Subject: Re: Melt solder DDS controller ?
From: Lew Paceley (lew@paceley.com)
Date: Fri Feb 27 2004 - 09:58:58 EST
Hi Jim,
The basic logic tradeoff is "how fast" versus "how far" and "how many"
but I can't see any issues other than the potential of some noise
pickup in switching a couple of (slow) relays. Twist the wires and/or
use a shielded cable and all should be well. If there's still a
problem add some bypass caps to ground.
You should only need the single 74LS42 and run one decoded line to
both relay drivers/switches for the band selected. I'd use a pair of
transistors as relay drivers/switches to split the current load of the
two relay coils. I like 2N7000's for this kind of "low side" task
(assuming your relay coil current draw is not too high) because they
usually work well with TTL input levels, have a reasonably low R_ds,
and draw no current except when they are on.
GL!
72/73,
*Lew*
N5ZE
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