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#028 MB102 3.3/5V Breadboard Power Supply

Demonstrates an 3.3/5V MB102 Breadboard Power Supply, using an Arduino to measure the voltage supply and plot the results with Processing.

The Build


Various 3.3/5V MB102 Breadboard Power Supplies are a widely available for providing regulated 3.3V and 5V for projects on a breadboard. For example, a search on aliexpress will find many sellers, and prices can be lower than $1USD a unit.

MB102 unit

It has some nice features (aside from the price!):

  • master on/off switch and status LED
  • power via 2.1mm plug or USB
  • power to each breadboard rail independently selectable as off, 3.3V, or 5V.

The test circuit simply drives 3.3V on one rail and 5V on the other, both powering LEDs. The Arduino measures the voltage of each rail through voltage dividers.

Measurement results:

Selected voltage Vout (DMM) Vout (Arduino)
=—————– ———— —————-
3.3V 3.33 3.31
5V 5.00 4.99

Here’s a sample trace of the two voltage rails as measured by the Arduino:

processing trace

Measure Voltage with an Arduino

Reading voltages with analog pins presents two challenges:

  • The internal ADC has 10 bit resolution (0 - 1023) with 2 bit precision, so the accuracy is 0.25%
  • The internal reference voltage (i.e. what 1023 will equal in volts) is nominally 5V but in practice is not very accurate (the board used for this test runs at about 4.93V)

For our purposes here, the ADC accuracy is not a big problem. To overcome the reference voltage accuracy issue, the sketch scales the voltage reading according to the actual reference voltage of the board. This must be measured with a multimeter and the vref_mv constant changed to this value (in mV).

For more detail, John Errington’s Experiments with an Arduino includes an excellent study of Precise voltage measurement with the Arduino.


The Arduino only acts as a measurement device in this circuit. Analog pins are used to read Vout via voltage dividers (Rd1/Rd2 and Rd3/Rd4). The voltage dividers are to ensure that the Arduino never sees more than half the battery voltage (4.5V) on the analog pin.

LEAP#090 PlotNValues (a simple Processing sketch) reads the data from the serial port and plots the input and output value over time, with some coloration effects thrown in for good measure. In other words, we’re using Arduino and Processing as a basic oscilloscope! And it kind of works, mainly because the frequency is so low.

The Breadboard

The Schematic

The Build

Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.