Make: Electronics by Charles Platt (read me a book .TXT) 📕

thing missing is a way to start the count unexpectedly.

Figure 4-40. Adding a bistable 555 timer to the reflex tester will stop the counter with a touch of a button, and keep it stopped.

R9, R10: 1K

IC6: 555 timer

The Delay

Suppose we set up yet another 555 in monostable mode. Trigger its pin 2 with a negative pulse, and the timer delivers a positive output that lasts for, say, 4 seconds. At the end of that time, its output goes back to being negative. Maybe we can hook that positive-to-negative transition to pin 4 of IC6. We can use this instead of switch S5, which you were pressing previously to start the count.

Check the new schematic in Figure 4-41 which adds another 555 timer, IC7 above IC6. When the output from IC7 goes from positive to negative, it will trigger the reset of IC6, flipping its output negative, which allows the count to begin. So IC7 has taken the place of the start switch, S4. You can get rid of S4, but keep the pull-up resistor, R9, so that the reset of IC6 remains positive the rest of the time.

Figure 4-41. The completed control section of the circuit, to be added above these timers.

R7, R9, R10, R12: 1K

R8: 2K2

R11: 330K

C1: 100 µF

C2: 68 µF

C3, C4, C6: 0.1 µF

C5: 10 µF

S1, S2, S3: tactile switches

IC5, IC6, IC7: 555 timers

This arrangement works because I have used a capacitor, C4, to connect the output of IC7 to the reset of IC6. The capacitor communicates the sudden change from positive to negative, but the rest of the time it blocks the steady voltage from IC7 so that it won’t interfere with IC6.

The final schematic in Figure 4-41 shows the three 555 timers all linked together, as you should insert them above the topmost counter, IC1. I also added an LED to signal the user. Figure 4-42 is a photograph of my working model of the circuit.

Figure 4-42. The complete reaction-timer circuit barely fits on a 63-row breadboard.

Because this circuit is complicated, I’ll summarize the sequence of events when it’s working. Refer to Figure 4-41 while following these steps:

1. User presses Start Delay button S4, which triggers IC7.

2. IC7 output goes high for a few seconds while C5 charges.

3. IC7 output drops back low.

4. IC7 communicates a pulse of low voltage through C4 to IC6, pin 4.

5. IC 6 output flips to low and flops there.

6. Low output from IC6 sinks current through LED and lights it.

7. Low output from IC6 also goes to pin 2 of IC1.

8. Low voltage on pin 2 of IC1 allows IC1 to start counting.

9. User presses S3, the “stop” button.

10. S3 connects pin 2 of IC6 to ground.

11. IC6 output flips to high and flops there.

12. High output from IC6 turns off the LED.

13. High output from IC6 also goes to pin 2 of IC1.

14. High voltage on pin 2 of IC1 stops it from counting.

15. After assessing the result, user presses S2.

16. S2 applies positive voltage to pin 15 of IC1, IC2, IC3.

17. Positive voltage resets counters to zero.

18. The user can now try again.

19. Meanwhile, IC5 is running continuously throughout.

In case you find a block diagram easier to understand, I’ve included that, too, in Figure 4-43.

Figure 4-43. The functions of the reflex tester, summarized as a block diagram.

Using the Reflex Tester

At this point, you should be able to test the circuit fully. When you first switch it on, it will start counting, which is slightly annoying, but easily fixed. Press S3 to stop the count. Press S2 to reset to zero.

Now press S4. Nothing seems to happen—but that’s the whole idea. The delay cycle has begun in stealth mode. After a few seconds, the delay cycle ends, and the LED lights up. Simultaneously, the count begins. As quickly as possible, the user presses S3 to stop the count. The numerals freeze, showing how much time elapsed.

There’s only one problem—the system has not yet been calibrated. It is still running in slow-motion mode. You need to change the resistor and capacitor attached to IC5 to make it generate 1,000 pulses per second instead of just three or four.

Substitute a 10K trimmer potentiometer for R8 and a 1 µF capacitor for C2. This combination will generate about 690 pulses per second when the trimmer is presenting maximum resistance. When you turn the trimmer down to decrease its resistance, somewhere around its halfway mark the timer will be running at 1,000 pulses per second.

How will you know exactly where this point is? Ideally, you would attach an oscilloscope probe to the output from IC5. But, most likely, you don’t have an oscilloscope, so here are a couple other suggestions.

First remove the 1 µF capacitor at C2 and substitute a 10 µF capacitor. Because you are multiplying the capacitance by 10, you will reduce the speed by 10. The leftmost digit in your display should now count in seconds, reaching 9 and rolling over to 0 every 10 seconds. You can adjust your trimmer potentiometer while timing the display with a stopwatch. When you have it right, remove the 10 µF capacitor and replace the 1 µF capacitor at C2.

The only problem is, the values of capacitors may be off by as much as 10%. If you want to fine-tune your reflex timer, you can proceed as follows.

Disconnect the wire from pin 5 of IC3, and substitute an LED with a 1K series resistor between pin 5 and ground. Pin 5 is the “carry” pin, which will emit a positive pulse whenever IC3 counts up to 9 and rolls over to start at 0 again. Because IC3 is counting tenths of a second, you want its carry output to occur once per second.

Now run the circuit for a full minute, using your stopwatch to see if the flashing LED