For example, with just relays and a few
diodes, you can build a circuit to work with
your existing alarm or keyless entry to start your vehicle. Add a capacitor and a
resistor to a relay and a diode and you've
created a timer to use with it. Or, by adding a relay to your head unit's remote turn on wire,
you can turn on several amplifiers, processors, lights, etc.
With Ohm's Law, you can calculate current, power,
voltage, and resistance such as the nominal
impedance load of multiple speakers to an amplifier, and much more.
Once you master
these, you'll think of all kinds of things to create and
add to your systems as well as your customers'.