Talking Ohmmeter Also Spits Out Color Bands For You

If you’ve got a resistor and you can’t read the color bands (or they’re not present), you can always just grab a multimeter and figure out its value that way. [Giacomo Yong Cuomo] and [Sophia Lin] have built an altogether different kind of ohmmeter, that can actually spit out color values for you, and even read the resistance aloud. It’s all a part of their final project for their ECE 4760 class.

The build is based around a Raspberry Pi Pico. It determines the value of a resistor by placing it in a resistor divider, with the other reference resistor having a value of 10 kΩ. The resistor under test is connected between the reference resistor and ground, while the other leg of the reference resistor is connected to 3.3 V. The node between the two resistors is connected to the Pi Pico’s analog-to-digital converter pin. This allows the Pico to determine the voltage at this point, and thus calculate the test resistor’s value based on the reference resistor’s value and the voltages involved.

A large fake resistor provides user feedback. It’s filled with addressable LEDs, which light up the appropriate color bands depending on the test resistor’s value. It’s capable of displaying both 3-band, 4-band, and 5-band color configurations. While six-band resistors do exist, the extra band is typically used for denoting temperature coefficients which can’t readily be determined by this simple test. It can also play audio files to announce the resistance value over a speaker.

It’s a neat project that surely taught the duo many useful skills for working with microcontrollers. Plus, it’s kinda fun — we love the big glowing resistor. We’ve featured some other fancy resistors before, too!

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Kelvin Probes Review Shows How 4-Wire Resistance Measurement Works

You might think the probes in the picture are just funny looking alligator clips. But if you watch [tomtektest’s] recent video, you’ll learn they are really Kelvin probes. Kelvin probes are a special type of probe for making accurate resistance measurements using four wires and, in fact, the probe’s jaws are electrically isolated from each other.

We liked [Tom’s] advice from his old instructor: you aren’t really ever measuring a resistance. You are measuring a voltage and a current. With a four-wire measurement, one pair of wires carries current to the device under test and the other pair of wires measure the voltage drop.

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Measure Resistance The Colourful Way

One of the first things anyone with an interest in electronics learns is the resistor colour code. The colour of the first band reveals the first figure, the second the subsequent figure, and the third a power-of-ten multiplier. At first you learn these colours, but eventually you just recognise the values through familiarity. You don’t have to think about multipliers when you see orange-orange-red, you just know that it’s a 3K3 resistor.

[Plusea] has come up with an entertaining interface for an ohmmeter, which instead of displaying the resistance on an LCD or a meter shows it as the colours of the code, via a set of addressable LEDs. The work is done by an ATtiny85 microcontroller, and the whole thing is mounted on a flexible PCB (fabrication of which is itself interesting, placing cut copper traces on a sheet of kapton and covering with a second kapton layer cut to be the solder mask). There is even a clever integration of a CR2032 battery holder from the PCB itself, though they admit that it could be made more compact with the use of SMD components instead of through-hole.

The write-up and associated photo album tells us a lot about the project, but is missing a crucial detail: a shot of it working. We’ll give them the benefit of the doubt on that front though, because we like the idea and its execution.

Strangely, this isn’t the first ohmmeter to use the resistor colour code in this way, we’ve previously brought you one featuring a light-up giant resistor.

Giant Resistor-shaped Ohmmeter


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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