Drive an LED direct from a PIC IO pin (ie no current limiting resistor)

[mister_e]

You get the idea.

Look Melanie Dice example, or do it simple, plug your LED directly to the PIC and have a PWM running out of your PIC, use a MOSFET to drive the common GND or VDD of your LED (whatever floats your boat).

You now have a variable intensity, using a minimum of parts... BUT, the more the battery get flatten, the less intensity you'll also have (assuming you don't use anything to monitor it... and if so... you end up with more extra hardware than a simple LDO regulator)

[HankMcSpank]

You get the idea.

Look Melanie Dice example, or do it simple, plug your LED directly to the PIC and have a PWM running out of your PIC, use a MOSFET to drive the common GND or VDD of your LED (whatever floats your boat).

You now have a variable intensity, using a minimum of parts... BUT, the more the battery get flatten, the less intensity you'll also have (assuming you don't use anything to monitor it... and if so... you end up with more extra hardware than a simple LDO regulator)

This is exactly the setup I'm proposing....use a transistor to control the 'effective' voltage the LEDSs 'see'. But for monitoring, I'll use a spare ADC pin on the PIc to monitor the unregulated supply voltage....and then adjust the PWM to the LEDs accordingly.

The problem with using an LDO regulator, is they're very inefficient.

[mister_e]

I would suggest to use a Mosfet with a decent small Rds, this way you could/should loose less voltage across than a regular BJT.

Add a small value resistor between the MOSFET source and GND, put a cap on it, feed it to the ADC and voila you monitor the current.

Tons of way to do it.

[HankMcSpank]

I would suggest to use a Mosfet with a decent small Rds, this way you could/should loose less voltage across than a regular BJT.

Add a small value resistor between the MOSFET source and GND, put a cap on it, feed it to the ADC and voila you monitor the current.

Tons of way to do it.

All good stuff there....have you any example of suitable mosfets you've used in the past? (just to give me a general vibe of the spec - I'm ideally looking for something in a SOT23 package)

Edit: Hmm: I'm thinking here that I'm only going to be switching 3.3V across the leds - do mosfets work ok that low (all the ones I'm seeing on digikey are typically 20V VDS or more ...ok so that's a max figure, but never having used modfets, are they ok with switching 3.3V well)

[mister_e]

Yish, me and remembering Mosfet Part # are two different animal

I would say go on Digikey website, type Mosfet SOT23, then shop by max current and Rds.

How much current you'll need?

[mister_e]

And don't forget to check the datasheet... Vgs...

[HankMcSpank]

Yish, me and remembering Mosfet Part # are two different animal

I would say go on Digikey website, type Mosfet SOT23, then shop by max current and Rds.

How much current you'll need?

Not much....8 possibly 12 leds....ay about 15mA each max ....say 180mA, I'll have a sniff on Digikey.....


Edit: Hmm: I'm thinking here that I'm only going to be switching 3.3V across the leds - do mosfets work ok that low? (all the ones I'm seeing on digikey are typically 20V VDS or more ...ok so that's a max figure, but never having worked with MOSFETS previously, are they ok with switching 3.3V well)

[mister_e]

Double check those

BSS138, 2n7002K, FDV305

[mister_e]

Edit: Hmm: I'm thinking here that I'm only going to be switching 3.3V across the leds - do mosfets work ok that low? (all the ones I'm seeing on digikey are typically 20V VDS or more ...ok so that's a max figure, but never having worked with MOSFETS previously, are they ok with switching 3.3V well)

You want to look for Logic Gate one and/or with a low Vgs... at least under the battery voltage