JohnW
(John)
The lights would have a micro controller based controller in them. These days it is way cheaper to use tens of thousands of transistors running some software in a micro-controller to do a simple job, even though it seems like overkill. A small embedded processor and a transistor costs much less than a dollar in volumes (cheaper than a mechanical switch) and once you've written the software, the per unit manufacturing cost for the software is zero. Once you are writing software it is pretty much free from a manufacturing point of view to add more features.
Why use a grounded line instead of a powered one? It is safer for the circuit.
One of the IO pins of the computer will be pulled up to 5V (or maybe 3.3V - called Vcc either way) by a high value resistor, so it is normally at Vcc. The IO pin itself will be in high-impedance mode (basically almost not connected to anything). As long as the pin stays between 0 and Vcc it draws almost no current (as in much less than 1 millionth of an amp) . The controller will detect when it is pulled to ground and probably cycle through one of its flashing options each time.
The key is that it is far more likely that a signal wire will accidentally short to ground than short to 12V. With this configuration, shorting to ground is normal and does no damage. A well designed signal circuit like this will also not be damaged by a short to 12V either since another high-value resistor will be used to isolate it. Signal circuits like this typically draw less than a milli-amp so there is no real danger either way.
When electronically switching power circuits in a vehicle, the standard is to run 12V to the device (light, solenoid, fuel injector, ignition coil, motor, etc), and ground it via a transistor inside the electronic control module. With a circuit like that, an accidental short to ground in the wiring will turn on the device, but not damage anything. A short in the 12V line before the device would just blow a fuse.
If it were wired the other way with the device grounded and the 12V power being applied by the big transistor in the electronic control module (the way things used to be wired with mechanical switches), a short to ground either in the wiring or inside the device, would cause a current spike that could destroy the transistor before the fuse blows and wreck the electronic module.
As previously mentioned, it does not matter where in the circuit that it is opened up. It can be at the ground end, the power end, or anyplace in the middle.
To add to that, remember that when they discovered electrons they got the charge wrong, so it turns out that electrical current (the electrons themselves) actually flows from negative to positive even though we almost always think of conventional current that flows from positive to negative just because it makes more intuitive sense for electricity to flow "downhill" (from positive to negative). In most cases, it makes absolutely no difference which way you think about it.
The most common exception for metal workers is when welding since there is significantly more heat transfer in the directions that the electrons are actually traveling in an arc (from negative to positive). Hence DC TIG welding normally has the electrode negative so the arc is transferring heat to the material instead of melting the end of the tungsten.
Why use a grounded line instead of a powered one? It is safer for the circuit.
One of the IO pins of the computer will be pulled up to 5V (or maybe 3.3V - called Vcc either way) by a high value resistor, so it is normally at Vcc. The IO pin itself will be in high-impedance mode (basically almost not connected to anything). As long as the pin stays between 0 and Vcc it draws almost no current (as in much less than 1 millionth of an amp) . The controller will detect when it is pulled to ground and probably cycle through one of its flashing options each time.
The key is that it is far more likely that a signal wire will accidentally short to ground than short to 12V. With this configuration, shorting to ground is normal and does no damage. A well designed signal circuit like this will also not be damaged by a short to 12V either since another high-value resistor will be used to isolate it. Signal circuits like this typically draw less than a milli-amp so there is no real danger either way.
When electronically switching power circuits in a vehicle, the standard is to run 12V to the device (light, solenoid, fuel injector, ignition coil, motor, etc), and ground it via a transistor inside the electronic control module. With a circuit like that, an accidental short to ground in the wiring will turn on the device, but not damage anything. A short in the 12V line before the device would just blow a fuse.
If it were wired the other way with the device grounded and the 12V power being applied by the big transistor in the electronic control module (the way things used to be wired with mechanical switches), a short to ground either in the wiring or inside the device, would cause a current spike that could destroy the transistor before the fuse blows and wreck the electronic module.
As previously mentioned, it does not matter where in the circuit that it is opened up. It can be at the ground end, the power end, or anyplace in the middle.
To add to that, remember that when they discovered electrons they got the charge wrong, so it turns out that electrical current (the electrons themselves) actually flows from negative to positive even though we almost always think of conventional current that flows from positive to negative just because it makes more intuitive sense for electricity to flow "downhill" (from positive to negative). In most cases, it makes absolutely no difference which way you think about it.
The most common exception for metal workers is when welding since there is significantly more heat transfer in the directions that the electrons are actually traveling in an arc (from negative to positive). Hence DC TIG welding normally has the electrode negative so the arc is transferring heat to the material instead of melting the end of the tungsten.
