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Topic: Powering Arduino Fio + amplifier + LED display from a single 3.7v LiPo (Read 2604 times) previous topic - next topic



I'm in the process of building a talking temperature guage for my vision impaired nephew.  Compoents being used are:

The circuit works well but only when the amplifier and the LED part of the LED display module are powered from a separate source - the 3.3V rail on the Fio is not suitable for what I suspect are voltage related reasons (the amplifier doesn't generate enough volume and the LED display cannot ligth up properly as some segments use two LEDs per leg and thus need around 5V to drive the LEDs properly).

I was able to successfully get things working (so I could concentrate on my code) by powering both the LED part of the display module and the amplifier circuit from an (unregulated) separate 7.4V lipo battery from an old camera, with a common ground.  The audio levels are perfect now and the LED is nice and bright.

So now that I'm looking to package it all up in a nice enclosure, I don't really want to rely on a second battery - so I was considering one of these (or appropriate substitute) to boost the Fio's regulated 3V3 line up to something acceptable for the LEDs and the amplifier:

  • http://www.pololu.com/catalog/product/799

  • http://www.pololu.com/catalog/product/791

The current draw from the LED display is considerably higher than that of the amplifier (I was quite surprised to see) - the audio amp draws 5.1mA at idle and jumps up to as much as 40mA during audio announcements.  The LED display, however, goes from 0mA at idle to a whopping 300mA during static number display - scrolling text load is something I haven't tested yet.  On that basis, feeding in 3.3V and selecting 9V output on either of the booster modules above should be able to provide enough current, assuming the 3V3 line on the arduino can supply enough current...

I have a feeling that this combined (boosted) load will overload the Fio's regulated 3V3 line if I attach a booster...  but I can't find any specs on the max current draw for that line and I don't know enough about reading schematics to work it out for myself - and I don't want to hook up the 3.7V LiPo battery to this booster circuit in parallel with the Fio because that may effect the charging scenario, plus I'd like the Fio to be able to turn on and off the amplifier when I'm in sleep mode (interrupt driven wakeup on button)...

I welcome and appreciate any suggestions from the more experienced out there than me - thanks in advance!




More thinking on this overnight...

According to the LED display's datasheet, "Also be aware that lots of LEDs happily eat lots of current".  Yay!

So while I measured the display at ~300mA, that was only with two digits displayed at a time, not scrolling text... even more good news (/sarcasm).

I used a calculator to figure out how to reduce the current drawn by the LED display from the second power source - and decided upon a 22? 1/4W E12 resistor (RRBG).  With this in line between the +V line of the 7.4V LiPo (now sitting at around 5.6V as it hasn't been charged in a while), the total current draw was reduced to 200mA, which is an improvement.

I'm going to modify my code to use scrolling text instead of static numbers and see if that increases or decreases the total current draw without the resistor in line and then see what I can do to reduce it further without impacting on the display integrity.


Ok so the LED display with scrolling text draws 50% more current than a static number display.  It peaks at 550mA in this mode.  The 22? resistor in line reduces this to peaks of 300mA but none of the 2 LED segment (tall legs in the digits) light up.  I also misread the voltage on the 7.4V LiPo - it is happilly showing 7.86V across the input of the circuit.  So my next trick is to see if I can configure the TLC5926 with a lower brightness setting... time to talk to the vendor :-)


I found in the LED display's datasheet that I could use PWM on the BL line to reduce the brightness of the display, I had been setting BL line to LOW...    I used analogWrite to set the PWM value to 200 and that made a huge improvement! 

The display is an acceptable level of brightness now while only drawing a maximum of 150mA from the 7.4V LiPo - which means I have a fighting chance of adding a booster circuit to the Arduino Fio's 3V3 line to bring up the JP2 input voltage to 5V while not drawing too much current so as to blow the Fio's 3V3 line...  unfortunately the Fio doesn't document what the max draw on that 3V3 pin is...

The only problem I have now is that the PWM signal on the BL line is now causing the to buzz while the LED display scrolls text - now to find out how to stop that :-)

Any hints appreciated!


So a bit of digging in the Fio reference circuit schematic suggests it uses a SOT23-5 as a voltage regulator, which suggests max combined current load of 800mA.  Given that the Fio has a max 40mA DC Current rating per each of the 22 I/O pins; that doesn't leave much for anything else...  just as well I'm only using six of them as outputs... so at a guess, I should have enough spare to feed 200-300mA into a booster circuit to drive the LED display.  I'll order one today - time to "suck it and see"...


For a battery-operated device, an LCD display is probably a better choice than an LED display, unless you need the extra brightness of an LED display.
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For a battery-operated device, an LCD display is probably a better choice than an LED display, unless you need the extra brightness of an LED display.

Yes indeed, under normal circumstances that's what I'd be using.  Unfortunately my nephew is limited in what he can see and read clearly, so it has to be a large LED display...

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