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[Andy40se]

Hi all 1st time here hope someone can point me in the right direction please.
I am trying to design something but have got stuck trying to find out how to do this.
My problem is.......
I need to be able to register a voltage on a given part of a circuit when an on button is activated. then if a variable resitors value is changed the voltage will change thus setting off an alarm. However the problem i have is the registered voltage will be different everytime the on switch is activated. so basicaly what i need is something along the lines of a wheatstone bridge circuit but that will automatically adjust itself everytime it is turned on. Is there anything like that out there or am i asking for something impossible? doesn't matter if it's done via hardware or software on a chip. thanks for taking the time to read this post.
Andy

 

[betwixt]

There are ways to do this.

In hardware:
What you do is split the voltage into two paths, one goes straight to a comparator circuit, the other goes to other input of the same comparator but via a low pass filter. Over time, the two voltages will be equal and the comparator circuit detects this. Because the LPF voltage is slow to follow the input, any instantaneous difference will be detected by the comparator. Eventually the LPF voltage will again be the same as the input and the comparator will cease to detect the change.

In software:
Use an ADC to measure the voltage and store the result, keep repeating this and comparing the new and old readings. You should allow a small difference because of ADC accuracy but basically, any significant difference in measurments indicates the voltage has shifted.

Brian.

 

[Andy40se]

Thanks very much for taking the time to post your reply brian.
Appreciate the help and am working on the hardware option as i type
Thanks again
Andy

 

[FvM]

The exact hardware equivalent of taking a reference on button press would be a sample and hold circuit. in "older days", they have been implemented with large capacitors (e.g. a 10 uF foil type) and low input current FET amplifiers. They have been able to keep a reference may be for hours with acceptable drift.

For infinite storage time, motor potentiometers have been used.