# Sensor voltage change

Discussion in 'Electronic Design' started by WAYNEL, Jul 8, 2005.

1. ### WAYNELGuest

Hi

I have a humidity sensor that produces a linear responce from 0 to 100%
RH calibrated at:

0.958v @ 0% RH
3.268v @ 75.3% RH

I have a PID controller that accepts 0 to 1v at the sensor input.

I am trying to produce 0v for 0% RH and 1v for 100%RH or 0.753 for
75.3%RH.

Can anyone propose a very simple circuit that will do something
similar. e.g using a transistor or opamp.

Cheers

WayneL

2. ### JonGuest

Use an op-amp.
Non Inverting input goes to ground
2 input resistors: R1 = R2 = any practical value.
1 Feedback resistor R3 = R1/(3.268-0.958) = R1/2.31
Sensor goes to inverting input through R1.
Offset voltage of -0.958 goes to Inverting input through R2.
..
The output voltage will be negative. Invert it with an inverting op
amp stage with a gain of -1. (Input R = Feedback R).
..
You could use any negative value of offset voltage, and then adjust R2
accordingly. For example, if you want to use -10V as the offset
voltage, then R2 would = R1 * 10/.958.

3. ### Fred BloggsGuest

View in a fixed-width font such as Courier.

3.068*R

5. ### Fred BloggsGuest

It's a DA. He wants Vout=(Vin-0.958)/(3.268-0.958)=(Vin-0.958)*(1/2.31)

6. ### Tony WilliamsGuest

Hang on Fred..... dVin/dRH is 2.31V per 75.3% change, which
is 3.068V per 100%.

For preferred values. 12k/3.9k is a ratio of 3.077, or about
0.3% error.

7. ### Fred BloggsGuest

Right- did not realize the OP was asking for both a circuit and a
transfer function. To the high school dropout:

View in a fixed-width font such as Courier.

..
.. |
.. |
.. |
.. X + - - - - - - - / E
.. | / |
.. | /
.. | / |
.. | /
.. | / |
.. 3.268 +- - - - - / C
.. | / | |
.. | /
.. Vsensor | / | |
.. | /
.. | / | |
.. | /
.. | / | |
.. 0.958 + - - -A - - - - -B - -D
.. | | |
.. | |
.. | | | |
.. | |
.. | | | |
.. 0 ------+----------+-----+-----
.. 0 75.3 100
..
.. RH->
..
..
..
..
.. ED CB
.. ABC similar to ADE => -- = --
..
..
.. 2.31
.. Then ED= ---- x 100 = 3.068
.. 75.3
..
.. which makes X= 3.068 + 0.958= 4.026
..
..
.. The RH sensor puts out 4.026V at RH=100%
..
..
..
..
..
.. DESIRED FUNCTION
..
.. |
.. |
.. 1V+- - - - - - /
.. | / |
.. | /
.. | / |
.. PLC IN | /
.. | / |
.. | /
.. | / |
.. | /
.. | / |
.. | /
.. 0V+------+------------+---------
.. 0.958 4.026 Vsensor
..
.. -----+------------+---------
.. 0% 100% RH
..
..

8. ### WAYNELGuest

Thankyou very much Fred Bloggs.

WayneL