On Mon, 19 Jul 1999 01:13:44 +1000 "Paul B. Webster VK2BZC"
<paulb@MIDCOAST.COM.AU> writes:
>OK then, noted that you want an emulation of the 10V analogue control
>voltages.  This should be quite easy as you generally don't need very
>fast changes (faster than say, 100 ms).
>
>  PWM or duty-cycle modulation is definitely the way to go.  Take the
>output from the PIC for each channel, filter it by a simple R-C filter
>(perhaps two-pole) with a calculated cutoff of about 20 Hz, then apply
>this to an op-amp driving a PNP power transistor (Darlington perhaps)
>supplied at 12V. The op-amp multiplies the 0-5V signal generated by
>the
>PIC by 2 using a 2:1 feedback divider.

        Any reason to use the transistor?  An op-amp will typically
output 20 to 25 mA, which is generally enough for 0 to 10V control of
light dimmers.

>[deletia]

>  If you have many channels, or you want to "remote" all this from the
>PIC, you may wish to use latched shift registers as output expansion
>from the PIC.  Mmm, maybe that's another story!


        I used a single PWM output of a 16c74 and demuxed it out to 48
channels.  A single resistor formed the R part of the RC filter (between
the PIC PWM output and the demux input).  Each demux output drives a
capacitor to ground which serves as the C in the RC filter and also
serves as a holding capacitor in a sample and hold.  This is followed
with a noninverting op-amp amplifier (gain of 10 or so to allow for
higher control voltage outputs... there's also a pot as a voltage divider
reducing the PIC PWM output voltage as necessary between the PIC and the
previously mentioned R).
        That's the basis of the 48 channel DMX decoder at
http://www.dovesystems.com .

Harold




Harold Hallikainen
harold@hallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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