with the necessary resistor.

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the input current vs output current graph suggests that with 1,*input voltage you can only draw 20mA... going to 64mA at 2,7V input voltage.

Also the chip is not short circuit protected, so perhaps make sure you monitor the output voltgae to shut down all when it drops below a certain level to keep your regulator from playing vulcano....

for the remainder it looks like a nice chip for low power apps.
In the past you could get free samples from maxim, I do not know if this is still available.

regards

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You can, but quite a lot of the samples (at least the chips i was interested in) are only surface mount models.

http://www.zen22142.zen.co.uk/Ci...

Seems like a good choice and pretty simple to implement, but would i need to put it before the power supply or after it - i would assume between the batteries and the PSU input?