DIY wrist strap tester replacement - Page 1
Forum section: electronics / projects

author: TechieTX
author_rank: Contributor
timestamp: May 03, 2018, 03:00:22 am
content: Background: we have a slew of these ACL 750 wrist strap testers at work. Frankly, they're cheap-@ss junk. If you open it up, it was probably designed in the '90s and never improved. The micro has the top sanded off, but it's obviously a PIC16C57. There is stuff bodged onto the board (maybe for the ridiculuous high R ranges), the internal wiring to the switches and connectors is 24awg solid insulated copper wire, and it just plain looks like a first-year electronics student project. You'd vomit if you saw it... there's even a strip of black electrical tape to keep that pushbutton switch in the middle from shorting out to components underneath it. The tape fell off and was rattling around in the case. Nice.Some of the units we have are starting to get insane... bad connections ruled out with new wires & solder, and cleaned very thoroughly with IPA so maybe the circuitry is taking it's final sky dive. Rather than spend$350USD eachto replace this trash, I figured it couldn't bethathard to duplicate ALL of them for the price of one. Measure 3 unknown 'resistors' from 1meg to 100meg and trip a piezo alarm if the measured value is above a selectable value or below 500K/750K ohms. Send those measured values over an RS-232 port to the computer for logging.You don't need accuracy. Heck, 10% or better on the resistance reading is fine by me. I'm just looking for dry skin, loose strap, or intermittent wrist strap cable, not a precision instrument. You aren't gonna GET precision when skin resistance is involved. ;-) I'm only interested in the 10meg wrist / 10meg foot range, as that's where all of ours are set to, but what I'm building will cover the OEM's range, as well.Quick sanity test: I wired up the circuit below on breadboard and it worked. The voltage follower is 'cos you can't shove a 100meg impedance signal into an A/D converter and hope to get anything useful. ;-) I got around 2% accuracy with a decent 5V supply and 1% 10meg resistor, testing various fixed resistors from 510K to 100meg. Theoretically it should read anything from 100K to 1gig ohm, 0.05V to 4.95V at the SENSE line.Looking at the toy box, I have a whole tube of ATmega328P chips that I bought to play with. So far the parts cost is under $15 each at Digi-Key. I wired the breadboard into an Arduino I have, and it reported the correct resistance values within the limits of that 10-bit A/D section. Cool, I can work with that.One microcontroller, an 8-pos dip switch, 6 LEDs, a couple of rail-to-rail low input bias opamps, a serial port, some power control circuitry and assorted snot. It's not rocket science, and it's certainly not worth $350USD. I likely won't populate the $0.50USD dip switch, I'll just use jumper wires.Next task: fire up the serial monitor and see what that RS-232 protocol looks like. I only have an hour or two free each week for this project; it'll take a while to finish the board layout and software. A month or two down the road I'll post a follow-up with the KiCad and firmware source codes, if anyone wants it. Hell, if you have any critique, I'm open to suggestions.The 750 board is freaking huge, so the prototypes won't be a $5 jlcpcb wonder. I could shrink the board size way down if the LEDs weren't so darned far from the power & serial port. Even with DIPs and through-hole resistors, it'd be 1/3 the size of what's in there if not for their crazy layout. I'll re-use the chassis, merely replacing the PCB inside.quote:Specifications:Accuracy: ± 10% (tester only)Environment: 32ºF to 100ºF (0ºC to 38ºC); 15% to 95% RHTest voltage: 19 volts DC ± (open circuit)Adjustable resistivity limits; defaults to ESDA or CE limitsConforms to ANSI / ESDA 20.20 standard and EU CE standardResistivity ranges:Wrist strapsHigh Range: 2M, 5M, 10M, 25M, 35M, 50M, 75M, 100M ohmsLow Range: 500k, 750k ohmsFootwearHigh Range: 2M, 5M, 10M, 25M, 35M, 50M,75M, 100M ohmsLow Range: 500k, 750k ohms

author: bson
author_rank: Unknown
timestamp: May 03, 2018, 05:45:22 am
content: Just stick a CP2102N on it and use USB cabling instead of RS-232. You can probably even power it over USB. I'm sure it would be pretty straightforward to create a bracket that fills the DSUB9 hole and offers a microUSB B connector on your PCB instead.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 03, 2018, 06:19:35 am
content: Given the low accuracy requirements, it could be done with a few bipolar transistors, a few resistors, and two LEDs. As a bonus, it would be trivial for the test current itself to switch the unit on so no power switch needed.

author: TechieTX
author_rank: Contributor
timestamp: May 04, 2018, 12:47:58 am
content: I think you folks are missing the point, two different ways.I want to use the existing software for this thing since we have automatic reports that are pulled out of the log files over the network. If I change to USB, then I have to re-write and then debug the PC end of the software. Not worth the effort for such a change (serial to USB). All I'm doing is duplicating the function of the existing board. If I was doing it entirely from scratch, then yes, USB would be the preferred implementation and I'd change to a different micro (like the AT90USBxxx) that had USB as part of the core logic and save an additional chip.And bipolar transistors? Really? Have youtriedrunning a 100meg ohm impedance signal into a BJT? We're talking differences of milliamps and nanoamps here. With a 100meg resistance, even 10 nanoamps of input bias (load) would shift the reading, and I'd have to calculate what it *should* be based on that parallel load. The 1meg base impedance on a darlington pair would utterly swamp the parallel 100meg resistor-equivalent. The MCP6242 has only +/- 1picoampsof input bias, so it won't load the reading. Heck, I could reasonably read well over a gigohm with the circuit above if it was needed, just by shifting that 10meg R1 to 100meg or higher. It'd probably have to be battery powered to read signals that low, though. I'm going to re-use their 12VDC wall-wart PSU.MOSFETs would work (it looks like that's what they did originally), but why bother when opamps are so easy to use nowadays?The LEDs are a nit... nobody actually looks at them, but if I'm gonna duplicate the circuit function, they're not a major inconvenience to add in. It adds cost due to the size of the board to match the holes in the chassis, but that's about it. If you did the whole thing in SMT it'd probably be only 4-5 cm square. Replacing the chassis would cost me as much as a bigger PCB would, so it's a wash.I'm making it more precise than it needs to be purely because it's cheap to do. I may end up using one of the spare boards for a mat tester here at home. I have a couple of left-over BME280 breakout boards, so I'll plan on leaving room for one of those as well. If I bolt on a couple of extra resistors with FET switches to boost the range, I could probably duplicate the function of one of these ohms-per-square meters:https://www.ebay.com/itm/272044615282but with actual readings, and not just one LED per order of magnitude.

author: Mr. Scram
author_rank: Super Contributor
timestamp: May 04, 2018, 12:53:51 am
content: I thought you were exaggerating, but that looks fairly terrible. That has a serious "hand soldered in the 80s by Chinese unskilled labour" vibe.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 04, 2018, 01:05:40 am
content: Quote from: TechieTX on May 04, 2018, 12:47:58 amBipolar transistors have been used to sense ion currents, which are a few orders of magnitude smaller than the test current in your application.http://www.techlib.com/science/ion.html

author: David Hess
author_rank: Super Contributor
timestamp: May 04, 2018, 09:04:00 pm
content: Quote from: TechieTX on May 04, 2018, 12:47:58 amThat is what superbeta bipolar operational amplifiers like the LT1008, LT1012, and LT1097 are for.I made good use of the National LM11 before the LT parts become available and before that there was the LM308. The better parts were useful up to at least 100 megohms.Linear Systems produces a discrete dual superbeta bipolar transistor for such applications with a specified input bias current of 700 picoamps maximum at 1 microamp collector current if you want to roll your own.QuoteSometimes super beta input bipolar operational amplifiers are used instead of FET input parts because their input bias current does not double every 10 degrees C but it probably does not matter here and I agree with you that a JFET or MOSFET input stage is the way to go for most low bias current applications including this one.My low input bias current part of choice is the National (now TI) LMC6081 with a typical input bias current of 10 femtoamps. The LMC6001 is the tested version of the same part if you do not want to test them yourself. The LMC6061 is a low power version of the LMC6081. I do not remember if there is an even lower power version.QuoteSuitable MOSFET input operational amplifiers have been around for a long time courtesy of RCA. Last time I checked, Intersil still made their CA series parts but of course now there are lots of options.QuoteWas there some requirement to make these tests at a higher voltage? If the ESD protection breaks down at high voltage, that could be a safety issue not revealed by a low voltage test.