Many analog circuits, for example opamps, require a negative power supply rail. e.g. 5V, 0V and -5V. This instructable describes how to create a -5V supply from a 5V and 0V only supply. This is particularly useful if you are using power supplied by a microcontroller board like the arduino.
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Step 1: Use an ICL7660S CMOS Voltage Conversion IC.
There are a number of circuits that will do this job. Some folk on the web seem to use 555 timers and some recommend rs232 TTL conversion chips. But the simplest solution seems to be a negative charge pump IC. I used an intersil 7660S (at least thats what I think it is - the label is very faint) from maplin. Here's a link to its datasheet:
1x icl7660S voltage converter.
2x 10 micro F electrolytic capacitors
Some breadboard and a few wires.
The IC pin connections are as follows (these come from the datasheet):
V on pin 8 and GND on pin 3
Connect the first capacitor between pins 4 and 2. The positive leg should be connected to pin 2.
Connect the second between pins 5 and 3. The positive leg should be on pin 3.
The negative power supply should be available on pin 5. This is the yellow wire in the photo.
I don't know how much current will provide but it should be enough to power an opamp or two. It probably says that somewhere in the datasheet.
The voltage levels it produced on the negative pin mirrored the positive pin to within 0.01 of a volt.
When I wired this circuit up to power an op-amp dealing with some audio, I found it produced a high pitched squeal at around 4.6KHz. Its a ripple voltage on the -5V output that is caused by the oscillator inside the chip. This is quite annoying, however the chip provides a boost pin (pin 1) that fixes this. If you wire pin 1 up to the +5V supply, the oscillation frequency is increased by a factor of about 4 or 5 and lowers the ripple so that it becomes inaudible. That wiring isn't in the breadboard image above. Adding a larger electrolytic capacitor on pins 3 and 5 also helps reduce the ripple. For my project, that wasn't necessary.
Step 2: Put It in Your Project
You can then build it into a power distribution board so that you can use the supply anywhere in your project. The photo contains a picture of a distribution board that will supply 5V, 3.3V, GND and -5V. The microcontroller board supplies the positive and ground. The chip gives me a negative that I will use in my next op amp circuit.
I've also placed the board in a central position in the enclosure to make it easier to supply other components.