Get to know the components
We begin by making a 'shopping list' of the components we need. Our simple circuit needs
four miniature relays, four BC107 transistors, four 4k7 0.25 watt resistors, four 1N4001 diodes, some phono sockets for
the video from the four cameras, a scart socket for the video out, some miniature sockets for
the switching signal inputs, a piece of printed circuit board and a box to put it all in.
We need to identify the leads on each component (the 'pinout') and we need to note which components have very short leads and will need accurately drilled holes for them to fit on the printed circuit board.
The component supplier's catalogue or website (for example Maplin Electroncs, here in the UK) usually gives the details. Of the four components shown on the left, only the 4k7 resistor can be fitted either way round. With the others, it's important to identify each lead and design the printed circuit board accordingly.