Make Electronics Experiment 18: Reaction Timer 

For anyone attempting this, I’d advise reading the errata on the website first: don’t connect pins 4 & 5 of the 4026’s to ground and swap the R10 1k resistors for for 10k.   The 1uF capacitor should be 0.1uF etc.  Also, to get mine to reliably work I had to remove the the positive pin connection on IC1 pin 16.