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The GPIO board has 4 high current driver circuits:
Circuit | CPU Port | AMP pin number |
| VB1 | PE0 (PE0 is input only, so another port must be jumpered to VB1 to use that circuit) | 24 |
| VB2 | PA0 | 25 |
| VB3 | PE4 | 26 |
| VB4 | PM2 | 35 |
The VB921 is a dedicated ignition coil driver in a TO-220 package. This part can flow up to 7.5 Amps, and it is self-limiting, so it will not flow more than that. It is capable of withstanding up to 340 Volts on the external circuit, making it ideal for direct coil control.
The above illustration is a general schematic. The following table gives the actual components installed in each circuit:
| Ra | Qb | Da | |
| VB1 | R5 | Q5 | D11 |
| VB2 | R6 | Q6 | D12 |
| VB3 | R7 | Q7 | D13 |
| VB4 | R8 | Q8 | D14 |
However, even though the VB921 is specifically designed for ignition coil control, it can be used for any external device where the current flow is less than 7.5 Amps.
The VB921 datasheet is here:
Besides the VB921 itself, the VB921 circuits have only one other component: a resistor to limit current from the processor (which should be 10 to 25 milliAmps). The specified 1.0K Ohm resistor will work in virtually all applications. Da is not installed with the VB921, since it is capable of withstanding several hundred volts of flyback EMF.
To use the VB921 output circuits, you supply power (12 Volts) to the device from a switched (and fused) source, and you ground the device thorough the GPIO VBx circuit.
The VB921, while very nice for controlling ignition coils, are expensive (~$6 each) and they are 'over-kill' for general 'non-coil' use. You can substitute a TIP120 NPN Darlington transistor for the VB921 in general applications. The TIP120 {497-2539-5-ND, ~75¢ each}, has the same pin arrangement as the VB921 (BCE), so it is a 'drop-in' replacement. The TIP120 can flow up to 5.0 Amps continuously, and 8.0 Amps peak current flow. If you install a TIP120, you should install the Da flyback 22 Volt Zener diode (1N4742ADICT-ND) since the TIP120 is rated to just 60 Volts on the collector. With the TIP120, the controlled device must limit the current (such as a solenoid or a relay), or have a current limiting resistor installed in-line with it.