position, i.e., two terminals, on or off, it’s a single throw (ST). If the switch has two positions, i.e., three terminals, it’s a double throw (DT).
The relay shown here is a SPDT (one switch, two positions). You can see that there are five numbered terminals. Two of them are used to activate the switch, and the other three make up the two-pole (double pole) switch. When you power up the relay (putting 12V across 85 and 86), it activates a magnet that pulls a switch inside the relay, and this switch then connects 30 and 87. If you disconnect the voltage between 85 and 86, the magnet stops, and the spring-loaded switch clicks back to its normal position between 30 and 87a.
You can connect relays to make complex conditional switches (“If the car is on and the window is down and the trunk is open, then honk the horn”). For some examples of interesting car relay applications, visit the12volt.com/relays/relays. asp.
Diodes are electronic components that ensure current only runs one way. They have a stripe indicating the negative terminal. The current can only flow from plus to minus, so you should put the stripe toward the direction you want the current to flow.
Let’s say you wanted to add a switch that keeps your radio on, even if your car’s keys are removed. You could just run a 12V line from your battery, through a switch, and to the “ignition” input on your radio. If you flipped that switch, your radio would turn right on. The problem is that current would flow back through the ignition wire and back into all the other devices it powered. The solution is to use two diodes: one between your new switch and the radio (stripe toward the radio) and another between the ignition and the radio (stripe toward the radio) to ensure current goes to the radio only.
Parallel ports are an easy way to control switches from an in-car computer (see MAKE’s Halloween Project, this volume, page 86). A parallel port has eight pins that can be set at 5V or 0V. Using a resistor and a small 5V “reed” relay (such as a RadioShack 275-232 or 275-310), you can program-matically activate or deactivate a switch by setting 0 or 1 on a parallel port line.
If you’re triggering a light load, such as a small
light, you can just connect the 12V through the relay. If you’re trying to switch a larger load, such as headlights or the door locks and windows, you’ll want to chain the relays together. The tiny 5V relay will pass 12V to the big 12V 30A automotive relay, which will control your heavy load.
You don’t have to hack a parallel port. There are many USB, serial, and parallel port-based relay controllers you can purchase and plug into your PC. These are already safety-fused and easy to program, and include example code. For instance, ontrak.net makes the ADR2010 that I used to control my Nash’s windows via an onboard computer.
I wanted to be able to activate and access my cars’ PCs from my house, so that I could connect via Wi-Fi to download music and grab email (via my company’s CarBot software) to be read to me as I drive. Instead of keeping the in-car computer on continuously (which would kill the car battery),
I bought a pager (service is about $4/month) to connect to the “ignition” switch of the in-car PC. The power supply for the PC (an M1-ATX from mini-box.com) has a mode where it will turn on and then stay on for two hours, giving me time to do what I need to do.
By rigging the pager buzzer up to the ACC/igni-tion pin on the PC’s power supply (isolated by a diode, so that ACC doesn’t fry the pager), I can turn on the PC in my car from anywhere in the city.
The PC, running Windows XP Pro, is configured to automatically connect to the internet (via Wi-Fi or EVDO) and run Trillian, a popular instant-messag-ing client. I can now dial a pager number to make my car computer wake up, go online, and start up IM — at which point I can log in and start controlling the machine.
POWER WINDOWS HACK I decided to upgrade my old Nash with power windows. The kit I purchased online took the good part of a day to install but was very simple electrically. It’s just a motor with two wires. If you apply 12V to one wire and ground to the other, the window goes up. Reverse the connection, and the window goes down. The rocker switches that came with the kit perform this reversal when you manually activate them, but to trigger this up-down with a computer required a more complicated arrangement of relays. You can see from the diagram (next page)
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