Experiment 18: Reaction Timer
176 Chapter 4
Refinements
Now it’s time to remember that what we really want this circuit to do is test a
person’s reflexes. When the user starts it, we want an initial delay, followed by a
signal—probably an LED that comes on. The user responds to the signal by press-
ing a button as quickly as possible. During the time it takes for the person to re-
spond, the counter will count milliseconds. When the person presses the button,
the counter will stop. The display then remains frozen indefinitely, displaying the
number of pulses that were counted before the person was able to react.
How to arrange this? I think we need a flip-flop. When the flip-flop gets a sig-
nal, it starts the counter running—and keeps it running. When the flip-flop
gets another signal (from the user pressing a button), it stops the counter run-
ning, and keeps it stopped.
How do we build this flip-flop? Believe it or not, we can use yet another 555
timer, in a new manner known as bistable mode.FundAmentAls
The bistable 555 timer
Figure 4-39 shows the internal layout of a 555 timer, as
before, but the external components on the righthand side
have been eliminated. Instead, I’m applying a constant
negative voltage to pin 6. Can you see the consequences?
Suppose you apply a negative pulse to the trigger (pin 2).
Normally when you do this and the 555 starts running, it
generates a positive output while charging a capacitor at-
tached to pin 6. When the capacitor reaches 2/3 of the full
supply voltage, this tells the 555 to ends its positive output,
and it flips back to negative.
Well, if there’s no capacitor, there’s nothing to stop the timer.
Its positive output will just continue indefinitely. However,
pin 4 (the reset) can still override everything, so if you apply
negative voltage to pin 4, it flips the output to negative.
After that, the output will stay negative indefinitely, as it
usually does, until you trigger the timer by dropping the
voltage to pin 2 again. This will flip the timer back to gener-
ating its positive output.
Here’s a quick summary of the bistable configuration:- A negative pulse to pin 2 turns the output positive.
- A negative pulse to pin 4 turns the output negative.
- The timer is stable in each of these states. Its run-time
has become infinite.
It’s OK to leave pins 5 and 7 of the timer unconnected,
because we’re pushing it into extreme states where any
random signals from those pins will be ignored.
1234 567FF8ABFigure 4-39. In the bistable configuration, pin 6 of a 555 timer
is perpetually negative, so the timer cycle never ends, unless
you force it to do so by applying a negative pulse to pin 4 (the
reset).