PIC Charge pumping?

[Mosaic]

Hi, I want to drive a 3.2VDC LED but my supply is 3V max.

I am using a 16F886. Would the following be possible?

Place a 10Uf capacitor in series with the 3.2VDC LED and a resistor valued to drop 2.8V @ 10mA. The capacitor is a ceramic non polarized. Connect to the PIC using 3 pins..1 pin (A) to capacitor (pin1) , 1 Pin (B)to Capacitor (pin2)/LED anode, 1 pin (C) to LED cathode via the dropping resistor.

Before flashing the LED, PIN A outputs Gnd, PIN B outputs 3V thus charging the cap to 3V.
Pin C is hi Z.

Then Pin B goes Hi-Z & Pin A goes 3V and then pin C goes Gnd thus delivering 6VDC across the LED & resistor to create a flash.

 

[mdorian]

You can use this circuit if you need only one led. Use timer interrupt and toggle the pin to power the led. For the circuit you describe the voltages on the pins will exceed the maximum allowed , see the data sheet.

 

[Mr RB]

I would use a small inductor and drive it in boost mode with the PIC output pin. As the LED clamps the boost voltage at 3.2v it won't turn on the PIC pin diode which turns on about 3.6v with a 3v Vdd.

If this is for battery use where the Vdd might be 3.4v or so you should use a small NPN or NFET to drive the boost inductor and a 1N4148 diode (0.7v) between the inductor and LED.

 

[be80be]

Here a workable design

 

[Mosaic]

I was hoping for a solution that would allow me to drive perhaps up to 8 or 16 LEDs using as many pins. or possibly multiplexing. Re post #2, how would the voltage that the pic pin sees exceed the rated spec?

 

[mdorian]

Then Pin B goes Hi-Z & Pin A goes 3V and then pin C goes Gnd thus delivering 6VDC across the LED & resistor

Pin B will have 6 V on it higher than Vdd+0.6.
Multiplexed? Why not?

 

[Mosaic]

ok , so if I use a diode inline with Pin B then Pin B could just go to gnd when Pin A goes to 3V and then the PinA/Cap combo will deliver abt 5.4V to the Load.?

 

[Misterbenn]

Why are you even bothering with this? just get better LEDs if your power supply is limited OR
use a higer voltage psu if the power supply isnt limited

 

[mdorian]

Yes use-it like any led matrix , column1 > Vdd , light the selected LEDs by lines to GND, a small delay, column 1 >GND , column 2 > Vdd and so on . I didn't put the limiting resistors due the lack of space on the sheet, put four resistors , one on each row . You can use polarised capacitors because they are smaller.
Misterbenn , the voltage is only related to the color of a LED. A white or a blue LED manufactured in Somalia will have the same voltage around 3.6V as one made in UK. To change Vdd he needs to change the microcontroller, however , if he have an higher voltage available, no more than 6V he can use-it for LEDs only, of course without multiplexing.