First I made the cake, and dyed a whole load of white roll-out icing green. I also dyed myself green which you would think would give away my secret mission but happily he is less observant than I feared. Then I iced the cake - this was my first mistake because I forgot to put something over the cake to make the roll-out icing stick! Usually either watered-down icing or jam is good.
I had a whole stack of packets of sweets to use to create the components from. I needed transistors (round liquorice with a flat bit sliced out of them), LEDs (jelly dolly mixtures), capacitors (round dolly mixtures), and resistors. The resistors were easily as complicated as the whole of the rest of the components put together and next time I will just buy as many colours of write-on icing as I need to do the stripes! In the event I made the stripes out of various things wrapped round marshmallows, including red laces, the offcuts of the green icing, bits of liquorice chopped really small, and (for the orange and brown) coloured bits of dolly mixture, separated from the white bits of sweet, mashed together, rolled out, and then cut into strips.
So, here's the finished article (and a closeup of a couple of the components)