“Before you move on to the next step, check that this circuit is working by connecting an LED with a resistor to the output of the 555 timer as follows: Connect the negative side (short leg) of an LED to the output on pin 3 of the 555 timer. Connect the positive side (long leg) of the LED to a 100 Ω resistor, and connect the other side of this resistor to the positive supply column. Connect your battery clip to one of the supply column pairs as usual. Then plug in the battery to check that the circuit works. If your LED blinks really fast, then you’re ready to move on. If not, recheck your connections to find out where the error is. When you know the 555 timer circuit works, unplug the LED, 100 Ω resistor, and battery clip.”