Digital Decade Resistance Box on the Cheap

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[Stynus] has finished a unique decade resistance box which doesn’t use conventional rotary switches to select the appropriate resistors. These switches are old fashioned and expensive, so [Stynus] built this decade resistance box that uses a microcontroller and a series of relays to switch the resistors.

Simply selecting a resistance on the screen tells the microcontrollers which resistors need to be switched in order to provide the proper resistance. The box uses relays to do switching instead of transistors because the transistors don’t handle high frequency AC as well as the relays. The device is powered by an 18V transformer and rectifier and, as a bonus, [Stynus] got all of his parts on the cheap which made this a great solution to the expensive resistance decade box problem.

This is a very well-polished piece of test equipment. We’ve featured other decade resistance boxes but never one that was controlled by a microcontroller. All of the PCB layouts and the code for microcontroller are available on the project site if you have a desire to make your own.

Measuring 185 µΩ In Circuit

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To measure resistance, you usually have to take the resistor to be tested out of the circuit, and sometimes that’s impossible. If you’re using a multimeter, measuring very small resistances is difficult to say the least. Combine both these problems – measuring microOhms in-circuit – and you have a problem that’s perfectly suited for the Mooshimeter.

Announced just a few weeks ago, the Mooshimeter is a two-channel multimeter that communicates with your cell phone over Bluetooth. It’s perfect for measuring current and voltage simultaneously, all while being tucked away in some place that’s either dangerous, inaccessible, or mobile.

The Mooshimeter team put together a great example of what can be done with their meter by measuring the resistance of a car battery grounding strap while behind the steering wheel. To do this, they put alligator clips across the grounding cable and clamped on a current meter.

Inside the car, they whipped out their cell phone and looked at the Mooshimeter’s output for the voltage and current measurement. The Mooshi app has an IV curve (with linear regression in the works), so simply dividing the current and voltage gives them the resistance of the battery’s grounding cable.

It’s a very cool and extremely simple demonstration of how cool the Mooshimeter actually is. Video of the demo below.

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Resistance decade box using DIP switches

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Here’s a simple piece of equipment which you’ll be proud to display on your electronics bench. It’s a resistance decade box. The concept has been around forever — it offers the ability to tune a wide range of resistance values just by adjusting the controls. We especially like the clean look of this one, and think the use of DIP switches is a nice touch.

Check out the toggle switch at the top. It lets you disconnect the resistance values from the output in order to test them with your meter. It may not seem like much, but fudging your switch settings could end up smoking your target project. The value of that feature isn’t lost on us.

The DIP switches are mounted to some Radio Shack breakout boards which work perfectly for hosting the resistors as well. This keeps the inside of the enclosure nice and tidy. The final touch is the printed face plate applied to the cover of the box.

Like we’ve said, this one is nice but our favorite is still this one that uses thumbwheel switches to dial in a value.

Giant resistor-shaped Ohmmeter

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The fun of having a giant resistor-shaped Ohmmeter is that it reads back the resistance by displaying the color code. If you’re not too hot with decoding those bands there’s a helper band to the right which will display the value numerically.

All of the electronics are housed in the opaque part of the resistor, making for a nice low-profile base. The bent leads are hollow and allow [Sebastian] and his friend to run power and measurement leads through to the power connector on the back and the pair of banana jacks near the front. Each translucent ring houses an RGB LED, except for the one on the right which has four 7-segment display modules embedded in it. An ATmega168 takes the measurements using its Analog to Digital Converter (ADC) to read the value from a voltage divider. You can see a quick demo of the Ohmmeter in the video after the jump.

This would be a fun thing to pair with that giant breadboard.

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LiPo internal resistance measurement tool

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This is a scratch-build meter for measuring the internal resistance of Lithium Polymer cells. [Bleuer Csaba] uses the LiPo cells for RC vehicles and thet take quite a beating from the motors  they’re supplying. This means that he only gets about 100-200 cycles out of each cell. To figure out where one is in its life cycle you can measure the internal resistance where a rising resistance indicates greater age. [Bleuer] mentions that you can buy a meter to do this for you, but what fun is that?

Since he’s rolling his own tool he defined his own parameters for the readings. After experimenting with different loads driven for different test periods he was able to extrapolate an equation that estimates the resistance measurement. As you can see in the clip after the break, this happens very fast. All he has to do is connect the cell and press one button. The measurements are made and various data points are displayed on the quartet of 7-segment displays.

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Measuring SMD parts with a home brew version of Smart Tweezers

SMD parts are great; they allow you to pack more parts on a board, do away with drilling dozens of PCBs, and when done correctly can produce a factory-quality board made in a home lab. There’s one problem with SMD parts; troubleshooting and measuring them. The ideal solution would be something akin to the Smart Tweezers we’ve seen before, but this fabulous tool costs three hundred bones. [Kai] came up with a much cheaper solution: home brew smart tweezers that can be built for a tenth of the cost as the professional model.

What [Kai] built is an LCR meter, basically a tool that measures inductance, capacitance, and resistance in a very, very small form factor. The technique of measuring a part’s properties involves feeding a set frequency into the device and measuring the phase, voltage and current coming out. It’s all wonderfully explained by [Dave] over at EEVblog in one of his earlier videos.

The hardware [Kai] is using includes an LCD display from a Nokia phone, an MSP430-based microcontroller, a very tiny opamp near the tip of one of the points of the tweezer, and a programmable gain amplifier used to measure the components. In testing, [Kai] can measure very low-value components with a +/- 2% accuracy, and larger, more realistic components with +/- 0.25% accuracy. An awesome accomplishment, and much better than the common Chinese meters that can’t measure in the nH/pF/mΩ range.

[Kai] hasn’t gotten his pair of smart tweezers working yet – he still needs to get the circuit up and running and write some software. We’ll keep our readers apprised of [Kai]‘s progress, though, and gently convince him to work with Seeed Studio or someone similar to get his version of Smart Tweezers onto maker’s workbenches the world over.

Multimeter add-on lets you measure tiny resistance values

This multimeter add-on is called the Half Ohm. It allows you to measure small resistance values, and can be used to track down shorts on a PCB.

The board acts as a pass-through for both probes. When your meter is set to measure voltage and nothing is connected to the probes the display will read out the level of the coin cell that powers the add-on. When you are probing, the value in millivolts is actually showing the resistance in milliohms. This works for any measurement less than one Ohm. Interestingly enough, it will help you zero in on a solder bridge. By probing the two shorted tracks you can find the issue by following the falling resistance values.

[Jaanus] published several posts leading up to the final version of the board. Check out this category link for his blog if you’re interested in reading through them.

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