Power from the adapter

The module and its two pairs of rails

The module is the standard Lonely Binary breadboard power adapter. It plugs across the end of the breadboard, with a block of pins in each pair of rails, and a slide switch for each pair decides whether that pair carries 5 V or 3.3 V. A red LED beside a switch means 5 V and a blue one means 3.3 V.

Both ends print + then −, reading left to right, and each pin goes into the rail the breadboard marks the same way. On most boards one way round is right for both pairs. If your board's rail printing is mirrored, one pair will be backwards whichever way you turn the module: the module's which way round page covers that case. This book assumes the ordinary one.

Take the cable out before you move a switch. The slide passes through a moment with nothing connected, and whatever is on the rail sees it.

What the meter reads

The 3.3 V is made on the module and sits at 3.3 V. The 5 V is not made at all: it is the USB supply, less a diode and a fuse between the socket and the rail, so it reads a little under 5 V, often about 4.7 or 4.8 V with a light load. Parts built for 5 V accept 4.5 V and up, so nothing is wrong. It does mean that the experiments calculate with what the meter says and not with 5.00.

The module's own handbook goes through why: why the 5 V rail reads low and the fuse and the diode. A USB port may put out anything from 4.75 V to 5.25 V, which is why your reading will not be exactly the one in the figure.

How much it can run

Two limits, and they are different. The whole module comes through one resettable fuse that holds half an amp, whichever pair of rails you are using. And the 3.3 V is made by a small regulator rated for 200 mA, shared by both pairs. A 3.3 V regulator of this kind draws from the 5 V rail the same current that it supplies, so the 3.3 V pair's current is part of the half amp too.

An LED with its resistor takes about 10 mA, so the 3.3 V side can light about twenty and the whole module about fifty, with some left over. A motor, a relay or a Wi-Fi board takes far more: for those, see the 3.3 V budget. For every experiment in this book the module is far inside its limits, and the fuse never has to act.

The first power-up

Do this once, before anything else is on the board. It proves the module, the cable and the meter in a minute, and it means that any trouble later is in the circuit and not in the supply.

Run it on your own module. If the reading is outside 4.5 to 4.9 V on 5, or well away from 3.3 on the other setting, swap the cable and the charger and measure again before suspecting the module. When both readings are right, unplug the cable and move on: every circuit in this book is built with the power off.