Newbie with a question... What is it?

[meether]

Hello all. I'm a newbie here and only have a limited knowledge of electronics, but I'm learning. I'm not sure if I'm in the right place for this question, but here it is. What do I have here? It's a capacitor meter of some sort, but I don't know the sort. I'm aware of different tests for capacitors... value, leakage, esr, etc. I'm also pretty new to drawing schematics, but I gave this a shot. So with that... Can anyone read it and tell me what it is? How to use it? I see there are a couple of trim pots... what might I have to do to calibrate it? I know... funky questions, but I figure there are enough knowledgeable folks on this forum that can decipher this simple circuit. Thanks for your time! (I hope the attachment came out OK)

[Gyro]

Welcome to the forum.

You have a venerable home constructed capacitance meter there. I wouldn't like to put an exact age on it, but at a guess, around 40 years. It just measures capacitance value, not ESR or leakage, as you've probably guessed, with ranges from 1nF (.001uF) to 1uF full scale.

You've done a pretty clean and neat job of a reverse engineered schematic there (apart from being upside down  ). The thing you're missing is translating the IC pins to IC function. The RS556 is actually an NE556 (RS components used to source ICs from various manufacturers and then mark them with their own 'RS' part number - probably one of the earliest examples of 'black-topping'!). The NE556 is a dual version of the well known 555 timer IC. You should find lots of example circuits and function explanations on the web (in addition to datasheets from various manufacturers. You won't find as many references for the 556 as it can just be treated as a dual 555. All you need its datasheet for is the pinout.

Once you re-draw the schematic using two functional timer blocks (with pin numbers) rather than just the package, things will make a lot more sense to you. I haven't confirmed it, but it looks as if one of the blocks is wired as an astable multivibrator, creating a constant fixed frequency pulse train. Then the second block is wired as a monostable multivibrator triggered by the first, where the pulse width is determined by the capacitor under test and the relevant range resistor. The higher the capacitance, the wider the pulses, and the greater the meter deflection.

The two trim pots do the same thing, one for the upper ranges but for the lowest range, parasitic effects (wiring capacitance etc) are becoming an issue so it has its own trimmer.

Hopefully this gets you most of the way there - as I say, re-draw the schematic using the IC functional blocks and maybe check out some 555 tutorials and everything should become clear to you.

[meether]

Hi Gyro. Thanks for the reply! I thought I had righted the schematic before downloading it. I hadn't noticed it was upside-down when I put it in my scanner.  The 556 chip has it's "dot" in the upper left corner. (when the schematic is right-side-up) That's what I "implied" with the little circle with the 1 in it. The rest of the pins are as they are on the chip. Is that what you need for the "IC pins to IC function"? I'll put in a pic of the pin functions. I did research that chip and found it as you described, and I have seen some diy ESR meters that use the 555 chip. I've read where an ESR uses AC across the capacitor to get that "Equivalent Series Resistance"... If I read the tutorial right, is it these ICs that make that AC for that?
And then... how does the milliamp meter translate to the cap value? And how would I go about checking and setting the calibration?

[Gyro]

You're welcome.  Yes, the dot is the pin 1 marker.

Yes, the internal block diagram, as you have shown is what you need, you can see that it is split into two 'halves, the only shared connections are supply and ground. They look a little squashed in the 556 due to the pinout*. I have attached an equivalent block diagram of one section from a TI 555 datasheet. This is drawn a bit more helpfully (inputs on the left and outputs on the right) which makes it easier to visualise and hang the external passive components on. I've deleted the 555 pin numbers so you can copy and add the 556 pin numbers for the appropriate half.

The way the meter translates is by integrating the width of the pulses. If the pulses are very narrow, then the average voltage is near zero so the deflection is very small, if the pulses are wider then the average voltage is higher, and so the meter deflection greater. The way to calibrate the meter would be to find a fairly accurate (eg 5% film) capacitor of 10nF, 100nF or 1uF to set full scale on one of the upper three ranges (as there is one pot, the appropriate capacitor should then show full scale on the other 2 ranges, within reasonable limits). For the bottom range, you will need a 1nF capacitor to set the other pot. You can then experiment with other capacitors, eg 22nF/220nF and 47nF/470nF, to see how linear the meter reading is. Off-hand, I'm not sure how linear the meter response will be (worst case, you might need to create a graph of meter deflection vs capacitance).

Yes, an ESR (Equivalent Series Resistance) meter uses an AC signal (often squarewave or 'modified sine wave'), normally around 100kHz (to cater for SMPS capacitors). These treat the capacitor as a 'shunt resistor' from the AC signal to ground. The magnitude of the signal that gets past the capacitor represents its ESR. There is a popular thread around about making a good workable ESR meter with just a 5 transistor circuit. It would be worth searching this out if you want to build one. [Edit: https://www.eevblog.com/forum/projects/5-transistor-esr-meter-design/ ]


EDIT: * While attaching the 555 block diagram I noticed that your 556 one doesn't show the internal voltage divider resistors on the inputs of the comparators (they are in there). This voltage divider resistor chain is important to understanding how the Threshold and Trigger inputs and comparators work!

[meether]

Yes, indeed. I mentioned I'm a newbie... Just to give you an idea how so... this is my first attempt at understanding what an IC is doing. You're helping there. Thanks. I did watch a tutorial vid on the 555 and I'm getting a better idea what's going on in there.
My experience is quite limited to "tweaking" tone and gain circuits a bit in guitar amps. Of course, that means modifying the sound with pedals and then modifying the pedals and the use of "wall wart" power supplies, and, of course, due to the fine construction of those power supplies these days... their failure... frequently due to a cap failure. I've never been one to "just replace it"... I want to understand and fix it. And of course, the more learned opens the curiosity to fix other similar things. So here the chain is taking me. One question begets an answer that invokes more questions. I enjoy the whole process!
And thanks for the calibration advice. I see the "range" would be considered "1" for a good cap of the value on the selector. Then, the "range" on the meter will have to be "translated" so to speak. And this would all be done with "reference" capacitors. I do understand how that works.
I have seen that 5 transistor ESR meter. (And others.) I've seen how some will make a new face for their meter for the very reason you mentioned. I might have to look into building one of those, or one like it...
Again, many thanks. I'm getting an understanding of what's going on with this particular meter, and that's a good place to start.