I've just finished constructing my first ever circuit board - it's one of the Jaycar "Short Circuit series, a "Knight Rider" LED scanner. It runs off a 9V battery, but I was wondering whether it could be powered by molex, so I could run it from my computer? I know Molex supplies 12V, rather than 9V, but I'm unsure whether the extra volts would damage the circuit...
On some of those basic circuits there can often be a pretty wide tolerance. Do the instructions specify an acceptable voltage range? Can you post a circuit diagram?
Simplest solution without knowing the characteristics of your circuit is to just use a 7809 regulator running off the 12v line to supply the 9v you are replacing.
Even from Jaycar a 7809 is only a couple of dollars. http://www.jaycar.co.nz/productView.asp?ID=ZV1509&form=CAT2&SUBCATID=974#1
TO-220 packaged 7809 from Jaycar, looking at the device from the front, middle leg to ground, left leg to the 12v, and the right leg gives you the 9v you want.
MOLEX Yellow -> 7809 Input MOLEX Black (either one) -> 7809 Gnd (middle leg) 7809 Output -> redwire of the old 9v connector MOLEX Black (the other one) -> blackwire of the old 9v connector
What is the part number of the 3 pin part at the one end of the board? Is it an LM7805 voltage regulator for the TTL ICs? And the LEDs are likely driven to 0V by open collectors so have a series resistor to +9V. What is the LED resistor value? LEDs do fine with up to 30mA (20-25mA is better) so if the LED resistors are over about 350 Ohm then you will be fine, otherwise you will have to change them maybe 470 Ohm of 510 Ohm.
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