Linear Variable Differential Capacitor

Heard of an LVDT? well this isn't one, its a nifty circuit I hacked off of someone elses page., here is some documentation for it.


This is the basic cirucit


These are the sensor parts



This is my implementation of the circuit

Measurements as it stands


Full left deflection.


Full right deflection.


Almost full middle deflectionisity.

Sorry, the wires were springy and I had to hold it down with a postit pad.

Now to mess with it

Now some mods, I replaced the 1N4004 with 1N4148, which are lower capacitance and better for high freq. The 74LS74 I was using to divide the 16Mhz/2 (to ensure 50% duty) was replaced by a 74S74 (better output driver). In the process I also swapped left and right


Full left deflection


Full right deflection



If it still works, you havn't messed with it enough.

See that 1K resistor going to ground in the schematic? it sets the zero. so now what I did was to pull it out, and make a voltage divider out of 2.2K resistors to set it at 2.5V instead. Why? this will keep all the output positive which is usefull for going into an adc on a microcontroller.


With the rightfullness deflectionisity


With the leftfullness deflectionisity

More? well, to be honest it would work better if you took a plastic straw, put tinfoil around it for the two capacitor plates, and used a grounded metal slug for the middle, cause then the plates wouldn't be able to seperate. (the voltage dips off with distance)

Want a closeup of that last circuit? what the hell, here you go...




But I didn't say anything about that 7474 thing!?  the 7474 is a D flipflop, it serves two purposes here

a) It divides the clock by two. This will garuntee the output is 50% duty, if your clock isn't 50% one side of the capacitor will be higher voltage than the other

b) it drives the sensor. aka current driver.

I have wired it as a T flipflop by connecting its data(D) [pin 2] line to its inverted output line (/Q)[pin 6] the clock signal goes in the flipflops clock (C) [pin 3] and the uninverted output (D) [pin 5] is used as the output.

Note that I'v been carefull to keep most all the circuit on the top power rails which have a bunch of decoupling caps, important for high frequency work.

Also note the scratch on my desk from scraping large microwave transformers, robot wheels and scrapers over it. if you look real close you can see that I'v also used the desk for 1:1 layouts for making things, the pencil marks can be hard to disingiush from the cut marks from me using it as a cutting table too.

The green wire is telephone wire. Not the light green wire, its not, just the green wire with the red stripes. well the brown wire is telephone wire too, but its not the same it came off spools. I have lots of pretty wires that I use to make things, I keep them in a drawr, ask if you would like to see some of the wire I used to make things. The polorized capacitor is reused, I pulled it of a circuit board from something, I'm not sure what, probably a crt monitor or printer, perhapps even a vcr. The green one is reused too, its one of the good ones, I think its out of an audio circuit, like an old tapedeck.