# Timing problem

Discussion in 'The Projects Forum' started by wayne_, Jul 1, 2013.

1. ### wayne_ Thread Starter New Member

Jul 1, 2013
4
0
Hello !
I have a Astronomical timing project.

The earth's resistance seems to change very precisely every second at 34.7 ohms; or 2083 ohms per minute.

I need a circuit or mechanical means to automatically increment this value until T = 12 Hrs. and at that point the resistance decreases linearly until T again = 24 Hrs. The maximum 'trimming' "R" is 3 M ohms. That is the changing quantity over 24 Hrs.
The total Earth resistance is roughly 150 M ohms +/- 1.5 percent or about 3 M ohms.

The cycle is continuous over 24 hours.

Any suggestions welcome!

Greetings!

2. ### crutschow Expert

Mar 14, 2008
13,011
3,233
I don't understand. You want to change the earth's resistance? Only someone with more power than you or I can do that.

Or are you trying to simulate this change is earth's resistance? How often do you want the simulation to change the value?

3. ### donpetru Active Member

Nov 14, 2008
186
25
I recommend you build an application with microcontroller. For example, with an 8bit Atmel microcontroller, you can use one channel ADC which to pick its resistance value of earth. Specifically, you will need to measure the resistance at some time or manually enter the value of that resistance in memory of the microcontroller. So, you have two options. After that, you have to create the software. It is not hard, back with more details and I can help you accomplish a first prototype.

4. ### wayne_ Thread Starter New Member

Jul 1, 2013
4
0
OK! Appreciate all this so soon!

Yes I wish to simulate the Earth Resistance.

I remember my "C" programming I learned long ago... I actually liked programming when I took computers then...

I was using an ancient 8086 Intel at 4.7 Mhz no graphics monochrome screen and 6 inch floppy drive. OS was DOS 3.3 and command line driven only....It was grossly ugly!

About ideally each second or multiple of - pragmatically probably each minute... New to this went to tech school when things were sort of old ... 1990. Albuquerque TVI ...
near Intel there.

I figure a 'resistance wheel and clock drive would be nearly 16 feet across so impractical for me.

Last edited: Jul 1, 2013
5. ### wayneh Expert

Sep 9, 2010
12,123
3,047
So you want a circuit that will change a resistance as a triangle wave (right?) at mean 150MΩ, amplitude 3MΩ, frequency = 1 cycle per day. Or 147MΩ plus a triangle wave of 6MΩ p-p.

I was once looking into a transconductance amplifier, LM13700, that can act as a variable resistor. See Fig 10 of the National data sheet. I don't know if you can use that to make a 6MΩ resistance swing, but it seems to me that it gets close.

6. ### wayne_ Thread Starter New Member

Jul 1, 2013
4
0
Really interesting device - we only touched on this sort of application.

Well only a swing - the variation - is only 3 M ohms per 24 hr. But, Ya the base resistance is still only 150 million ohms... funny how earth can be so tiny in fluctuating yet constant...

The low pass filter pulse width modulation embodiments are real kool.

I also like reference log current source...

A low pass filter applies to my application; the earth has a TC of 6540 seconds and also similar to the Sun's cycle.

So to use this as lo pass filter like @ 6.8 mfd cap, and 151 M ohm resonant R/C filter works OK.

The effect I want now though is a counting/ramping resistive plot; perhaps I will find all of this later.

Appreciate all the ideas here:

Wayne_