Fork me on GitHub

Project Notes

#027 Power Supply - LM7805 Stable 5V Source

Demonstrates the LM7805 5V regulator, using an Arduino to measure the voltage supply and plot the results with Processing.


The LM7805 provides regulated 5V, ideal when current requirements are moderate (up to 1A with heatsink). Note that other chips in the LM78xx range are available to provide other voltages.

For the chip under test, we get a stable voltage just a tad under 5V, as measure with a multimeter and the Arduino:

Vout Derivation
4.98 actual (DMM)
4.982 actual (Arduino)

Here’s a sample trace of the voltage 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 pin is used to read Vout via a voltage divider (Rd1/Rd2). The voltage divider is to ensure that the Arduino never sees more than half the battery voltage (4.5V) on the analog pin.

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

About LEAP#27 Power
Project Source on GitHub Project Gallery Return to the LEAP Catalog

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

LEAP is my personal collection of electronics projects, 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 (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.