Experiment 18: Reaction TimerChapter 4170Experiment 18: Reaction TimerBecause the 555 can easily run at thousands of cycles per second, we can use it to measure human reactions. You can compete with friends to see who has the fastest response—and note how your response changes depending on your mood, the time of day, or how much sleep you got last night.Before going any further, I have to warn you that this circuit will have more connections than others you’ve tackled so far. It’s not conceptually difficult, but requires a lot of wiring, and will only just fit on a breadboard that has 63 rows of holes. Still, we can build it in a series of phases, which should help you to detect any wiring errors as you go.You will need:• 4026 chip. Quantity: 4 (really you need only 3, but get another one in case you damage the others).• 555 timers. Quantity: 3.• Tactile switches (SPST momentary switches). Quantity: 3.• Three numeric LEDs, or one 3-digit LED display (see the shopping list at the beginning of this chapter). Quantity: 1.• Breadboard, resistors, capacitors, and meter, as usual.Step1:DisplayYou can use three separate LED numerals for this project, but I suggest that you buy the Kingbright BC56-11EWA on the shopping list at the beginning of this chapter. It contains three numerals in one big package.You should be able to plug it into your breadboard, straddling the center channel. Put it all the way down at the bottom of the breadboard, as shown in Figure 4-31. Don’t put any other components on the breadboard yet.Now set your power supply to 9 volts, and apply the negative side of it to the row of holes running up the breadboard on the righthand side. Insert a 1K resistor between that negative supply and each of pins 18, 19, and 26 of the Kingbright display, which are the “common cathode,” meaning the negative connection shared by each set of LED segments in the display. (The pin num-bers of the chip are shown in Figure 4-33. If you’re using another model of display, you’ll have to consult a data sheet to find which pin(s) are designed to receive negative voltage.)Switch on the power supply and touch the free end of the positive wire to each row of holes serving the display on its left and right sides. You should see each segment light up, as shown in Figure 4-31.Each numeral from 0 to 9 is represented by a group of these segments. The segments are always identified with lowercase letters a through g, as shown in Figure 4-32. In addition, there is often a decimal point, and although we won’t be using it, I’ve identified it with the letter h. Figure 4-31. After putting a 1K resistor between the common cathode of the display and the negative supply voltage, you can use the positive supply voltage to illuminate each segment in turn. agdbfcehFigure 4-32. The most basic and common digital numeral consists of seven LED seg-ments identified by letters, as shown here, plus an optional decimal point.