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Geophone Circuit

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aevans17

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I am trying to build a simple seismic circuit to measure the amount of vertical motion in the floor as people walk by my cubicle at work. I bought a geophone (a magnetic sensor that measures resistance between two magnets and thus measures vertical motion.) I want to interface this with my PC and so I am thinking of using either the Parallel Port, Serial Port, or possibly Game port of my PC to send in the data. At first I wanted to simply use the game port and connect the geophone as the Y axis of a joystick (since a joystick is simply a potentiometer that goes from 0 - 100K ohms). This did not work so well because the geophone at 5V only measures about between 1-2K ohms of resistance. I found a circuit on the net that converts the resitance to DC voltage between 0 - 5V.

http://www.eas.purdue.edu/~braile/edumod/educseis/educseis.htm

However, the PC interface costs $100 bucks and is not very cool. I wanted to build myself and so bought a ADC and found another circuit that converts the voltage to a binary number between 0 and 255. (8bit ADC)

http://www.iguanalabs.com/adc2051.htm

I am having trouble getting the ADC converter to work with my PIC 16F84A, but I think that I can figure this one out on my own. I was wondering if anyone new of a circuit that acts as a resitance multiplier so that if the output from the Geophone is only between 1k OHM and 2kOhm the resulting output of my circuit would be between 5K - 10K ohm or possibly 50K - 100K ohms??? I am a software guy and pretty new at hardware. I know how to program the PIC and I think that I can get it to work with my PC Parallel port once I figure out how to get a 10Mhz clock connected to my PIC in order to send the data from the ADC to the PC within 10us which is the time out for a read from the LPT data port.

Thanks for any help!!!
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It would be simpler to use the PC serial port - and more than fast enough for what you want.

You don't need a resistance multiplier, all you need is a voltage amplifier (easiest using an op-amp) - the A2D needs a voltage, not a resistance. To generate a voltage from the changing resistance you could simply connect it in series with another resistance across the supply rails - the voltage at the midpoint will then vary as the resistance varies. For a somewhat 'nicer' design, incorporate the sensor in a Wheatstone bridge circuit and use a differential amplifier.

To simplify the PIC side you could also use one with in-built A2D, many of which are 10 bit (rather than 8 ) - the 16F819 is 18 pin, like the 84, but includes A2D.
 

aevans17

New Member
Thanks Nigel! The circuit that I have currently converts resistance to voltage for the ADC. I didn't know anything about PICs when I bought my programmer so I just bought the intro programmer which came with the 84A so I have stuck with that. Had I known more I would have bought a PIC with builtin ADC and UART which would have made things really easy. I basically interface the circuit from this link and pass the input into the ADC which is set up to interface with the PIC.
http://www.eas.purdue.edu/~braile/edumod/educseis/educseis.htm

The circuit that I have uses an OP amp to amplifier the signal, a potentiometer to adjust the voltage approiately so that 2.5V will be output when the Geophone is at rest, and then another IC to stablize the voltage between 0-5V.

I have downloaded the serial code for from your tutorial page and I plan to use that to send my bytes to PC, however it is slightly easier to send the data to Parallel port because then I don't have to worry about sending over serial and I don't need to interface with a MAX232 in order to convert to the correct voltage levels of -10V/10V. Thanks for the suggestions, and for all of the samples and tutorials from your website. Once I get the thing to work I will post my completed work!!!!
 

Nigel Goodwin

Super Moderator
Most Helpful Member
aevans17 said:
http://www.eas.purdue.edu/~braile/edumod/educseis/educseis.htm

The circuit that I have uses an OP amp to amplifier the signal, a potentiometer to adjust the voltage approiately so that 2.5V will be output when the Geophone is at rest, and then another IC to stablize the voltage between 0-5V.
The circuit from that link looks fine, it does everything you need - providing a 0-5V signal from the sensor.

I have downloaded the serial code for from your tutorial page and I plan to use that to send my bytes to PC, however it is slightly easier to send the data to Parallel port because then I don't have to worry about sending over serial and I don't need to interface with a MAX232 in order to convert to the correct voltage levels of -10V/10V. Thanks for the suggestions, and for all of the samples and tutorials from your website. Once I get the thing to work I will post my completed work!!!!
You don't have to use a MAX232, if you are using software to generate your serial link you can invert it using the software and feed it directly to the serial port - although it's 0-5V, instead of -12/+12 - it works fine. For the other way you can use a resistor to limit the current into the PIC pin, again inverting it in software.

To use the parallel port you would have to write code for both ends of the link, depending how you did it - either as serial through the parallel port, transferring data as 4 bit 'nibbles' (using handshake input lines), or using the port in an 'enhanced' mode EPP etc.[/quote]
 

Clyd3

New Member
WHERE DO I GET A GEOPHONE

Thanks for all the useful info from your posts, which resulted in my memory bank gaining a few extra MB of info (approx 450%)...
I found this topic extremely interesting and now need to get a geophone for myself, but I dont have a clue where to start looking... please help...
 

aevans17

New Member
Geophone Completed and Connected to PC!!!!

Nigel thanks alot for all your help!!!! Serial really prooved to be the correct form of data transmission because Parrallel port was too difficult to syncronize with my circuit. I got my graphing software up and running yesterday and the guys here at work really enjoyed seeing people walk past our cubicles and make the graph really move. I wrote my graphing application in C# using the .NET framework so that it runs well on XP. I used an article posted at MSDN to get the code to read the serial port, I will post link later, and then wrote my own Windows Forms custom control to do the graphing. I will post all of the code for both the PIC and my application as soon as I work out the bugs. I will also attempt to create a circuit diagram of the completed project in case anyone else wants to build it.

Right now I have all of my IC's, resistors, and PIC on two bread boards with lots of jumper wires. I wanted to put this circuit on a single board, but I don't know what the best process is for doing this. I know that you can buy perf-boards and then transfers, but how do you put the transfers onto the perfboard??? Also where is a good place to get the materials to do this??? I know that they sell some types of perf-board at Radio Shack here in the US, but are there better products (easier to use) available elsewhere or online?

Thanks in advance for any information you can provide.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: Geophone Completed and Connected to PC!!!!

aevans17 said:
Right now I have all of my IC's, resistors, and PIC on two bread boards with lots of jumper wires. I wanted to put this circuit on a single board, but I don't know what the best process is for doing this. I know that you can buy perf-boards and then transfers, but how do you put the transfers onto the perfboard??? Also where is a good place to get the materials to do this??? I know that they sell some types of perf-board at Radio Shack here in the US, but are there better products (easier to use) available elsewhere or online?
Personally I just use Veroboard, a 0.1 inch (2.54mm - ever noticed how stupid metric can be?, still, I bought a pound of bacon tonight - stuff the governments metrification program!) matrix board with copper strips running in rows - you can see the sort of thing in my tutorial examples.

Many people create their own PCB's, either rubbing down transfers, or designing it on computer and using a photo process. Both board then need etching and drilling. I've never had the patience for this :lol:
 
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