One of the advanced features of the Infinitybox system is the ability to have a simple connection to almost any switch that can be used to start the car. There are literally thousands of switches that could be used, allowing each builder to select one that meets the design aesthetic of their car.
This is a great feature, but there is a potential problem. First, a little background- as documented in the Infinitybox manual, all inputs to the Mastercell must be ground-switched. In other words, the switch for all inputs must close the circuit, using the wire from the Mastercell, to ground. Any externall-sourced voltage on the circuit to the Mastercell can damage it, or more subtly, cause erratic action of the switch.
It's that erratic switch action that concerns us here.
It's vital that the switch you choose doesn't alow any stray voltage to the Mastercell. In the simple case of an unlit switch, this is pretty easy. But some of the fancier switches from the OEMs have been known to leak voltage from the lighting circuit of the switch to the actual switching circuit.
Such leaks are bad, not only because there is the possibility of damage to the Mastercell, but more importantly, because they can confuse the Mastercell into thinking that the switch status has changed. The result? Your engine might start, even with no one in the car. If you left the car in gear, and didn't have a clutch safety switch installed, your car could start, and drive itself into the nearest solid object. Like another car at a car show, which would make your new car a star on the Internet fail video circuit. You don't want that.
So, for engine start situations, it's especially important not to allow any stray voltage to the Mastercell input. Even small transients can fool the Mastercell.
Really, any switch with any kind of light (an LED surround, or lit-up status lights, etc) is a potential candidate for causing this problem. Some switches where this has been known to be a problem include the popular Honda S2000 switch, the Nissan GT-R-style switch, and the VW Touareg/Phaeton switch. All have internal lighting circuits that have been known to, in some circumstances, leak voltage to the switched circuit. Many others probably have the same problem.
Luckily, you don't have to give up your switch choice to use it safely with the Infinitybox system. The simple solution is to use a diode in series with the Mastercell wire for the start button/switch.
Installation is easy- just insert the diode in the circuit between the switch and the Mastercell. Since diodes are directional (they let voltage flow in only one direction), you have to install it in the right orientation. In this case, wire it between the Mastercell input and the connector on the switch, with the line on the diode towards the switch.
You should use a diode like this for every switch with a lighted circuit that controls an input to the Mastercell if the circuit is critical.