# mili-voltmeter (mV) DC

hello.

I want to make a voltmeter that measures voltage in mili range with maximum 900 mV input.

this is testing of soil electrical resistance .

give a voltage to soil and measure out-come in mili volt.

I really have no idea how to make it with arduino. I have arduino uno.

please help me and tell me how to make it and show it on lcd 2*16 step by step

thanx

voltage in micro range

How micro? Millivolt, micro- or nano? AC or DC? You can just describe shortly your project, so other people can figure out details. 8)

mili volt DC

I want to measure resistance of soil.

give 5V from arduino to it and measure the out coming in mili volt (mV) range

And then start looking at Analog to Digital converter chips. www.analog.com would be a good start. Can get I2C interface, SPI interface, 8-12-14-16-20-24 bits, all kinds of speeds. 24 bit would theoretically get you down to 5V/(2^24) = 0.298uV resolution (if I did the math right) with 0-5V range and clean layout.

Ah, measuring dirt. Resistance is Ohm. What do you mean by non ohm?

I'm not very familiar with I2C and bytes. would please explain more?

Measuring resistance device, is not quite a voltmeter , but rather current meter. You know how to use a google, right? http://www.smeter.net/grounds/earth-electrode-resistance.php http://answers.yahoo.com/question/index?qid=20071006050754AAUdrHT And if there is no answer, I'd suggest to change a subject of the topic: "How to measure soil resistance with arduino?" If there is a person who did something similar, I'm sure he would give you a tip.

I stuck my multimeter into various potted plants around the house, I see resistances of 100Kohm to 200kohm, the lower readings coming from plants that were watered recently.

I stuck my multimeter into various potted plants around the house, I see resistances of 100Kohm to 200kohm, the lower readings coming from plants that were watered recently.

alimami,

That being said, try using a 100K or 200K ohm resistor as a pull-up to 5VDC, on one of the analog inputs of the Arduino. Then connect a soil probe to the same analog input. Connect another soil probe to the ground of the Arduino.

Try experimenting with adding soil and letting it try, and polling the analog input. The resistance divider between the pull-up and the soil's "resistance"(depending also on distance between probes) should vary depending on soil moisture. You may want to use a potentiometer as your pull up, and try to dial in your ideal divider value, because the resultant soil resistance has many variables.

Quote I stuck my multimeter into various potted plants around the house, I see resistances of 100Kohm to 200kohm, the lower readings coming from plants that were watered recently.

alimami,

That being said, try using a 100K or 200K ohm resistor as a pull-up to 5VDC, on one of the analog inputs of the Arduino. Then connect a soil probe to the same analog input. Connect another soil probe to the ground of the Arduino.

Try experimenting with adding soil and letting it try, and polling the analog input. The resistance divider between the pull-up and the soil's "resistance"(depending also on distance between probes) should vary depending on soil moisture. You may want to use a potentiometer as your pull up, and try to dial in your ideal divider value, because the resultant soil resistance has many variables.

Or couldn't you just change the distance between the probes? We don't know how close they where in your potted plants. That sounds like an inherently inaccurate way to measure dirt resistance. That being said, I have no idea on how to make it any better.

I found a cheep-o multimeter at a nearby minimart for less than \$15. I'm not sure how accurate it is. It's certainly cheep enough (for me) to use permanently as a dirt-o meter.

My tests had the probes about a handwidth apart, pretty much either side of the plant in those pots.
Hope Depot here also sells Sperry Multimeters, analog meters that are \$11-12 or so.

If OP alimami is looking to get digital data and make some decision on that, then Jshwaa’s idea of using the dirt as part of a voltage divider seems pretty easy to implement.

Either way I don’t feel like you could get this off the ground and have ‘reliable’ data to gather, without some serious experimentation to find a ballpark resistance, based on the distance between the probes(handwidth is fine but is better defined in inches or centimeters), the composition of the soil being sampled, and of course the amount of water in the composition. But you’d also have to take into consideration the possibility of water pooling and skewing the readings, and many other possible and unforseeable variables that could make getting ‘accurate’ data very difficult.

I would try and use this method as a way of ‘thresholding’ the reading and finding the right value that resembles a relative ‘dry’ or relative ‘moist’ condition. After all, this is just for watering plants, correct?

Find the ballpark value that is indicative of ‘moist’, and a ballpark value that is indicative of ‘dry’ by experimentation, and in code use ‘greater than’ or ‘lesser than’ comparisons to decide whether to water, or not to water; and when watering, when to stop watering. Leave adjustment points (potentiometers) in your design and tweak it in. If you find that adjusting is no longer needed, measure the pot values and replace with discrete resistors.

Once you have the circuit tweaked to where you can accurately determine when your plants need watering and when they don’t, some interesting data to gather would be how often it waters and how much water it dispenses per watering.

But to find the exact voltage that means ‘dry’ or ‘moist’ must be found experimentally.