Two switches, two rails
The standard board has one slide switch per side of the breadboard. Each picks 5 V or 3.3 V for its own pair of rails and lights a red or a blue LED to say which. The two sides are independent, and both share one ground.
One switch per side
The left switch sets the left pair of rails and the right switch sets the right pair. Slide one towards the 5V printed beside it and that side's rails carry the USB supply, straight from the diode. Slide it towards 3V3 and they carry 3.3 V from a small regulator in the middle of the board.
Each switch has two poles, and the second one is what lights the LEDs. With the slide on 5V, a red LED beside the 5V label lights; on 3V3, a blue LED beside the 3V3 label. Four LEDs in all, and on a working board exactly one per side is lit.
The LED is the check
The switches are small and easy to knock, and the two positions look the same from most angles. The LED does not. Before you plug in anything that minds its voltage, look at the LED on that side: red is 5 V, blue is 3.3 V.
This matters most for 3.3 V parts. An ESP32 module, most modern sensors and many displays are built for 3.3 V and are damaged by 5 V on their supply pin. Wire them to a side showing blue.
Why two independent sides
Because the common mixed circuit has both: a 5 V part — a relay module, an HC-SR04, a character LCD — and a 3.3 V board reading it. With one side at 5 V and the other at 3.3 V, each part gets its own rail, and the − rails are one ground so the signals between them mean something.
That shared ground is not a detail. A signal is a voltage measured against ground, and two parts that do not share one cannot agree on what a HIGH is.
Mixing voltages still needs care at the signal wires. A 5 V output into a 3.3 V input is the same problem as 5 V on its supply pin, just smaller. A level shifter or a divider belongs between them — see the reuse links for how.
When it does not work
A slide switch left partway between its two positions can connect neither: no voltage on that side's rail, and no LED. Push the slide firmly to one end. If it is at an end and still dark, check the other side — if that is dark too, the board has no power, and the cable or the fuse is the place to look.
Towards the 5V label printed beside the switch. The LED is the part to trust: red means 5 V is on that side's rails, blue means 3.3 V. Check it before you plug in a part that minds, and measure with a meter if you are not sure.
It will not hurt the adapter, but it changes the voltage under whatever is wired to that side, and the slide passes through a moment with nothing connected. A microcontroller will reset; a 3.3 V part switched to 5 V can be damaged. Unplug the cable, move the switch, then plug in again.
Yes. All six − pins are the same ground, the USB supply's ground. That is what lets a 5 V part on one side and a 3.3 V part on the other talk to each other: signals need a shared ground to mean anything.
Both 3.3 V sides share one small regulator, and it is smaller than most people expect.
The 3.3 V budget →Edit this page — content/books/bbpwd/two-switches-two-rails.mdx
Questions about this product
See what other owners have asked, and read their solutions.
This page covers several products. Choose yours to see the right discussions.
Discuss this article
Ask about this page. The answer stays here, on the page it belongs to, for whoever hits the same wall next.