It would be better to use an EEPROM, but I already had some UV erasable EPROM's.
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Signing UpStep 1: Parts
-An EPROM (in my case a M2732A-4FI)
-LEDs and resistors for them
-A 5V power supply
-Wire
-Bread Board or any other board
-if you want, you can add switches







































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As for getting 21V cheaply, simply, and easily, you could try a couple of voltage doubler circuits hooked up to an oscillator. 5V from, say, a 555, doubled to 10V, then to 20V. The current would be minimal; check the datasheet for the exact specs on your chip.
OR....
Hook some 9v batteries in series: 9v, then 18v, then add two AA batteries for 21v. Might want to test it with a multimeter first to make sure the batteries aren't too high to start with.