If this is your first 'scope, and given the uses for it you mention, my advice is to get a decent analog unit on ebay. I have an inexpensive DSO and an old Tektronix analog and I use the Tek far more than the DSO. The reason? Mainly because the persistence of the analog scope allows me to see the spikes and glitches in much better detail. DSOs with persistence are coming down in price but you're looking at about 7-800$. You can find a good analog scope, e.g., Hameg or Tektronix 2-ch 50-60MHz for less than 100$ and you'll learn a lot about how scopes work.
And you can add the Saleae logic analyzer mentioned above and still come out spending less than a cheap, entry-level DSO.
My two cents, anyway.

Hi Baravantan,
To answer your last question, yes, I have measured what seem to be good values in the milliOhm range. I have a 100mOhm precision resistor and have measured many capacitors with what appears to be reasonable and repeatable results.
I took a quick look at your code and all appears to be OK with the actual calcESR routine. I didn't look at your PCB layout so I can't say more than that in my proto-board build I used the schematic exactly as in the original post EXCEPT that I used a 10microFarad cap and a 110Ohm resistor in the pulse circuit (47micro and 100 Ohm in the original).
I did notice in testing that the whole thing is very sensitive to the declared supply voltage and the value of this resistor. And I noticed in your code the following...

double Vin_LO = 4700; // milliVolts
uint16_t Rs_LO = 110; // Ohms

Are you actually using a 110 Ohm resistor? And is your supply voltage really only 4,7 V? Unless these (and the Vref voltage) are well defined, the calculation can be very non-linear.
Anyway, hope this helps.

Wow, what timing! You posted while I was composing. Please read my update.
I think your simulation is too simplified. I would be interested in your opinions if you would try mine.

Hi bseishen,

If you read my original text (kept intact, below) then I am sorry. I 'double-thinked' myself into some confusion. I will try for a better take.
First, the Schottky diodes only limit the maximum voltage available to the DUT. Here's what happens. The transistor Q1 turns on and a rising wave edge goes through R8 and is pretty much unaffected by C2/C1. So, R8 is effectively in series with the DUT. This forms a simple voltage divider and the rising wave edge sort of shelves at the voltage determined by the divider formed by R8 and the DUT. For a DUT with, say, 100 milliOhms, that is a pretty small voltage, i.e., a few milliVolts. Since we're dealing with capacitors here, there is some continuing charging that will mess up the measurement if we wait too long to make it. I put together a LTSpice model to play with this. Attached is a closeup of a pulse from a sample run with a DUT of 100uF and .1 Ohm ESR. The yellow trace is the R8 side of C2 and the blue is the DUT side. (I had looked at the same thing on a 'scope and the simulation agrees really well. I don't know why it didn't sink in earlier. It is interesting to do a run with a resistor as DUT. The 'shelving' is more apparent.)
So, pretty much ignore what I said before. Part of my confusion was from using a different power source after changing the diodes. This shows that the value for Vcc in the code is quite critical for consistent and linear results. Also, it is now apparent how two ranges work by adjusting the voltage divider.
I hope this helps. My understanding is better, anyway.

---- here is the original text ----
I went back and really looked at this as well as reading the original document by Dr. Le Hung (link in szmeu's first post). I have to admit that after looking closely, I really don't understand this completely either. Clearly, the pulse can only have an amplitude determined by the schottky forward breakdown. In my circuit I used germanium diodes and that was about 300mV. I just swapped them for 1N5822 (as in the orignal schematic) and now have a pulse of 220mV and my calibration is way off. What seems apparent now, is that the voltage specified in the code as the 'supply' voltage, i.e., 5V, is just a number. I mean that it has to be adjusted to correspond to something that, well, I don't know exactly what it specifies. Dr. Le Hung calls the pulses current pulses. There must be something to that since the two ranges do work (there is a factor of ten, approximately, difference when the pulse 'comes from' the 100 vs. 1000 Ohm resistor).
Try playing with the voltage value and see if you can dial something in. I haven't tried that yet but it will be my next step to try and figure this out.
I am first a software guy and the hardware part of this is now not as clear as I thought. Maybe someone else can add to this.

Hi again,
Yeah, it looks like you should be calling the lcd constructor with LCD pins 4,6,11,12,13,14 or for your assignments arduino pins 12,10,5,4,3,2. And I think the LCD pin 5 needs to be tied low.
-Totoro

Hi durpex,
I don't have the spec for the LCD here but something does look odd. You have pin 6 on the LCD connected to pin 10 on the arduino, but no where in the code are doing anything with pin 10.
(?)
- Totoro

Hi GD,
Since your temperature is in celcius, just calculate the dew-point with the temp and humidty values (don't try to convert the humidity). Then just covert the  dew point result to F.

Sreg,
Great to hear you got it working!
You mentioned above that you return -1 when millVolts is zero. I wondered why, so I took a look at your code. You have helped me to avoid a bad thing, the potential divide-by-zero bug. Thank you!
- Totoro

bobale,
Sorry, I am just a breadboard/point-to-point/proto-board type. My main need for an ESR meter is to troubleshoot old audio gear and test equipment.

Sreg,
First, just to be sure we're on the same wavelength, short pins 1 and 2 on CN1 (on szmeu's schematic) together and put the  device-under-test (DUT) between these shorted pins and ground. For now, just ignore the possibility for a four-wire setup (actually three wire with the circuit from szmeu).
OK, you've done that. Now, with a 30mV pulse across your DUT, you can do the math and you should have approx. 600milliOhms +/- for the DUT. This assumes that at top of the 100 Ohm resistor, R8, (where it connects to the emitter of Q1) you see a 5V pulse.
If that's the case, then you may want to make sure that the oversampling is set up correctly for your device. You may need to change some of the register set stuff in the original code. This stuff...
  sbi(ADCSRA,ADPS2);
  cbi(ADCSRA,ADPS1);
  sbi(ADCSRA,ADPS0);
Refer to the datasheet for your device and the Atmel document that szmeu mentions.
Then again, it could be something simple like the discharge transistor, Q2, is always on. Or you didn't change the value of Vref in the code. You mention the internal ref on your device is 2.56V. In the code it's 1.1.
This is all just guesses on my part. Maybe you have checked all this stuff. My 2 (euro) cents worth, anyway.
Good luck!
- Totoro

I am happy to post my code. You can download it from here.
http://vondervotteimittiss.com/belfry/downloads/ESR_Meter.ino.zip
Just keep in mind, it is a work in progress. As it stands, it works with the 'high-current' resistor in the circuit (the 100 ohm resistor which is actually 110 ohm in my circuit and in my code I refer to this as the low range).
As I mentioned, the schematic I used is the same as szmeu originally posted. (OK, not exactly. I use a 110 ohm resistor, because I had one, and a 10microFarad capacitor, because I had one. Adjust the resistor value accordingly in the code.) The pulse generation part of the circuit as szmeu posted is really elegant and needs no change.
The only change I made is in the display. In my breadboard, I use a three-digit, seven-segment display and have programmed the Arduino drive it. (Adjust the SEG_ON/OFF, DIG_ON/OFF defines as appropriate for your display setup and anode/cathode digit switching transistors.)
I am currently working on getting the 'low-current' mode (the 1000 ohm resistor), what in my code is the high range, to work. That should let me measure higher values of ESR, as szmeu suggested.
Please keep in mind that I am new to micro-controller programming! If you have any questions about the code, I'll try to answer as best I can.
And if you have any suggestions how I can improve, please let me know.
Thanks for your interest.
- Totoro

Hi all,
A few weeks ago I was looking for an ESR meter to build and ran across this thread. Thank you szmeu! I have built this up and it works very well.
I just wanted to share some observations. First, the hardware is a great circuit, simple and effective. Thanks again. And the circuit is very tolerant of the non-polarized capacitor value (47microFarad in the original). I used a 10micro I had and it works fine.
Second, you are correct that the pulse is not from a current source and what you want is to calculate as a voltage divider. (BTW, the original code will get you close enough to serve the purpose, especially at low values of ESR.)
Let's say that Vin is your source voltage (near 5V here), Vout is the value between the two resistors (you measure that at AIN0), Rs is the upper resistance (the 100 Ohm resister) and the lower resistance, the ESR, we'll call Rm. The formula is given as follows.
Rm = Rs / ((Vin/Vout) - 1)
It is important to not mix units, so here is an example. Say your AIN0 measurement, 'milliVolts' variable in the code, is 20. So, Vout is 20mV. Using the value of 5V for the source voltage, Vin, we need to express that in millVolts because that's the unit we're using for Vout. So, Vin is 5000mV. Rs is 100 Ohms so the result will be in Ohms.
Rm (in Ohms) = 100 / ((5000/20)-1) = 100 / 249  = 0.402 Ohms
To get milliOhms, multiply times 1000 or 402 milliOhms!

Here is my re-write of the ESR calc routine. This combines the oversample routine with some things were outside in the original code. A couple of lines are re-ordered. The big calculation difference is at the end.

Code:

// ESR calculation routine
// discharge, start a current pulse, measure the voltage at the ESR_PIN, stop the pulse
// repeat 4096 times and average the results ('oversampling and decimation')
// refer to Atmel Application Note AVR121: 'Enhancing ADC resolution by oversampling'
// http://www.atmel.com/images/doc8003.pdf
double calcESR()
{
  unsigned long accumulator = 0;
  unsigned int sample = 0;
  int i = 0;
  // oversampling 4096 times (for 16 bit is 4^(desiredResolution - ADCresolution))
  while ( i++ < 4096 )
  {
    digitalWrite( DISCHARGE_PIN, HIGH ); // discharge the capacitors
    delayMicroseconds( 600 ); // discharge wait time
    digitalWrite( DISCHARGE_PIN, LOW ); // disable discharging
    cli(); // disable interrupts for the pulse measure
    digitalWrite( PulsePin, PulseActive ); // start current pulse
    delayMicroseconds( 1 );
    // on the scope it appears that after enabling the pulse
    // a small delay is needed for oscillations to fade away
    sample = analogRead( ESR_PIN ); // read ADC value
    sei(); // measured, enable interrupts
    digitalWrite( PulsePin, ! PulseActive ); // stop current pulse
    accumulator += sample; // accumulate
  }
  // sampling is done
  esrSamples = accumulator >> 6; // decimate the accumulated result
  // calculate voltage on AIN0 pin...
  milliVolts = (esrSamples * vRef) / 65.536;
  // calculate ESR in milliOhms...
  double Rm = Rs / ((Vin / milliVolts) - 1); // Rm is in Ohms
  return Rm * 1000; // milliOhms
}

This now consistently measures my 0.1 Ohm and 1 Ohm precision resistors within about 2%. And a 4.7 Ohm resistor reads 4.68. Super!

BTW, I used all those extra pins on the Arduino to drive a 3-digit, 7-segment display. (That's why the code above has interrupt disable/enable calls to protect the pulse measure from the display multiplex interrupts.)

- Totoro

Nick, that was a quick answer! Thank you very much.
"Think of it as a form of documentation."
OK, that works for me!
- Totoro

Hi all,
I'm new to the forum and to Arduino development. I have gotten pretty far thanks to wealth of information here. I am sure this has been asked and answered before but I haven't been able to get the correct search phrase to find it, so excuse me for asking again.
I want to include a contributed library header in my library header. I have written a library named DS1821 for the same-named Maxim/Dallas Semi temperature sensor and want to include the OneWire library (my class inherits the OneWire class). Both libraries, my DS1821 and OneWire, are in the /Users/me/documents/Arduino/libraries directory (Mac OS X 10.6). Arduino is 1.0.1.

In my header, DS1821.h, I have the lines

#ifndef DS1821_h
#define DS1821_H
#include <OneWire.h>
. . .
#endif // DS1821_h

However, my application won't compile unless I include both headers.

#include <OneWire.h>
#include <DS1821.h>
. . .

While this works just fine, what should I do to not have to include the OneWire header in my application code?
Thanks for your help!