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 hasn’t
yet been calibrated. It’s 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 1μF 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’d 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 1μF capacitor at C2 and substitute a 10μF capacitor. Because you are multiplying
the capacitance by 10, you’ll 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 10μF capacitor
and replace the 1μF 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 drifts gradually
faster or slower than once per second. If you have a
camcorder that has a time display in its viewfinder,
you can use that to observe the LED.
If the LED flashes too briefly to be easily visible,
you can run a wire from pin 5 to another 555 timer
Fig. J: Adding a bistable 555 timer to the reflex tester
will stop the counter with a touch of a button, and keep
R9, R10: 1K
IC6: 555 timer