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Subject: Re: Humidistat with digital display

Date: 03/01/00 at 2:22 AM
Posted by: Donnie James
E-mail: donniedj@hotmail.com
Message Posted:

In Reply to: Humidistat posted by Colin on 02/29/00 at 11:07 AM:

Yes, I will help. My quick schematic is at http://www.anglefire.com/de/donnied/humidi.gif Sorry for the pencil and to to clear graphics. Concerning humidity % display, use a 3 1/2 digit digital panel meter. A voltage divider of 3.3 on the Vsensor's output goes to the input voltage of the digital display. A reading of 1.000 = 100% humidity, .230 = 23%. Calculation: 3.3Vmax / 3.3 = 1Vmax, hence 100% max.

Concerning a humidity trigger method 1, 80% of 3.3V = 2.64V; therefore, a reference voltage(Vref) >= 2.64V needs to be set at the inverting pin (-) of the op-amp or comparator in the schematic. The non-inverint pin (+) receives the output from Vsensor (sensor voltage 0-3.3v,not from the voltage divider). As Vsensor rises to 2.64 or any desired trigger point the op-amp/comparator will go high to activate a relay, inturn turning on your fan via ac or dc power.

Method 2 for trigger, take Vsensor's output to the voltage divider on the base of Q1,transistor. Make sure Vsensor goes to R5 and not the node between R5 and R6. Adjust variable resistor R6 so that Q1 comes out of cut-off mode when Vsensor >= 2.64V. Now, either a Triac optoisolator or relay is used to control fan.

R1 = 2.3K R3 = 18K R5 = 1K
R2 = 1K R4 = 5K pot R6 = 1K pot
Q1 = 2n222, 3904, any NPN transistor, or mosfet

Important! That diode I included is for voltage spike protection with any relay connected across any ic device. Use proper Relay power ratings. DO NOT ATTACH the input ground of the Triac-Opto to any side of the Triac-Opto's output side!, same for Relay. I am sure you know this, just a warning for watchers of this post.

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