# 555 timer as a humidity sensor

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?

• What order of magnitude resistance are you using (kilo, mega)? Can't you increase that? Sep 15, 2014 at 15:20
• Can you use the C-mos version (C555) with higher valued resistors? Sep 15, 2014 at 15:22
• 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" Sep 15, 2014 at 15:23
• 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) Sep 15, 2014 at 15:27
• @user2633388 are you sure that you're not overcomplicating things by putting the two sensors together in such an arrangement? Sep 15, 2014 at 15:41

• 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 $2^N$. Sep 15, 2014 at 15:45