If you were wondering how to use the open collector drivers on the Gertboard version 1, then you have come to the right place. As shown in the diagram, the red area marked out is the open collector driver section.
The ULN2803APG is a Darlington driver IC that provides eight sink drivers. If you are new to the concept of sinking current then the Current Sinking and Sourcing in TTL Circuits article might help. Each driver sinks up to a maximum of 500 mA, which is enough to drive a high power LED, or a low power relay.
These drivers perform the Boolean NOT logic operation; hence, an input signal of 3.3 V (logic 1) at the input pin 5 produces a corresponding output signal of 0 V (logic 0) at the output pin 14. The zero volts potential therefore provides a ground for the current to sink.
Inductive loads such as relays and motors usually generate a back emf, and consequently these drivers have an internal protection diode. The supply voltage is fed through pin 10 and the IC accepts between 5 V to 30 V, however 9 V is typically used.
Open Collector Input / Output
The input signal if fed through the vertical header socket J4. You can input 3.3 V signals from the Raspberry Pi directly here using jumper wires. The horizontal line of headers marked J12 are the output pins.
Jumper Configuration Circuit
This circuit diagram shows how the header pins connect to the ULN2803APG chip. In this example, an input signal at pin RLY5 on J4 takes a signal path through the Darlington driver IC between pin 5 and pin 14, to appear at pin RLY5 on the J12 socket.
For more detail: Gertboard Open Collector Drivers: ULN2803APG