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RC time constant help

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blueroomelectronics

blueroomelectronics

Well-Known Member
It's basic electronics but I'd like to check I have my calculations right.
I'm working with an HS1101 humidity sensor (160pf to 200pf) and a resistor 549K 1% The power supply is 5V and the threshhold is 3.125V I've arrived at these values

0% = 0.0862ms (160)
50% = 0.0969ms (180)
100% = 0.1077ms (200)

Vc = V * (1- e^(-t / R*C))

An RC time calculator
**broken link removed**
 
Thanks Roff, here's the HS1101 hooked up to a 555 astable. I'd like to avoid the 555 and use a comparator (internal to the PIC micro)
The idea is to short out the HS1101 and measure the time it takes to charge to 3.125 volts.
**broken link removed**
 
That should work. Did you read in the datasheet about the resistor from pin 5 to GND on the 555 being there for temperature compensation? Do you propose to do by with some other method, or just ignore it?
 
I've not worked with the hs101 but have done several capacitive sensors and in general your approach is the one I use. One thing to consider is the capacitance of the leads. Especially if they are fairly long. I had some success with building in calibration code. Remove the sensor, run the cal code. That gives you the time constant to subtract from the readings when the actual sensor is in place. It's not perfect since it's not temp/humidity compensated but it's not bad either. In one project, the leads contributed about 70 pf and the min capacitance of the sensor was around 100.
 
blueroomelectronics said:
The 1M resistor on the control input, how does changing the 2/3 control do temperature compensation?
Click to expand...
From the datasheet:
R1 unbalances the internal temperature compensation scheme of the 555
in order to introduce a temperature coefficient that matches the
HS1100/HS1101 temperature coefficient. In all cases, R1 should be a 1%
resistor with a maximum of 100ppm coefficient temperature like all
other R-C timer resistors. Since 555 internal temperature compensation
changes from one trademark to one other, R1 value should be adapted
to the specific chip. To keep the nominal frequency of 6660Hz at 55%RH,
R2 also needs slight adjustment as shown in the table.
Click to expand...
The internal resistors of the 555 are silicon, which have a tempco of around 0.7%/degree C. The internal divider will, ideally, be perfectly stable with no external resistance added. When you add external resistance with ~zero TC, you upset the stability of the divider.
 
blueroomelectronics said:
Thanks Roff, do you use PICs at all?
Click to expand...
Not yet. I bought one of your Inchworm kits and some PICs a few months ago, but haven't gotten around to it yet. Right now I'm trying to get an old 31" TV out of my shop. It's too heavy for me to move. I've been salvaging parts from it, but the CRT is about 95% of the weight. I'm thinking about building a steel box around it and then shooting it through a small hole. :D Then I could dispose of it in small pieces.
 
Roff said:
Then I could dispose of it in small pieces.
Click to expand...

Apart from the obvious enjoyment of shooting a CRT, in a country where you can :D - if you really want to just break it up, a hammer is all you need. First knock the very pip (like a nipple) off the tube neck, it's under the plastic moulding that fits over the pins. This softens the vacuum, over a few seconds if done carefully, and you can then simply break the glass with a hammer.
 
Nigel Goodwin said:
Apart from the obvious enjoyment of shooting a CRT, in a country where you can :D - if you really want to just break it up, a hammer is all you need. First knock the very pip (like a nipple) off the tube neck, it's under the plastic moulding that fits over the pins. This softens the vacuum, over a few seconds if done carefully, and you can then simply break the glass with a hammer.
Click to expand...
Thanks, Nigel. I didn't realize there was a safe way to release the vacuum. I did a little research and wound up punching a little hole in the anode plate with an ice pick. Worked like a charm! It hissed quietly for a minute or two and then stopped.
 
The pip is where they suck the air out, it's originally a longer thin tube which the air is sucked through, then it's melted to seal it, cut it off, and form the 'pip' - so it's the weakest part of the tube.
 
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