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Reaction Timer
The circuit below can be used to test a person’s reaction time to a precision of ±0.01 seconds.  
The time is indicated by a series of seven leds showing the output of a binary counter (4060).  
For convenience, I put these leds on a separate strip of vero board.  
The two D type flip-flops are in a 4013.  
   
The led shown on the circuit diagram is the stimulus*.  
Switch S1 starts the timer and illuminates the stimulus led.  
Switch S2 stops the timer.
   
 
   
To calibrate the timer  
1. Remove the 4013 from its holder; this allows the oscillator in the 4060 to run continuously.
2. Connect the battery and then connect an oscilloscope across the first led (that is, the led which is connected to pin 7 of the 4060).
3. Adjust the variable resistor to produce an oscillation of time period 10ms.
 
   
Vero board, copper side  
 
   
   
Vero board, component side  
 
   
L0 to L6 represent the anodes of the leds which indicate the reaction time... be careful to get them in the right order!  
   
* Interestingly, when I replaced the stimulus led with a piezo-electric buzzer, most people were found to have a slightly shorter reaction time...  
   
The photos below show one version of the reaction timer.  
Face presented to the person being tested, showing stimulus led and buzzer.  
   
 
   
Face seen by the person conducting the test, showing on/off switch, optical/audible selector switch and leds.  
Also shown are the start and stop push button switches.  
   
 
The next photo shows the result for a person having a reaction time of 18 hundredths of a second, 0.18s  
 
   
The person having the following result was a little slower at 0.22s  
   
 
   
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