Brian Clewer writes:
> Can anyone help me with a vaguely PIC related problem...?
>
> I have a project in which I need to control the brightness of a 12V lamp
> from a PIC.  The lamp has to go from maximum brightness to the off state.  I
> am planning on implementing this using varying mark-space ratios so to
> minimise power loss.
>
> Can anyone advise me on a cct. for doing this?  I am close by using a E2POT
> digital potentiometer from Xicor and a CA555 timer, but this will only give
> me a mark:space ratio of 1:1 at one extreme.  i.e. I can only get it half
> dimmed, or half bright depending on the polarity of the output. However,
> this cct. does have the advantage of only using two pins on the PIC.
>
> If anyone could advise of a timer circuit with which I can use the E2POT and
> still obtain a full range of mark:space ratios (with minimal component
> count) I would be very grateful.  One additional point is that mains supply
> is not available, so this cannot be used as a frequency generator.

You can make a PWM signal (fully variable mark-space ratio) using the
555 timer and pot that you have and an additional comparator. The
trick is that you ignore the square wave output of the 555
completely. The timing capacitor on the 555 is linearly charged and
discharged at the frequency of the 555 so that it has a triangle wave
on one side of it. Connect the triangle wave to one
comparator input and the pot wiper to the other input. The output of
the comparator will be a PWM signal with a duty cycle that is
controlled by the pot wiper voltage. The limits of the triangle wave
are 1/3 and 2/3 of the supply voltage so to have a full range of duty
cycles the pot wiper must also cover this range of voltages. The catch
is that the comparator may not have the same 200 mA drive capability
as the 555. Comparator could be an LM311 or an emitter-coupled pair of
transistors.

> Thanks a lot!
> Chris Windram

No problem.
Mal Goris

--
http://www.nfra.nl/~mgoris/