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.

PIC programmable power supply

This programmable power supply is the perfect addition to your bench tools. [Debraj Deb], who previously built a whole house power monitor, designed this build around a PIC 18F4520 microcontroller. The desired voltage is set with an attached keypad, resulting in a digital output on the 8-bits of port D. The port connects to another protoboard with an R-2R digital-to-analog converter resulting in the target voltage. A set of transistors amplifies the current and a power transistor then takes care of the final output. After the break you’ll find two videos, the first walks us through the hardware and the second demonstrates the device in action, along with measurements of its performance. This certainly provides a lot more functionality than an ATX power-supply conversion.

Update: A big thanks to [Debraj] who sent us a code package as well as the schematic (PDF) used during testing. We’re having trouble getting the code package up for download right now. Check back later, hopefully we’ll have it up soon.

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LED battery level indicator

[Kc7fys] came up with a this simple battery level indicator. It uses a single LED to display a battery’s voltage; if the voltage exceeds 12V, it glows green. If it is below 11V, the LED glows red. Anything in between generates an orange glow. The meter is built around an LM358 chip per this schematic, but his actual build looks pretty sloppy because of the dead-bug assembly (check out NASA’s pretty version). Nonetheless, it works, so clean it up and build one if you want to put it (or your batteries) to the test.

Tube amp monitor

Sparkfun contributor [Pete] really loves tube amps, but he’s a very safety-conscious guy who doesn’t like being electrocuted. This is a problem, since tube amps are usually very high voltage, and a small mistake can be fatal. To deal with this voltage issue, he built a tube amp with a control system built around a 6DOF v3 controller board. The control system is there mainly in case of a failure, automatically shutting off the high voltage transformer in any such event. It has the added benefit of filtering any 60Hz noise from getting into the audio, which happened before he installed the control system.

In addition to regulating power, the controller board also monitors bias points in the output tubes and displays its diagnostics on an LCD. Aside from getting great sound from the tube amp, [Pete] made it look great too, installing colored LEDs under the tubes. We love his design: just because safety comes first it doesn’t mean cool-factor can’t come in a close second.