Adjustable Voltage And Polarity Tester


Instructables user [Rudolf] wrote in to share a handy little tool he created with ham radio operators in mind. Now and again, he found himself connecting to an unknown power supply, and rather than blow out all his expensive radio gear, he decided to put together a simple polarity and voltage tester that can be easily carried out in the field.

The tester features a pair of powerpole connectors, which are used quite often for connecting HAM gear. A PIC12F675 runs the show, acting as an adjustable comparator for detecting voltage levels. By default, his probe glows amber when the supply voltage is below 11.5V, turning green when the supply is between 11.5V and 15V. When the detected voltage is too high, the built-in LED glows a bright red. When the polarity is reversed, the LED flashes red regardless of the supply voltage.

All of these trigger levels can be set in the PIC’s code, which [Rudolf] is kind enough to include on his page, along with schematics for making your own.

20 thoughts on “Adjustable Voltage And Polarity Tester

  1. Very very nice! Clean interface with the LED and power connectors, and nice design and board. I really wish this wasn’t behind instructibles though – this is a project that I wanted to check out, but due to a promise never to use instructibles, I won’t be able to. One thing I might emphasize for anyone else who wants to use the circuit is to read carefully the section detailing the voltage divider – it determines the range of input voltages that you can hook this detector up to. Thanks for this!

  2. @vtl: Yes, and his ham radio does what cellphones are for.

    I am in search of a free program to help me reverse-engineer circuits (a see-through image of the PCB in the article could be autotraced). Any hints?

  3. Why not use an AC/DC bridge, a voltage regulator, and potentially a voltage booster to guarantee he has the proper polarity and voltage no matter what he plugs in to (within reason)?

  4. @haweyaez1

    Is an AC/DC bridge anything like the bridge rectifier he has in the schematic? Other than the boost circuitry you mentioned, everything has already been implemented in his circuit.

  5. @Colecago, @vtl – Yes, using a multimeter would most likely be a bit more helpful in determining the exact voltage but lets put a scenario into play here:

    Scenario 1:
    Imagine yourself out and about when you spot this new ‘Used Junk/Electronics Store’. You say to yourself
    “Hmm… used junk eh? I’m gonna have to check this out.”
    You walk into the store to find a shelf with unmarked power supplies, then you think
    “I’ll just pull out my trusty (cumbersome) multimeter and check out what this thing puts out!”
    You fumble around a few moments then you figure the voltage is ~12.3V.

    Scenario 2:
    Imagine yourself at a friend’s apartment whom had just purchased an unmarked power supply. You ask him what its estimated electric potential energy is and he fumbles around with his trusty multimeter. Meanwhile you know you have your “Headless Electric Potential Estimator and Polarity Analyzer” (HEP-EPA for short) in your pocket for times like these. Before he gets a chance to plug his multimeter in to check, you’ve already determined its polarity and made sure it is between 11.5V and 15V, which is ideal for your HAM equipment.

  6. Seriously, I’ve got an autoranging multimeter that’s the size of a thick credit card and it also checks diodes, fuses, and reads resistance…

    So, now ypu’re at that swap meet and you find a table with some transformers and power supplies. You quickly determine that the poer supply is borked due to a bad fuse, short the fuse and determine itis a 14.5 volt ac transformer and use it to determine that the transformers are actually 8k:16ohm output transformers. Serious win for the guy with the $9 pocket multimeter.

    Honestly, this is a solution looking for a problem.

  7. if you are allowed to open up the radio without voiding warranty or fcc license you can check the wires from the plug to the board.

    usually the + is red, connected to the fuse

    the – is usually connected to the ground of the device and to the traces on the board that run along the outter edge of the board.

    a good radio will have a diode to protect it in some way.

  8. Great hack good sir, I built a similar but less sophisticated one of these 19 years ago when I became a Ham. Back then we weren’t aware of Anderson Powerpole connectors, and we used Molex connectors on our low voltage emergency Ham radio gear, so I took a new connector and I soldered in an LED that lit up Flashing Red for incorrect polarity, Green for correct, and Orange if AC was on the line. I fit all the components inside the Molex connector, so all one could see was the LED, it was sick.

    Of course, this was for emergency communications where out in the field one may find equipment/power supply’s that are missing polarity information, and my tiny Molex LED test connector took up virtually no space and saved my gear more than once, it’s no toy. I also built and carried a tiny polarity reverser Molex connector/adapter so if I did encounter it I was prepared for it.

    Yes, I have an ancient Fluke 87 but no way am I taking a DMM no matter how small, out on an emergency communications event where every ounce counts against you in the field, my fail safe LED test connector is smaller and more robust.

  9. This obviously won’t do anything that a multimeter can’t, but it’s the sort of thing that’s easier to use in less-than-ideal-conditions (in a hurry, in the dark, etc.)

    Obviously if you never leave the confines of your brightly-lit on-the-grid workshop, this is just a toy.

    (Also, the metaphor is inexact, but just because you have a pair of vise grips doesn’t mean that you shouldn’t have a hammer and a decent set of wrenches or any of the other tools that vise grips are a poor but functional substitute for.)

  10. >…emergency communications event where every ounce counts against you in the field,

    Drama much? I was unaware there was a weigh-in on Field Day, much less actually deducting points for extra weight. Maybe you plan to fly to your Field Day date and are afraid of excess baggage fees?

    That being said, I own a half dozen multi-meters and still use this: regularly. Chill with the hatin’.

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