We used: borrowed rechargeable batteries, put into a 4xAA battery pack; reclaimed bits of wire from another project, 25-30 cm long, with a small croc clip soldered on one end, and bare wire soldered to make a stiff pointy end at the other (to poke into the playdough); LEDs which were these ones and a basic playdough mix.
A great table based activity to run at our club. A good combination of squidgy playdough and DIY technology. Despite carefully printing off the instructions and making the dough in advance, Chrissy (the adult running the activity!) could not get the LEDs to light up – one of our genius Junior Helpers sat down, read the instructions, and worked it out. After that it was dead easy for everyone.