test eqiupment

2 Articles

LED Probe

LED Probe: A Smart, Simple Solution For Testing LEDs

June 14, 2025 by 13 Comments

If you’ve worked on a project with small LEDs, you know the frustration of determining their polarity. This ingenious LED Probe from [David] packs a lot of useful features into a simple, easy-to-implement circuit.

Most multimeters have a diode test function that can be used to check LEDs; however, this goes a step further. Not only will the probe light up an LED, it will light up no matter which side of the LED the leads are touching. A  Red/Green LED on the probe will indicate if the probe tip is on the anode or cathode.

The probe is powered by a single CR2032 battery, and you may notice there’s no on/off switch. That’s because the probe enters a very low-current sleep mode between uses. The testing intelligence is handled by either an ATtiny85 or, in the newest version, an ATtiny202, though the basic concept and design are compatible with several other chips. All the design files for the PCB, the ATtiny code, a parts list, and a detailed explanation of how it works are available on [David]’s site, so be sure to check them out. Once you build one of these probes, you’ll want something to test it on, so explore some of the LED projects we’ve featured in the past.

A Magic Eye Tube Does All The Work In This Kit

March 11, 2025 by 16 Comments

We’re used to low cost parts and a diversity of electronic functions to choose from in our projects, to the extent that our antecedents would be green with envy. Back when tubes were king, electronics was a much more expensive pursuit with new parts, so designers had to be much more clever in their work. [Thomas Scherrer OZ2CPU] has just such a design on his bench, it’s a Heathkit Capaci-Tester designed in 1959, and we love it for the clever tricks it uses.

It’s typical of Heathkits of this era, with a sturdy chassis and components mounted on tag strips. As the name suggests, it’s a capacitor tester, and it uses a magic eye tube as its display. It’s looking for short circuits, open circuits, and low equivalent resistance, and it achieves this by looking at the loading the device under test places on a 19 MHz oscillator. But here comes that economy of parts; there’s no rectifier so the circuit runs on an AC HT voltage from a transformer, and that magic eye tube performs the task of oscillator as well as display.

He finds it to be in good condition in the video below the break, though he removes a capacitor placed from one of the mains input lines to chassis. It runs, and confirms his test capacitor is still good. It can’t measure the capacitance, but we’re guessing the resourceful engineer would also have constructed a bridge for that.

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