# Help with Art of Electronics problem 10.33 (third edition)

Discussion in 'Homework Help' started by umichfan1, Aug 4, 2016.

1. ### umichfan1 Thread Starter Member

Jun 16, 2012
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I am working through problem 10.33 in the new (third-edition) Art of Electronics textbook. It states:

Design a reaction timer. “A” pushes his button; an LED goes on, and a counter begins counting. When “B” pushes her button, the light goes out and an LED display reads the time, in milliseconds. Be sure to design the circuit so that it will function properly even if A’s button is still held down when B’s button is pushed.

I am assuming that the display is in hexadecimal (as in Fig. 10.90 of the text), and my solution uses a single-pulse generator (“one-shot”) whose circuit diagram is given in Fig. 10.75 of the text. I also am omitting the LED light for simplicity. I’ve come up with a solution that I believe works (attached), but I am new to digital electronics and would therefore appreciate feedback on:

1) whether it is correct

2) how it could be improved

Thank you!

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2. ### AlbertHall Well-Known Member

Jun 4, 2014
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I'm afraid that won't work. The contacts of the push buttons will bounce, possibly over several milliseconds. So you could easily get several pulses, at 1kHz, fed to the 'D' type when the A push button is pressed.

Consider a flip flop set by A which enables the counter. Then a similar flip flop set by B which resets the A flip flop.

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3. ### AlbertHall Well-Known Member

Jun 4, 2014
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Sorry, that's not quite right, I just noticed that they are one-shots, but then I don't understand what the 'B' input on the one-shot is doing as the clock has a constant 1kHz.

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4. ### umichfan1 Thread Starter Member

Jun 16, 2012
32
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Yeah accidentally I left out a few assumptions. First, good point about the switch bounce. I forgot to mention that for simplicity I'm assuming that the pushbuttons in my circuit are already debounced (there are several simple circuits for debouncing switches in chapter 10 of AoE).

Also, the "one-shot" box in my circuit diagram I am assuming is Fig. 10.75 in AoE (attached). Perhaps I used the wrong terminology here...should I instead call this a "single-pulse generator"?

In my solution, after "A" presses their button, that will result in a pulse being sent to the D-flop, which will enable the counter. My thought is then that when "B" presses their button, that will result in another pulse being sent to the D-flop, which will in turn disable the counter. Does that make sense?

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5. ### AlbertHall Well-Known Member

Jun 4, 2014
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Given all that I think it works

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6. ### umichfan1 Thread Starter Member

Jun 16, 2012
32
0
Thanks, I appreciate it!