Retrotechtacular: Weston Electrical Instruments

A ‘meter is one of the most important tools on any electronics bench. After you’ve exhausted your five senses trying to figure out what’s happening in a circuit, firing up the old ‘meter is usually the next step. Meters are largely digital nowadays, but their analog ancestors are still widely available. We have a chemist and inventor named [Edward Weston] to thank for the portability and ubiquity of DC measuring equipment.

After immigrating to the United States from England with the degree in medicine his parents wanted him to earn, [Edward Weston] asserted that he was more interested in chemistry. His career began in electroplating, where he soon realized that he needed a reliable, constant current source to do quality plating. This intense interest in power generation led him to develop a saturated cadmium cell, which is known as the Weston cell. Its chemistry produces a voltage stable enough to be used for meter calibration. The Weston cell is also good for making EMF determinations.

Within a few years, he co-founded the Weston Electrical Instrument Corporation. The company produced several types of meters along with transformers and transducers known for their portability and accuracy. In 1920, [Weston & Co.] created this 1920 educational film in cooperation with the United States Navy as part of a series on the principles of electricity.

The viewer is invited to consider the importance of measurement to civilization, most notably those fundamental measurements of length, mass, and time. [Weston] positions his electrical measuring instruments at this level, touting them as the international favorite. We get the full tour of a Weston meter, from the magnet treated for permanence to the specially designed pole pieces that correctly distribute lines of magnetic force. What education film about electromagnetism would be complete without an iron filings demonstration? This one definitely delivers.

Continue reading “Retrotechtacular: Weston Electrical Instruments”

Exploding Multimeter Battle Royale

If you check out eBay, Amazon, or the other kinda-shady online retailers out there, you’ll quickly find you can buy a CAT III (600V) rated multimeter for under $50. If you think about it, this is incredible. There’s a lot of engineering that needs to go into a meter that is able to measure junction boxes, and factories in China are pushing these things out for an amazing price.

Over on the EEVBlog, these meters are being pushed to the limits. Last month, [joeqsmith] started a thread testing the theory that these cheap meters can handle extremely high voltages. A proper CAT III test requires a surge of electrons with a 6kV peak and a 2 ohm source. With a bunch of caps, bailing wire, JB Weld and zip ties, anyone can test if these meters are rated at what they say they are. Get a few people on the EEVBlog sending [joeqsmith] some cheapo meters, and you can have some real fun figuring out how these meters stack up.

The real experiments began with [joe smith]’s low energy surge generator, a beast of a machine that can be measured with an even beastlier high voltage scope probe. This is a machine that will send a voltage spike through anything to short out traces on poorly designed multimeters.

How did the cheapo meters fare? Not well, for the most part. There was, however, one exception: the Fluke 101. This is Fluke’s My First Multimeter, stuffed into a pocketable package. This meter is able to survive 12kV pulses when all but two of the other brands of meters would fail at 3kV.

What’s the secret to Fluke’s success? You only need to look at what the Fluke 101 can’t do. Fluke’s budget meter doesn’t measure current. If you ever look inside a meter, you’ll usually find two fuses, one for measuring Amps and the other for all the other functions on the scope. There’s quite a bit of engineering that goes into the current measurement of a meter, and when it goes wrong you have a bomb on your hands. Fluke engineers rather intelligently dropped current measurement from this budget meter, allowing them to save that much on their BOM.

There’s an impressive amount of data collected by [joeqsmith] and the other contributors in this thread, but don’t use this to decide on your next budget meter; This is more of an interesting discovery of how to make a product that meets specs: just cut out what can’t be done with the given budget.

Fail Of The Week: The Deadliest Multimeter

Need a good multimeter? The Fluke 17B is an excellent basic meter that will last your entire career. It’s also $100 USD. Need something cheaper? Allow me to introduce the AIMOmeter MS8217. On the outside, it’s a direct copy of the Fluke 17b, right down to the screen printing but understandably lacking the yellow enclosure. $30 USD will get you an exact copy of a Fluke 17B, it would seem. Right? Not a chance. [electronupdate] did a teardown of the AIMOmeter, and while this meter looks like a Fluke on the outside, it’s probably going to kill somebody.

The teardown begins with a look at the ratings on the back of this off-brand meter. It does have two fuses, but the engraving on the back strangely claims ‘Wrrebt insurance limit’. If anyone has any idea what a ‘wrrebt’ is, please leave a note in the comments. The only references to this word in Google are mis-OCRed blackletter type in a book from the early 1800s.

Opening up the meter reveals – surprisingly – two real fuses in the meter. There were no markings on the bigger fuse, which could be a problem for verifying if the fuse is of the proper value. That’s not really a problem, though: the fuse isn’t even between ground and the amp probe socket. Yes, this fuse is completely useless, and testing the resistance with the fuse out of the circuit confirms this.

After putting the meter back together, [electron] tests the accuracy of the meter. With a 1 mA current source, the mA setting seems to work, but when testing the larger Amp range of this meter, the results display in milliVolts. Don’t worry, there’s an easy fix for that: just press the dial down just right and the correct setting will be displayed. Wow.

You get what you pay for, and if you only ever use an AIMOmeter for measuring Arduinos and batteries, you might – might – be alright. This is not the kind of meter you want to measure line voltage, motors, or anything else with, though.

Give your Multimeter a Wireless Remote Display

Multimeters are one of the key tools in a hardware hacker’s bench. For 90% of us, the meter leads are perfect for making measurements and looking over at the results. Sometimes you need a bit more distance though, and for that, [Ken Kaarvik] has created the Multimeter remote display. Remote displays are pretty handy when you want to measure something several feet away from your bench. They’re also great if you need to check something in an enclosed space, like a server rack or a refrigerator. Fluke actually sells multimeters with wireless displays, such as their model 233.

The key to this project is the FS9721 LP3 chip by Fortune Semiconductor. (PDF link) The FS9721 is essentially a system on chip (SOC) for multimeters. It contains a digital to analog to digital converter, an LCD driver, and a microcontroller. It also can send data out over a 2400 baud serial link. Two of [Ken’s] multimeters, the Digitek DT-4000ZC and a Fluke 17B, both have this chip. The Digitek has a 1/8″ plug for connecting to the outside world, while the Fluke requires some simple hardware mods to enable data output.

Since this was his entry for the Trinket EDC contest,  [Ken] connected the serial output of the FS9721 to an Adafruit Pro Trinket. The Trinket formats the data and sends it to an  nRF24L01+ 2.4GHz radio module. The receiving end has an identical radio, and another Pro Trinket. [Ken] actually built two wireless displays. One is a dual-boot Game Boy advance which has a really slick background on the color display. The other receiver utilizes a 128×64 OLED. The trinket, nRF24L01+ and display all fit neatly inside an Altoids tin.

Click past the break to see both wireless remote displays in action!

Continue reading “Give your Multimeter a Wireless Remote Display”

Fixing A Multimeter’s Serial Interface

[Shane] bought a multimeter with the idea of using its serial output as a source for data logging. A multimeter with a serial port is a blessing, but it’s still RS-232 with bipolar voltage levels. Some modifications to the meter were required to get it working with a microcontroller, and a few bits of Python needed to be written, but [Shane] is getting useful data out of his meter.

The meter in question is a Tenma 72-7735, a lower end model that still somehow has an opto-isolated serial output. Converting the bipolar logic to TTL logic was as easy as desoldering the photodiode from the circuit and tapping the serial data out from that.

With normal logic levels, the only thing left to do was to figure out how to read the data the meter was sending. It’s a poorly documented system, but [Shane] was able to find some documentation for this meter. Having a meter output something sane, like the freaking numbers displayed on the meter would be far too simple for the designers of this tool. Instead, the serial port outputs the segments of the LCD displayed. It’s all described in a hard to read table, but [Shane] was able to whip up a little bit of Python to parse the serial stream.

It’s only a work in progress – [Shane] plans to do data logging with a microcontroller some time in the future, but at least now he has a complete understanding on how this meter works. He can read the data straight off the screen, and all the code to have a tiny micro parse this data.

Voltset Multimeters at World Maker Faire

Many tents at World Maker Faire were divided up into booths for companies and various projects. In one of these tents, we found the Voltset booth. [Tom, Ran, and Michael] were on hand to show off their device and answer any questions. Voltset is essentially a multimeter which uses your phone as a display. It connects to an Android phone via USB or an optional Bluetooth module.

Now we’d be a bit worried about the risk of damaging our phones with a voltmeter electrically connected via USB. However, many people have an old phone or retired tablet kicking around these days, which would be perfect for the Voltset. The Bluetooth module alleviates this problem, too – though it doesn’t fix the issue of what happens to the multimeter when someone decides to call.

Voltset isn’t new; both the Voltset team and the similarly specced  Mooshimeter were also at World Maker Faire last year. In the interim, Voltset has had a very successful Kickstarter. The team is accepting pre-orders to be shipped after the Kickstarter backers are sent their rewards.

voltset-2[Tom] told us that the team is currently redesigning their hardware. The next generation prototype board with more protection can be seen in the far right of the top photo. He also mentioned that they’re shooting for 5 digits of accuracy, placing them on par with many bench scopes. We’re skeptical to say the least about 5 digits, but the team is definitely putting their all into this product. We’ll wait until the Kickstarter backers start getting their final devices to see if Voltset is everything it’s cracked up to be.

Cheap Multimeter Gauges Embedded Idle Time

How often is your microcontroller actually doing something? You can find out by measuring idle time, but how exactly do you do this? [Jack Ganssle] shows that simple embedded applications can toggle a pin when idle, which can then be measured. More complex applications like those using a Real Time Operation System can do the same by making use of the idle hook. But what can you do to make this toggling pin feedback actually mean something?

His solution is to repurpose an analog multimeter. The meter is interface with the toggle pin and a trimpot calibrates the needle. This way the needle jumps when the processor is busy and returns to zero when idle. What a great tip for getting a little more feedback about what’s going on inside of that black plastic IC package. It’s not surprising to find such a clever hack from one of the Hackaday Prize judges.

While you’re in the analog multimeter aisle you might want to pick up a couple of extras for more alternate data displays.

Continue reading “Cheap Multimeter Gauges Embedded Idle Time”