[sdiy] lowpass gate etc - the sounds of capacitors

[mikei at marketbridge.ca]

No scope involved... unless you want it to be  :D

The capacitor is used as the timing cap in a CMOS 555 astable. A 
frequency counter is used to measure the pulse repetition rate. It is 
possible to set the CMOS 555 up for 50% duty cycle - or you can hook up 
a CD4013 to the output to get precise 50% duty cycle (in case your freq 
counter is finicky about duty cycle).

Good for relative measurements eg. matching... select the caps that put 
out the same Hz (or measure period if you want). With an assortment of 
1% caps on hand for calibration checking you can get a good estimate of 
the actual capacitance value - taking into account the pin capacitance 
of the CMOS 555 and capacitance of the wiring etc.
Cheers, Mike


Harry Bissell Jr wrote:
> 
> You'll need a real accurate timebase on your scope
> to resolve the pulse width.
> 
> The bridge mode 'might' be more accurate...
> 
> H^) harry
> 
> --- mikei at marketbridge.ca wrote:
> 
> > "555" timer with frequency counter approach is good
> > but use a CMOS 555
> > timer instead (such as TLC555  TS555  ICM7555) as
> > the CMOS unit
> > operates with much smaller leakage currents, giving
> > less error with
> > smaller capacitances than the more pedestrian
> > bipolar 555. Have some 1%
> > polystyrenes on hand for a calibration check.
> > Mike
> >