Archaeology Resistivity Meter

Looks good.

Do you think we need a spec next ?

Might be worth hanging on a couple of days to get comments on the block diagram and thoughts so far.

Things to think about on spec:

Operating temperature range

Endurance on one fully charged battery

Max output current and voltage (if 20mA RMS and nearly 60V pk-pk won't do it's going to get a lot more expensive )

Input common mode range (max +/- 14.5V including signal) ref instrument internal ground.

Max signal combined signal and common mode +/- 29V differential

Input impedance - could be 100M if wanted but I think this is a bad idea - should be lower , maybe 1M - problem is that you want the option

of input caps for AC coupling. If you have 100M input Z then you get -3dB at 10Hz with a 160pF input cap, but its impedance at

150Hz is 6.6M which will make things noisy. I'd like the capacitor impedance to be no more than 50k at 150Hz = 22nF, with 100M input that gives

a time constant of 2.2s so it may take ages to settle, I'm tempted to go for 10M or even lower input resistance to reduce that effect. (NB, the settling time

of the AC coupling RC is nothing to do with the normal settling time of measuring - its to do with how long it takes the system to settle after a large DC

bias is applied to the input.) Ideas / comments etc welcome. (AC coupling will be switchable.)

Input protection - static discharge (8kV ?, 16kV ? - full lightning ? (please not !) - any ideas from the field.

Operator controls - buttons - membrane key pads are expensive - nice push buttons are also expensive -  any ideas.

Would a touch screen be any good in the field ?

Well there's a good collection of random thoughts and comments.

MK

Ken,

I'm just catching up, sorry to hear of your loss.

Michael, will post some feedback on various points in a moment

Dave

Thoughts on power sources.

Sony consumer-camera-style batteries are a great source, I've used them for probably nigh-on twenty years for portable kit where they're suitable. My introduction came via a broadcast TV production client for whom I used to design and manufacture equipment used for various motor- and power-sport broadcast, both telemetry/data-logging and video capture. In days of yore, pre solid-state recorders, the client used to use the Sony GVD miniature DV decks in resiliently-mounted housings, and for commonality, I designed my kit to use the same NPF batteries.

Just a couple of issues to be aware of:

Electrical: A few packs had/have inbuilt over-discharge protection, but many (especially non-Sony clones) don't, so the power input will need to sense and then shut-down before over-discharging the pack, as Michael just mentioned (an acquaintance last year came to me with a cheap LED lighting panel powered by NPFs, and he was bemoaning how the batteries were no longer taking/holding much charge despite only having a dozen or so cycles on them...). There needs to be ample warning, you don’t want to have to stop and do a battery change mid-grid as stopping mid-grid for even a few minutes can introduce a discontinuity (the equipment may resume measuring accurately, but the ground conditions may have changed).

Mechanical: Most of those packs don't inherently have complete physical attachment. When the slide onto a holder, the terminals at one end connect with those in the holder and the battery is constrained by a cover or hatch; on the NPF style there are also a couple of slotted-hooks like those on, say, some PC case covers to help keep the battery aligned. Those are usually sufficient to hold the battery in the plane of the contacts and a modest spring pressure usually keeps the contacts connected. However, if the battery slides back out away from the contacts, the connection is broken and the hooks can disengage. Sony recorders with exposed batteries had a mechanical latch which sprang-up once the battery was in and would stop it sliding backwards; but many of the third-party plates don't have that feature. Those plates are great for charging and for low-dynamic power uses, but not so good as-supplied in higher-dynamic environments. Need to remember that an instrument such as this is dropped thousands of times every day – 400 or 800 times per 20x20m grid. Whilst lifting / pulling the probes out before moving them is usually a smoother movement, the probes are inserted into the ground by swinging the frame forward and dropping it with additional downward push – usually it decelerates relatively smoothly as the probes penetrate, but sometimes it jars when a stone is hit.

Dave

Hi Dave,

That's very useful information. I've noticed the same thing, my low-cost charger doesn't have that latch either.

However the battery-plate that I've ordered from aliexpress does have it (at least, it looks like it from the photo), so I'm hoping that works. Still, I wouldn't trust that on it's own with this cheap battery plate, and there may need to be some 3D-printing for a screw-on battery cover perhaps (maybe with foam padding), or some damping ideas, since so many knocks can't be healthy for the battery.

There are better battery plates on Amazon, but the price is higher. Since the battery is a standard voltage dual-cell device, worst case the battery and the plate could be swapped out for a different option when testing too, or constructors may be invested in a different battery system (e.g. Canon) and so they might use that anyway.

Although, not that I'm an end user for this project, but if it were me I'd prefer Sony over Canon because the battery has easier-to use connections (large round sockets, whereas Canon batteries [at least the ones I've seen] use metal leaf connections).

The aliexpress battery plate (search term is Andoer Battery Adapter Base Plate Battery Plate for Lilliput FEELWORLD Monitor for Sony NP-F970 F550 F770 F970 F960 F750 Battery in case the link goes bad) arrived.

It is made as well as any typical consumer item. The battery I tried is held with zero wobble, and the latch mechanism worked to prevent it sliding off. The wire is heavy gauge too, and battery contacts are nicely made.

There are photos below but in brief, I think it is good value, I wish I'd ordered another couple.

The yellow arrows show the slide-on hooks, red arrows show the latch.

The latch spring is visible here:

The battery contacts have springy curved metal like banana plugs:

Some epoxy glue on the back of the pins could be useful maybe, for extra strength. The connector is JST PH.

The aliexpress battery plate (search term is Andoer Battery Adapter Base Plate Battery Plate for Lilliput FEELWORLD Monitor for Sony NP-F970 F550 F770 F970 F960 F750 Battery in case the link goes bad) arrived.

It is made as well as any typical consumer item. The battery I tried is held with zero wobble, and the latch mechanism worked to prevent it sliding off. The wire is heavy gauge too, and battery contacts are nicely made.

There are photos below but in brief, I think it is good value, I wish I'd ordered another couple.

The yellow arrows show the slide-on hooks, red arrows show the latch.

The latch spring is visible here:

The battery contacts have springy curved metal like banana plugs:

Some epoxy glue on the back of the pins could be useful maybe, for extra strength. The connector is JST PH.