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Capacitance meter with two 555 timers
Steve Greenfield | April 15th, 2008Use your DMM with this to measure unknown capacitors. Measures from a fraction of a pF to a few uF.
I left a few things off of the schematic, as I drew this in 1985 for my own use. I’d previously built (and sold) one built with a 50uA meter built-in, all with parts from Radio Shack.
You’ll need to use a 5V regulator such as an LM7805, LM78M05, or LM78L05 because calibration will be off as the batteries age if you don’t regulate the voltage. The output of the regulator should be bypassed with an electrolytic capacitor of 2.2 to 10uF. Tantulum is better than aluminum electrolytic, but either works.
How it works:
The first 555 timer is a clock (astable) that puts out very short negative going pulses.
A negative pulse triggers the second 555, which is connected as a monostable. The second 555 puts out a positive pulse, the time determined by the value of Cx (the capacitor being tested) and the scaling resistor selected by the rotary switch.
With regular pulses from the first 555, and the On time of the second 555 determined by the value of Cx, the duty cycle of the output of the second 555 is proportional to the amount of capacitance at Cx.
Output at pin 3 of the second 555 goes through a 4.7K resistor and small electrolytic capacitor (anything from about 2.2uF to 10uF is good) smooths the pulses to an analog voltage that is proportional to Cx.
The potentiometer labeled Trim is to calibrate full scale and should be a small trimmer resistor. A 20 turn is even better. The potentiometer labeled Zero should be a front panel control and is used to zero the display before calibrating.
With a DMM connected on the 200mV scale, the lowest scale (10M ohm) will be 200pF full scale, reading as fine as 0.1pF. On the highest scale (1K ohm), full scale is 2uF on 200mV, or 20uF on 2V scale.
See the schematic for more information.
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A clever design using easily available parts! Very useful.
The schematic (IS that one?) above, the size of a postage stamp, is, of course, illegible at that reduction.
HOW can on PRINT a 8½ x 11 copy of the schematic? I printed the text OK.
You have to click on the schematic, and you’ll get one that you can print full page. Unfortunately, you probably won’t be able to print directly from your browser as it may cut the bottom off.
It is 949×1420 pixels.
If you go to the Electronics_101 group on Yahoogroups, you can find more info on building this project.
Click on my name to go to the group site.
This can also be used as a noncontact or contact capacitance switch by connecting a comparator in place of the meter. Detect someone touching a doorknob or panel, or detect someone getting close to a door by taping a sheet of foil to the inside (won’t work on a metal door).
I’ve used this circuit as a liquid level detector. Drop two insulated wires into a container, spaced about a 1/4 inch apart. Zero the meter reading with the tank empty. Fill the tank, calibrate to 100mV, which will now stand for 100%. Tada! As long as the wires are straight and parallel, it should be very linear.
You can also build a Theremin using two of these. Build two of these circuits. Run the output of one to a VCO (Voltage Controlled Oscillator), the other to a VCA (Voltage Controlled Amplifier). Tada! Frequency and volume control. Connect each to it’s own antenna/hand sensor.
Steve, great circuit, very versatile.
About the liquid level application,did you mean Unisulated wires?
Aircraft fuel quantity systems use one unisulated metal tube inside another, they have a very linear capacitance.
Thanks!
No, I did mean insulated. Thinner insulation is better.
If you use uninsulated, it will only work to measure liquids that are good insulators, such as fuels and oils. But uninsulated won’t work in water, since pretty much any water you’ll be measuring levels on will have at least trace amounts of contaminants, making it at least somewhat conductive, which would change the readings depending on how non-pure.
The two sense wires can be coiled to increase the surface area change per change in level, useful if measuring the level over only a few inches in a very small tank.
For each wire, fold a wire in half and insert into the tank. That way there is no end in the fluid that must be sealed.
The wires must be far enough apart so that droplets can’t stick between them or liquid wick up between them and give false readings.
If the fluid is conductive, distance between the sense wires will make little difference. In a conductive fluid, the capacitance is formed by the wire through the insulation, conductive fluid as a resistive conductor, to the insulation on the other wire. On the 200pF scale, the timing resistor is 10M so as long as the path through the conductive fluid is 1% of that, it makes no appreciable difference to the reading. This depends on the fluid, of course.
Anyone having problems downloading the full size schematic?
You can find it in the Electronics_101 files section:
http://groups.yahoo.com/group/Electronics_101/files/555_Timer_IC/
Or from my website:
http://www.polyphoto.com/tutorials/Dual555CapMeter/Dual555CapMeter150dpi.jpg
Steve
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Steve,
Thank you for your clarification.
One more question, have you ever done anything with PLL circuits?
PLL? Not really. I’ve built a few 567 tone detector circuits, but just by plugging the numbers in.