[sdiy] Resistor switching and breadboards

[harrybissell at wowway.com]

Two answers (inline)


n Wed, 05 Dec 2007 18:52:42 +0000, flightofharmony wrote
> Two separate questions:
> 
> 1) When using, say, a 4066 to switch a resistor into and out of a 
> circuit using square wave PWM for "variable" resistance, assuming 
> switching frequency is sufficiently high, are there any transients 
> or glitches that may appear?

Of course there will be. The switching frequency will usually be capacitively
coupled into the circuit.  That may cause errors, noise, aliasing, etc. There
are better switches with "low charge injection".  In some cases polarity
counts... sometimes power from Vdd ends up in your circuit via the switch.
Read the fine print on the data sheets.


> 
> 2) Breadboards. Grrr. I am constructing a synth and playing with a 
> 555-based ADEG. I am also using one opamp out of an LM3900 to buffer 
> the output. When I set the circuit to retrigger, it won't - unless I 
> touch the lead of the feedback resistor connected to the inverting 
> input. Then it works fine, either using my finger or needlenose 
> pliers. Whaddahell!? I know breadboards are notorious for loose 
> connections, stray capacitances and other such joys - any ideas 
> about this one?

It might be that solderless breadboards suck (well of course they do) but it
may also be that you are injecting noise (making the circuit switch) or stray
capacitance which makes it work.  See if your scope probe causes the effect...
then maybe add a 50pF capacitor from that point to ground to simulate stray
(added) capacitance.

Or get a good, soldered breadboard. I like Vectorboard and T42-1 terminals.

otoh yesterday I found one connection I failed to solder (not causing the
problem but ~how~ embarassing... :^)

H^) harry



> 
> ~flight
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Harry Bissell & Nora Abdullah 4eva