LED brightness/PIC10F etc Button cell 3v/CR2032/2016

Hi I have a job to provide a LED signal when event occurs. It needs plenty of LED brightness which means we have to use Red and Green LED only. Also, the main problem is no LED are happy with 3v driven by PIC micro - they are below the output curve - 4.5v is much better.

I dont believe that there is any need of series limiting resistor at 3v button cell supply; am confident that the PIC is more than capable of handling the few ma when switched on, despite all app note showing series resistors. When the PIC is set as output, it has series resistors in the drive output in any case.

Interested in any others comments on this typical scenario, bearing in mind minimum parts count for maximum LED output, with constraint being it must be button cell and preferably only one battery (more than one leads to reliability problems in battery connectors, and discharge linearity.
Cheers,
Alistair George.

The average forward drop of a Red/Gren LED is 1.2 Volts.
A Pic can source/sink ~20mA from an I/O pin.
So, if you plan to drive a standard led from a 3V supply at ~20mA, your series resistor would be:

(3 - 1.2) / 0.02 = 90.

So a 91R or 100R would be the value to use.

Hi Alistair,

AlGeorge said:

the main problem is no LED are happy with 3v driven by PIC micro - they are below the output curve - 4.5v is much better.

Click to expand...

Look into charge pump voltage doubler (couple of caps, couple of diodes) or inductive step-up (aka boost, it may be possible to get away with one MOSFET and one inductor).

- Nick

btbass said:

So a 91R or 100R would be the value to use.

Click to expand...

Apologies I should have figured nobody would take my OP as correct. Series resistors are NOT required because CR2032 Maximum pulse discharge current: 15 mA. So effectively, you have a series resistor in the battery.

As an end note; unless someone suggests better:
Tried charge-pump/voltage doubler. It works but the extra parts 2 electros 2 diodes, and the physical size of electros prevent its use on a low cost SMD board.
The boost circuit similar.
The best option so far is to select the lowest voltage, highest output LED. Also to connect it to VDD rather than VSS seems to help even though PIC specs say the drain and source figures are the same.

Anyone can contribute to this thread recommending 3v high output led - I understand white tends to give out most light. Have found red to be well down on green. Most interested in suggestions of supplier for right angle high output low voltage led. Thanks.