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I want to use a 555 timer to measure humidity using a humidity dependent capacitor which goes from about 300-> 400 pF.

This makes the frequency way to high for me to measure. Is there some similar cir cuit I can use to bring the frequency down to more like hundreds of Hz?

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    \$\begingroup\$ What order of magnitude resistance are you using (kilo, mega)? Can't you increase that? \$\endgroup\$
    – clabacchio
    Sep 15, 2014 at 15:20
  • \$\begingroup\$ Can you use the C-mos version (C555) with higher valued resistors? \$\endgroup\$
    – Wouter van Ooijen
    Sep 15, 2014 at 15:22
  • \$\begingroup\$ You could use the 555 to drive a counter or shift register, which would act to divide the frequency. I usually start by looking at wikipedia "List of 7400 series integrated circuits" and look for the word "counter" \$\endgroup\$
    – gbulmer
    Sep 15, 2014 at 15:23
  • \$\begingroup\$ i want one of the resistors to be a thermistor which can be say 47k, that means i need 10M or so at the other resistir. i thought resisitors were a bit dodgy with such high resistance (because theyll more likely conduct around the sweat on the casing) - or i need it well disconnected from the environment. thats just some hazy memory from 20 years ago - so maybe it is ok with R1=10M, R2=47k, C=300pF which would give 300Hz or so (dependent on C) \$\endgroup\$
    – user2633388
    Sep 15, 2014 at 15:27
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    \$\begingroup\$ @user2633388 are you sure that you're not overcomplicating things by putting the two sensors together in such an arrangement? \$\endgroup\$
    – clabacchio
    Sep 15, 2014 at 15:41

1 Answer 1

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You could use a counter after the 555. For example, a 74HC4040 would allow you to reduce the frequency by as much as 4096:1, so you could use a reasonable resistor value in the 100K range (assuming a CMOS 555). If you are using higher than 5V, a CD4040 would work.

enter image description here

You can pick whichever output you like to select the output frequency in steps of 2:1. For example, if the input frequency is 20kHz, you could select 10kHz, 5kHz, 2.5kHz, 1.25kHz, 625Hz, 312.5Hz, 156.25Hz, 78.125Hz, 39.0625Hz, 19.531Hz, 9.765625Hz, or 4.8828125Hz by selecting one of outputs QA to QL.

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  • \$\begingroup\$ ultimately i want to measure the frequency and time the up/down with a phone microphone - so 44.1 kHz sample rate \$\endgroup\$
    – user2633388
    Sep 15, 2014 at 15:32
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    \$\begingroup\$ Or use a CD4060 and loose the 555. \$\endgroup\$
    – Wouter van Ooijen
    Sep 15, 2014 at 15:36
  • \$\begingroup\$ Well, you can pick whichever tap you like. From a division of 1 (bypassed) to 4096, in steps of 2:1. 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048 or 4096:1. \$\endgroup\$
    – Spehro Pefhany
    Sep 15, 2014 at 15:37
  • \$\begingroup\$ I want to analyse the output to measure both R1 and C (thermistor and humidity dependent capacitor) \$\endgroup\$
    – user2633388
    Sep 15, 2014 at 15:39
  • \$\begingroup\$ If your thermistor is a compensation part, the compensation will be included in the divided frequency. It's a mathematical division. Hz out = Hz in divided by \$2^N\$. \$\endgroup\$
    – Spehro Pefhany
    Sep 15, 2014 at 15:45

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