Help reading schematic.

Hi, I am trying to add this circuit into a project, and I'm wondering what the (-) power input should be hooked up to? The idea of the circuit is to produce an ac current, so does that mean I need a dual rail power supply to make this work? I am really having a hard time understanding what it is for. Thanks!

http://www.emesystems.com/images/smx/SMX555sc.gif

White are common
Red are positive
I don’t know what black make
Green are output.

This is an oscillator (150 kohms and 0.1 uF)
In someway the sensor “disturbs” the frequency and you can measure the frequency change at the output (open collector)(use internal pullup)

Pelle

if you developed the habit of posting detail of what you are actually doing, and what you are using.

So, by some detective work we know it is a "SMX Interface for WATERMARK sensor or other moisture probes", and let's have the circuit out in front of us:

Now I trust you have carefully read the SMX short manual and the SMX long manual to get an idea of it. OK, I do see a minor problem - one wire is either yellow or white, but that does not matter.

Now, it does not "produce an AC current", so we do not need to worry about that. (It is designed to apply only AC to the sensor, in order not to cause electrolysis, but that is a different matter.)

You want to wire it up for "Digital Frequency Output", with the white or yellow wire to ground, the red wire to a 5V supply and the green wire to an Arduino input which you want to use for measuring the frequency. The black wire can be connected to the white/ yellow wire, but that is not necessary.

Easy?

Thank you for the replies. It sounds like the black wire is unnecessary for my application. I was planning on using a pull-up resistor to interface with the analog input on the arduino. I am going to make the circuit, omitting the black wire and see what happens. I thought that maybe the black was ground and white was -5 v but I guess I was wrong. I will report back with my results. Thanks!

No

The signal from this circuit are digital so do not use analog inputs.
The voltage regulator seems to be hard to find so search for another 3, 3.3 or 5 volt regulator.
(5 volt directly from your arduino)

Pelle

The black wire might be to connect to earth, just a thought. Remember circuit ground isn't the same as earth, and in these days of isolated SMPS's its usually a floating ground.

The 1k resistor takes any mains coupled on to circuit ground away from the input capacitors (Often there's a fraction of a mA of mains on a SMPS output due to leakage capacitance, a 1k resistor to earth would reduce any ac pickup to less than a volt.)

[update: So I bothered to look at the manual - turns out the sensor's current consumption is proportional to the signal too, so the 1k resistor can be used as a shunt resistor to convert that current to a voltage in the range 0.2 to 1.0V...]

jackricci:
It sounds like the black wire is unnecessary for my application.

As MarkT observes, the black wire and its associated resistor is included simply as a simple way of “reading” the current drawn by the “sensor”. The idea would be that you connect it (as per figure 2 in either manual) to ground and the white/ yellow wire to your Arduino input with a 100µF capacitor also from this to ground and the red wire to 5V.

jackricci:
I was planning on using a pull-up resistor to interface with the analog input on the Arduino. I am going to make the circuit, omitting the black wire and see what happens.

Nothing to do with a “pull-up resistor”, you would in fact do what I (and the manual) have just described, however the output is only going to be 0.02 to 1V so you would probably want to select the “band gap” internal reference in the Arduino MCU.

jackricci:
I thought that maybe the black was ground and white was -5 v but I guess I was wrong.

Black usually is ground, and in that case it is deliberate; black and red are the supply, yellow/ white is the voltage signal.

I would however recommend that you do not use the current sensing arrangement; it makes much more sense to use the frequency output on the green wire and count pulses (dare I say it, using interrupts :D) in a 20 millisecond interval. If you are using this to measure soil moisture, you are obviously going to do this very infrequently and can do it when you are not performing any other important (time-sensitive) function.