# Old Wattmeter Uses Magnetics To Do The Math

Measuring power transfer through a circuit seems a simple task. Measure the current and voltage, do a little math courtesy of [Joule] and [Ohm], and you’ve got your answer. But what if you want to design an instrument that does the math automatically? And what if you had to do this strictly electromechanically?

That’s the question [Shahriar] tackles in his teardown of an old lab-grade wattmeter. The video is somewhat of a departure for him, honestly; we’re used to seeing instruments come across his bench that would punch a seven-figure hole in one’s wallet if acquired new. These wattmeters are from Weston Instruments and are beautiful examples of sturdy, mid-century industrial design, and seem to have been in service until at least 2013. The heavy bakelite cases and sturdy binding posts for current and voltage inputs make it seem like the meters could laugh off a tumble to the floor.

But as [Shahriar] discovers upon teardown of a sacrificial meter, the electromechanical movement behind the instrument is quite delicate. The wattmeter uses a moving coil meter much like any other panel meter, but replaces the permanent magnet stator with a pair of coils. The voltage binding posts are connected to the fine wire of the moving coil through a series resistance, while the current is passed through the heavier windings of the stator coils. The two magnetic fields act together, multiplying the voltage by the current, and deflect a needle against a spring preload to indicate the power. It’s quite clever, and the inner workings are a joy to behold.

We just love looking inside old electronics, and moving coil meters especially. They’re great gadgets, and fun to repurpose, too.

# Watt Meter Build Walks You Through Power Measurement Basics

You almost never hear of a DC Watt Meter – one just does some mental math with Volts and Amps at the back of one’s head. An AC Watt Meter, on the other hand, can by pretty useful on any workbench. This handy DIY Digital AC Watt Meter not only has an impressive 30A current range, but is designed in a hand-held form factor, making it easy to carry around.

The design from Electro-Labs provides build instructions for the hardware, as well as the software for the PIC micro-controller at its heart. A detailed description walks you through the schematic’s various blocks, and there’s also some basics of AC power measurement thrown in for good measure. The schematic and board layout are done using SolaPCB – a Windows only free EDA tool which we haven’t heard about until now. A full BoM and the PIC code round off the build. On the hardware side, the unit uses MCP3202 12 bit ADC converters with SPI interface, making it easy to hook them up to the micro-controller. A simple resistive divider for voltage and an ACS-712  Hall Effect-Based Linear Current Sensor IC are the main sense elements. Phase calculations are done by the micro-controller. The importance of isolation is not overlooked, using opto-isolators to keep the digital section away from the analog. The board outline looks like it has been designed to fit some off-the-shelf hand-held plastic enclosure (if you can’t find one, whip one up from a 3D printer).

Although the design is for 230V~250V range, it can easily be modified for 110V use by changing a few parts. Swap the transformer, change the Resistive voltage divider values, maybe some DC level shifting, and you’re good to go. The one feature that would be a nice upgrade to this meter would be Energy measurements, besides just Power. For an inside look at how traditional energy meters work, head over to this video where [Ben Krasnow] explains KiloWatt Hour Meters

# Attacknid Becomes Laser Death Drone

[styroPyro] liked his Attacknid, but decided it needed just a bit more blue death ray laser. We’ve seen [styroPyro’s] high-powered laser hacks before, but this time he’s taken to hacking one of [Jaimie Mantzel’s] Attacknid robots. According to one of the top comments on [styroPyro’s] video—a comment by Attacknid inventor [Jaimie] himself—the robots were meant to be hacked, and [Jamie] is ecstatic.

[styropyro] removed the disk shooter from his Attacknid and used the fire control circuit to activate a 2 watt blue laser. A low powered, red laser pointer serves as a laser sight, allowing you to aim at your target before unleashing the beefy blue laser. As the video shows, 2 watts is a heck of a lot of power. The Attacknid easily pops balloons and sets fire to flash paper. As usual, we urge you to use caution when handling 2 watt lasers, which fall under Class 4: aka the most dangerous class of lasers. Goggles, skin protection, and safety interlocks are the order of the day. [styroPyro] has been working with high power lasers for a few years, and seems to know what he’s doing. That said, we’ll leave the burning lasers to the professionals.

# [Ben Krasnow] Explains Kilowatt Hour Meters

[Ben Krasnow] is back, and this time he’s tearing down a kilowatt hour meter (kWh). While not as exciting as making aerogel at home, or a DIY scanning electron microscope, [Ben’s] usual understated style of explaining things makes a complex topic simple to digest.

These old mechanical meters have been a staple on the sides of houses and businesses since the dawn of commercial power. We always thought the meters were a basic electric motor. Based upon [Ben’s] explanation though, these meters are a complex dance of electromagnetic fields. Three coils create magnetic fields near an aluminum disk. This creates eddy currents in the disk resulting in a net torque. The disk spins, turning a clockwork and advancing the dials.

Why three coils? One is a high turn high gauge voltage coil, and the other two are low turn low gauge current coils. The voltage coil has to be phase shifted 90 degrees to create the proper torque on the disk. Confused yet? Watch the video! [Ben] does a much better job explaining the field interactions than we could ever do in text.