This project comes from my desire to have a simple power supply for my electronics prototypes without having to build a regulated power supply for each of them. This will free up some valuable real-estate on the breadboard when I'm prototyping and, being a switchable design, means that I can quickly, and easily change from source to 5V or 3.3V with a flick of a switch.

I've also built in a connection to a 3 wire digital voltmeter (such as this one on eBay) so that I have an easy to read aide memoir ... so I don't blow up sensitive components that require 5 or 3.3V.

The overall design is pretty simple and has a pretty low component count.

I built this for just over $2 from parts that I've purchased from eBay over time.

To build this circuit, you'll need the following parts:

  • 1 x DC Barrel Jack ($0.32)
  • 3 x 100 nF Electrolytic Capacitors ($0.09)
  • 1 x LM7805 5V Regulator ($0.44)
  • 1 x LM1117 3.3V Regulator ($.40)
  • 1 x toggle switch ($0.28)
  • 1 x 3P1T Horizontal Slide Switch ($0.15)
  • 1 x 2P Female Socket ($0.13)
  • 1 x 3P Female Socket ($0.14)
  • 1 x 4P Female Socket ($0.14)
  • 1 x 80 x 50 mm single sided PCB

All prices are in AUD and are the price per item that I paid when buying on eBay. The total cost is $2.37. Additionally, the Voltmeter cost me $1.88, bringing the total cost to $4.25.

Step 1: Design

The overall design of this circuit is a waterfall supply. The source power is reduced by the LM7805 Regulator (5V), and the 5V is reduced by the LM1117 Regulator (3.3V). To provide a switched supply, the source power is connected to Pin 1 of the 3P1T switch, the 5 Volt supply is connected to Pin 2 and the 3.3 Volt supply is connected to Pin 3.

The Common pin of the slider switch is connected to output pins 1 and 3 while GND is connected to output pins 2 and 4. This gives 2 pairs of Power and GND output to connect to the 2 pairs of power rails on your breadboard.

Because there are 2 regulators in series, you can drop one of the 100 nF capacitors that would be required if these regulators were used individually, that's because the output of the LM7805 is already "smoothed" by its 100 nF capacitor. When I tested this circuit on the breadboard, it appeared that the 100 nF capacitor on the LM1117 wasn't actually needed, without it I still got a pretty steady output.

I'm going to connect a 3 wire digital voltmeter (when it arrives) to the circuit so that I can have a spiffy voltage display happening. To accommodate this, I've provided a 3 pin female socket at the bottom of the board. The voltmeter takes power (and ground) from the source power supply and is connected to the 3P1T Common/Output Pin 1/3 trace so that it will measure the voltage on the output side of the circuit.

As I haven't got the module yet ... I haven't pre-drilled the mounting point. I'll do that when the module arrives.

This is a single sided PCB design and I have 5 jumpers on the board. I have purposely left a lot of blank space in the bottom middle of the board to accommodate the voltmeter module.

The irregular shape of the board came about because the PCB was originally intended to go into a small (80 x 50 mm) project box. This circuit isn't going into a project box, though.

Step 2: Test Output

I connected output pins 1 and 2 to my multimeter and then power the circuit up with a 12V DC wall wart.

The first image shows the output where the switch is set to Pin 1. This will pass-through the power straight from the wall wart to the output pins. Here, my wall wart supplies 12.23 Volts

Picture 2 shows the output when Pin 2 is active. This is the output from the LM7805 and gives me 4.98 V.

Picture 3 shows the output when Pin 3 is active. This is the output from the LM1117 and gives me 3.28 V.

Finally, the last picture simply shows the output when the circuit is turned off ... that is, when the toggle switch is switched off. No power ... nothing to see ;)

When I get the digital voltmeter module, I'll take some more happy snaps and update this instructable.

Anyway, that's it for this instructable. I hope that you find it useful.

Step 3: Added Digital Voltmeter

As promised, here are some images of the mini digital voltmeter added to the circuit.

I'm reasonably pleased with the voltmeter and the overall performance of the circuit. Of course, the voltmeter will be shoehorned into other projects too! (such as the 12V NE555 PWM Controller circuit).

Well, that's it for now. I'm moving on to some other projects that I have on the drawing board.

<p>Not bad, thanks for sharing. However, these days I'd recommend that you use switching power supplies for higher efficiency. They are noisier, but for digital circuits they are ideal.</p>
<p>Thanks carlos66ba. The cost and size of this circuit make it more suitable, but thanks for the recommendation.</p>

About This Instructable



Bio: I have been working in IT since the mid 1980's. Most of that has been database and application development. I've been working on ... More »
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