Initially I was tempted to buy a ready made one at DealExtreme, but as that did not seem to be identical to the one that they pictured, I decided not to. the problem was that the one in the picture did not cover the breadboard, whereas the one that was send out -according to customer reviews- díd cover the breadboard partly. Add to that that I would have to wait for the delivery if ordered at DealExtreme, I decided to make one myself, a better one than before and the one at DealExtreme gave me some inspiration.
This circuit is a 5 and 3.3 Volt PSU that can easily be used for mini breadboards. It is a more or less standard 5 and 3,3 Volt circuit, but the trick is in the PCB that can be inserted in the PSU rails of the breadboard. The breadboard I am using has a top and a bottom rail that are isolated from each other. With this circuit and -and not in the least the PCB- it is possible to give each rail a different voltage (5 or 3.3V) or have both rails at 5 Volt.
The circuit is fed from a wall-wart. Though I have soldered the wires in place, there still is a diode for reverse polarity protection as many wall-warts can be reversed (in polarity) by a switch. The Vin from the 7805 (and LF33) does not come with a big capacitor but it has a small decoupling capacitor. The 5 Volt has again a decoupling capacitor and a 100uF for smoothing the signal. An LED with current limiting resistor indicates proper functioning. The 3 connectors surrounding the 7805 are forming a 3 pin header on which the raw Vin, the ground and the 5Volt signal are available.
The 3.3 Volt from the regulator is smoothed with a 100uF capacitor and led to a jumper block that selects between 5 and 3.3 Volt. Most breadboards have two power rails that each have a positive and a neutral. In making a PSU that inserts securely in the breadboard it has to use at least 3 spaces. Initially I tried with one, but that just led to enormous frustration. Because 3 positions deep is a lot to lose on a small breadboard, the PCB need to have some 'arms' to insert in the power rails, leaving the rest of the breadboard free and uncovered. The Fritzing file of the breadboard is found here. If you don't have fritzing and don't want to install it, the print desgin can also be found here. After etching, carefully remove the section of the PCB that falls over the body of the breadboard. Do it such that a minimum of material is removed to keep the arms sticking into the breadboard remain as sturdy as possible. Then mount the parts and solder those. For the two 2x3 pinheaders that stick in the breadboard, make sure that they match up before soldering them.
The use of the breadboard is not completely full proof: switching between 5 and 3.3 Volt for one rail is done with a jumper and the only indication that 3.3 Volt is in use is an LED that is a bit less bright. If you use both voltages and would still have the rails connected by a wire of some sort, then there will be a short. also, when using both 5 and 3.3 Volt parts, make sure to not mix up the voltages and blowing up some parts.
2x 100nF capacitor
2x 100uF capacitor (10V)
1x LF33CV (or other 3.3 Volt regulator)
3 prong male pinheader 2x
2x 1k resistor
Just a remark on the 100uF capacitors: I used those because I had them lying around. I think the datasheet of the L7805 only requires a 33uF capacitor and frankly I also have used then without a capacitor :-)