Twelve volts was a big deal when domestic manufacturers adopted it in the '50s. Doubling their voltage from 6 made electrical systems comparatively efficient and the increased power ushered in a new era of luxury and convenience.

Understanding that, what could possibly compel any of us to make a device that reduces the operating voltage in any part of our cars? Oh let us count the ways.

Because not everything in an early car can be converted to 12 V, the aftermarket offers convenient voltage drops. But convenience comes at a price: the cheap ones are notoriously inefficient and don't work on everything; the ones that will work on everything cost quite a bit of money. But we can build our own versions for a fraction of even the cheap ones. That's reason one.

Several automakers clung to 6V instruments for decades after they adopted 12V charging systems. Being mechanical, the voltage drops they employed eventually wear out. Replacements aren't cheap and being copies of the originals means they wear out too. But we can build solid-state versions that promise to last probably forever in automotive terms and for considerably less money. That's reason two.

Reason three is a result of new technology. Even the most technophobic among us rely on GPS to get around or listen to our MP3 players through our in-car radios. If you're one, you can bear testimony that their power supplies look bad and take up a lot of space.

That needn't be the case. Most of these devices rely on the Universal Serial Bus (USB) 5V standard so an oversized central power supply and USB ports wired into the car could replace all those warts-only you have to make them. And we can do just that for about the cost of a fast-food combo meal.

This and next month we'll show you how to address these issues. As most of us aren't electrical engineers, we stripped these tutorials to their bare essentials. In some cases all you have to do is copy the designs.

These solutions cost a fraction of what store-bought devices cost-provided they even exist. And let's face it, champagne taste on a beer budget is what motivates most of us. Think of it as hot rodding, geek style.

The electrical version of a thumb pressed over the end of a garden hose, resistors restrict the amount of power that can pass through a circuit. While resistors still enjoy great popularity as current regulators for wiper and blower motors in cars swapped to 12 V, probably their greatest utility nowadays is in lighting.

We'll use LEDs for illustration because (a) they operate at little voltage, (b) they're useful as all hell, and (c) their current draw remains consistent. Consistency is especially important because we calculate a resistor's values upon the device's current draw.

Here's what a resistor looks like in real life and in schematic:

Resistor calculation relies on five elements:
• Vs or supply voltage is your car's voltage
• Vf or device voltage is the device's voltage rating
• I or device current is the rate of flow expressed in amperage
• R is the resistor's impedance expressed in ohms
• W is the resistor's power capacity expressed in watts

For our purposes let's say we want to use a 3.6V, 25mA resistor in a car that has a 12V charging system. That the system's voltage exceeds the LED's voltage means we have to choose a resistor to protect it.

First we must establish by just how much the circuit's voltage exceeds the device's voltage. To do that, we subtract the device's voltage (3.6) from the 13.6 V that a car's charging system produces (we call it 12 V but charging systems can nudge 14 V, so better to calculate for the maximum realistic voltage).

The voltage difference between 13.6 and 3.6 is 10. To determine how much impedance a resistor needs to prevent that extra 10 V from killing our LED we divide that figure by 0.025, the numeric expression of 25mA. A 400-ohm resistor would knock the extra 10 V from our 13.6V supply.