LED driver from 3V supply

Check this out, you can use a serial link to control 8 lines of

800mA+ currents with built in protection etc..

Jamie

I think there was a big thread about this very problem a couple years back. It ended up with a big blowout knockdown argument between the usual suspects (as many threads seem to end up), but before that happened there were some useful circuits posted.

Basically, use a square wave output from the uC to make a charge pump, or something like a CD40106 hex schmitt trigger oscillator to do the same.

**So non rechargeable, like the Energiser L91 type ?

Cell voltage is about 1.3V at the end point, so I hope you are not expecting to drive blue or white LEDs with just two of them.

** That is still the simplest way.

** Not everything you dream of exists.

... Phil

Three small mosfets and three, or maybe just one, resistor should work. No base resistors needed.

What do you expect the voltage drop of the LEDs will be? Are the LEDs identical?

20mA is quite a lot. Do you really need to drive them that hard? Usually a few mA is plenty for an indicator that does not require warnings about eye safety.

It might make more sense to spend a few cents more on the LEDs and drive them directly. You might only need the LEDs and one resistor.

If you realy do need 20mA- you don't really have enough voltage to drive blue/white LEDs from the cells directly. If you use 5V you can just throw a 10 cent ULN2003A in there (plus LEDs and resistor(s)) and you're done. It draws just leakage when the outputs are off. Of course the 20mA will cost you more like 35mA off the 3V.

--sp

--
Best regards,  
Spehro Pefhany 
Amazon link for AoE 3rd Edition:            http://tinyurl.com/ntrpwu8 
Microchip link for 2015 Masters in Phoenix: http://tinyurl.com/l7g2k48

[battery voltage 3V or so]

There's some switchmode options, like MIC2287, that can drive/regulate current, basically small switchmode current sources.

It's easier, though, to do it with transistor switches and resistors; driver ICs are overkill unless there's some oddity (like, you want regulated

20 mA through a UV LED with 3V forward drop, from a 3V source).

Lateral question:

Can you choose a different CPU with a better GPIO spec? (eg Atmel have AVRs (tinys and megas) in the range of 40mA per GPIO pin with some additional cumulative limits)

Or a specialist LED driver chip:

if you don't mind clocking the data in serially. Advantage: no LED series resistors needed either.

Failing that, I'd probably go rummaging around the 4000-series CMOS logic chips to find a hex (inverting) buffer with a suitably high output current.

Use LETs instead of LEDs and drive the base through a resistor.

Cheers

--
Syd

TWENTY MILLIAMPS??? Are you nuts? Or are you setting up an airplane landing strip? A few hundred microamps will do for indicator purposes.

Well, the OP might have a large stock of old LEDs and some of those were barely visible even with a 20mA drive!

Yes.

No blue or white.

The Microcontroller operates from 2.0-3.6V. Someone first wanted to use alkaline D cells but I convinced him that this was not a good idea due to the more linear nature of the alkalines.

That's what I thought.

Too bad.

Unfortunately they are not identical. I'll need three separate series resistors.

You're right about the base resistors, I just automatically thought I needed them but for slow speeds there's no need.

Quad FET arrays seem to be rare. For the pilot run we are hand assembling and want to minimize component count even though the 3904 arrays are much more expensive than individual transistors.

How about a hex inverter logic chip? You could parallel sections to get a bit lower voltage drop.

One 7438 (quad nand) logic gate should work, too.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

On Monday, July 6, 2015 at 4:19:28 PM UTC-7, sms wrote: [about switching and biasing LEDs]

How about biased transistors? It's energy-efficient to limit the base current