112 Nicolas Collins
Note that cheap wall-warts will always have some AC ripple in their voltage
output. And that a circuit powered from such a supply may “hum” slightly.
Until you get good at designing or finding better power supplies a battery will
usually sound cleaner. One easy fix that sometimes helps is to add a big
capacitor, say between 100-100uF, between + and - supply on your circuit board
(Figure 1.) Big capacitors, called electrolytic capacitors, have polarity, just like a
battery, which is marked on the body. Make sure you connect - to -, + to +.
Placing an additional 0.1uF capacitor between the + and ground supply pins of a
chip also helps lower noise and reduce “crosstalk” between different parts of
your circuit.
Finally, you should add a “Regulator” chip, as shown below:
7809 RegulatorThis chip filters out the last of the ripple and sets the voltage to a precise level,
which you specify when you select the chip. A “7809” will put out 9 volts, just
like the battery we’ve been using for our circuits. It needs an input voltage of at
least 3 volts more than it puts out (i.e., 12 volts), so measure the output of your
wall wart with your meter to make sure it is high enough. You can get
regulators for a wide range of voltages, should you need them for future projects,
but these basic design principles remain the same. Bolt the regulator tab to a
piece of metal to help keep it cool.