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I want to use the 555-timer as a capacitance sensor for (10pF ~ 250pF) range. Is there a way to make the frequency change with respect to capacitance change more while keeping the frequency between 1Hz and 30 Hz?

In other words, can I increase \$\Delta\$ f/ \$\Delta\$ C while having a restriction on the frequency range?

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  • \$\begingroup\$ Maybe show the circuit you use but if it's just based on a simple RC charging circuit that causes the 555 to oscillate you may be out of luck. You may, however, be able to improve your detector i.e. improve the way you measure the frequency change so, more details here please. \$\endgroup\$ – Andy aka Feb 6 '17 at 9:02
  • \$\begingroup\$ You are also going to be dealing with stray wiring capacitance plus the input pin(s) capacitance of the 555. There is a high possibility that these will total up to a value that is more than the low end of the measurement range you desire. \$\endgroup\$ – Michael Karas Feb 6 '17 at 10:03
  • \$\begingroup\$ Given the low capacitance values to measure and very low frequency you request, I think using a 666 chip in a pentacle-shaped PCB is more appropriate. More seriously, why this frequency restriction? \$\endgroup\$ – dim Feb 6 '17 at 10:10
  • \$\begingroup\$ And what value of resistance have you calculated you'll need to get the 555 oscillating at 1 Hz with a 250pF capacitor - according to my calculation R1=R2 = 2000 Megohms - good luck with that! \$\endgroup\$ – JIm Dearden Feb 6 '17 at 14:12
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    \$\begingroup\$ I can't help you with the sensitivity (it's a single RC time constant so sensitivity is not really up for grabs easily) but you can use lower value resistors by simply following the 555 output with a divider- eg. a CD4040 would allow you to operate at \$2^{12}\$ = 4096 times the frequency at the 555 with the same flash rate at the divider output. \$\endgroup\$ – Spehro Pefhany Feb 6 '17 at 16:08

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