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From: Peter Bennett on 19 Feb 2010 00:56
On Thu, 18 Feb 2010 21:03:12 -0800 (PST), CJ
<ferrari.secret.santa(a)gmail.com> wrote:

>On Feb 18, 10:33�pm, "Phil Allison" <phi...(a)tpg.com.au> wrote:
>> "CJ"
>>
>> >The microcontroller power supply is independent from the LED power
>> > supply. �Previously I was using a 5V microcontroller and a supply
>> >voltage of 4.2V for the blue LED's. �This worked OK.
>> >I changed to a different microcontroller that only runs on 3.3V, and
>> >of course I can't turn off the PNP transistor because I can only put
>> >3.3V to the base through the microcontroller.
>>
>> ** Never thought of placing a diode between the uC and the base of each PNP
>> ??
>>
>> > I thought I can change to P-channel MOSFETS with a logic drive. �I
>> > bought some Diodes Inc DMG6968U (P-channel enhancement mode mosfet).
>>
>> ** That number is for an N channel fet.
>>
>> .... �Phil
>
>On the diode, I thought about it, but the transistor is a PNP driving
>the high side, and it's 4.2V in vs 3.3V on the uC, so in order to turn
>it off I would need to pull it high with a resistor to the high side
>supply... but then when I try to drive it low with the uC, the diode
>is going to prevent that, isn't it?

Install the two diodes in series with cathodes toward the uC - this
will allow the PNP base to go up to ~ 4.7 volts when the uC output is
high. A pull-up resistor on the base will ensure that the PNP turns
off. You should probably also have a resistor in series with the
diodes.



--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
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