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Archaeology Resistivity Meter

kltm

Hi I'm looking for ideas on an update to a resistivity meter for archaeology. The only published designs for diy were in 2 magazines. One was published in 1997 and the other in 2003. I have copies of both articles available. The reason behind this is the current high cost of available equipment, usually well beyond the reach of most archaeological groups. I've attached a basic block diagram. In the first magazine article the meter is very basic. It relied on the operators to write down the reading given as the survey was taken. Given that a normal survey grid is 20m x 20m and 1 reading is taken on every sq mtr there would be 400 readings to write down and then input into a program used to interpret the results. The later article is really an update to the first where a PIC has been added to record the readings. This again is prone to error, because eadings are taken manually by pressing a button.

I'm sure given the advances in electronics there must be better ways. 

 

 

 

image

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  • in reply to kltm

    Hello Ken,

    mk2 block diagram.

    Several changes,

    The DAC is driven by the FPGA

    The ADC has changed type to a TI single channel audio type (much cheaper (about £3.30) and OK for our purpose I think.)

    The ESP chip provides Bluetooth and WiFi, chosen because its widely available, dead cheap and there is a lot of app material on the web.

    The battery type is defined as Sony NP-F970 equivalent, 5AH gives about 35Whrs, and I estimate a WiFi and display on power

    consumption of 3W so the available batteries should give at least 10 hours per charge. I bought 2 and a charger for about £40.

    The display is ideally a Riverdi 4.3" which uses the Bridgetek controller and has SPI interface. You can get a bare bones one for about £36

    but the nice one with the touch screeen and flat glass front (like the PI display) is about £50. Or you can use a sub £5 quarter VGA type from

    China - but I won't be writing code for it image

     

    Any illegible and unguessable features, please ask.

     

    image

     

    MK

  • in reply to michaelkellett

    Hi Michael,

     

    I ordered a battery plate from aliexpress, it has landed at an airport so hopefully I should have it in a few days, and can report if it's any good. It was $8.95 including delivery

    (there's dozens of different ones on aliexpress and amazon, this one was about the lowest-cost).

  • in reply to shabaz

    That'll be great if it's OK - on Amazon they want silly money for them.

     

    I've got one of my batteries on charge and I'll set up some stuff to test its capacity.

     

    MK

  • in reply to michaelkellett

    Looks great, I'll draw it up this evening.

  • in reply to michaelkellett

    Hi Michael

    Just posted block diagram. I hope there's not too many mistakes.

     

    Ken

  • in reply to kltm

    Great, thanks.

     

    Might be worth adding:

    the ADC type - PCM4201

    note "overload monitor" on the analogue signals going into the level shifters from the input and output amps (I forgot them on the second diagram)

     

    MK

  • in reply to kltm

    Hi Ken and Michael,

     

    Nice diagram!

  • in reply to michaelkellett

    Michael,

     

    Quick observation on the block diagram, ADC monitors voltage probes, but one thing that I cannot spot is provision for measuring/monitoring the injected current? If running in mode of conventional resistivity kit (square wave with constant current) then need a way to set and then monitor; if square wave with constant voltage, or a non-square-wave other waveform, need to continuously measure current to compute res?

     

    Dave

  • in reply to davemartin

    Well, sort of yes and no, the output amplifier is a current source, (see posts 42 - 44 in this thread), so the amplifier input is a voltage from the DAC and low pass filter

    but it forces a current, proportional to the input voltage, into the load. If you try an force too much current the amplifiers might not be able to generate a high enough output

    voltage, which is why there is voltage feedback to the processor analogue ports (which will use the processors rough and ready ADC to keep a check that the voltage

    isn't too high).

     

    So the output current is continuously referred to the demand voltage, by the nature of the feedback arrangement of the amplifier. (If you are interested in looking more deeply into this

    try Googling "Howland Current Pump", my circuit is a differential version of that venerable design.)

     

    The block diagram shows a digital gain control from the processor to the output amplifier, by switching the current sense resistors and  varying the codes to the DAC we will be able

    to set the current anywhere between 20 mA and 50uA with reasonable accuracy and good repeatability.

     

    MK