Friday

Ellen and Liz visited Dr. Davis in the hospital at noon and went home for the afternoon. They decided to get in a two mile run together. They put on their running outfits and started out.

No matter how large or small it is an electric switch either is on or off. A connection is either made or not made between two points in the circuit. Current will either flow or it will not.  This one was very small and it was probably the simplest of electric switches. Once turned on it locked in the on position and would not turn off.  Current would flow till the battery went dead. 

The pressure from the first step Liz made drove a small copper pin resembling a thumb tack but about one fourth the length through two pieces of copper separated by a thin plastic film completing a circuit between the two plates. The pin was tapered slightly. Once it was driven home it would not come out to turn off the circuit. The current from two button batteries would flow till the circuit performed its task or the battery was exhausted. Within a minute three other like but independent circuits were energized by identical switches, one in her right shoe and the other two in her mom’s shoes..

As each circuit came to life an eight bit binary counter on a chip was set to 00000000 by the power on. Another switch closed for an instant indicating it had sensed an impact of over one and a half G’s. It was a simple device, a spring contact that hit another contact. It was much like a doorbell push button but much smaller. The pulse from the switch was routed to a circuit. The pulse incremented the binary counter to 00000001. Subsequent pulses incremented the counter, 00000010, 00000011, 00000100, 00000101, 00000110, 00000111, and 00001000. After two hundred fifty pulses the counter read 11111001, then on subsequent pulses, 11111010, 11111011, 11111100, 11111101, 11111110, 11111111, and finally to it rolled to 00000000. The switch had sensed 256 steps.

On the two hundred fifty sixth step the counter went to 00000000 and the carry line of the counter emitted a pulse. This was routed to a circuit that turned on a transmitter that sent a ten second radio signal.

The other three circuits were also counting. Two of them were at 11110000 and 11110001, the third was at 11111101. The others would never reach the overflow count.

The unit that transmitted finished the transmission ten seconds.It sent a pulse one second later to a transistior. Radio receivers in the other three units that had not sensed the counter overflow heard the ten second transmission. When the transmitter stopped the other three circuits armed.  One second later the timer sent a pulse to a special transistor. In all four units the transistor switched the current from the battery to an electro-chemical device. An instant later one and two thirds ounces of an explosive more powerful than C4 detonated. The circuits were designed to detonate all four explosive charges within twenty five milliseconds, one fortieth of a second. It actually happened in less than three milliseconds.