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  1. Path: sparky!uunet!zaphod.mps.ohio-state.edu!rpi!batcomputer!cornell!uw-beaver!news.u.washington.edu!carson.u.washington.edu!whit
  2. From: whit@carson.u.washington.edu (John Whitmore)
  3. Newsgroups: sci.electronics
  4. Subject: Re: How would you detect lack of a square wave?
  5. Date: 3 Jan 1993 23:21:42 GMT
  6. Organization: University of Washington, Seattle
  7. Lines: 35
  8. Message-ID: <1i7se6INNr2b@shelley.u.washington.edu>
  9. References: <1993Jan2.172332.21070@socrates.umd.edu>
  10. NNTP-Posting-Host: carson.u.washington.edu
  11.  
  12. In article <1993Jan2.172332.21070@socrates.umd.edu> jimbo@socrates.umd.edu (Jim Bogard) writes:
  13. >What's the best way to detect the presence/lack of a square wave?
  14.  
  15.     One way is to use a 'missing pulse detector' with
  16. a one-shot (which has already been mentioned).  Another,
  17. more common in cheap consumer gear, is to use a transistor
  18. and rectify the AC...
  19.  
  20.                                 +5V
  21.                                  |
  22.                                  R
  23.                                  |
  24.                           +------+--------output
  25.                           |      |
  26.                           =     /
  27.                           |    |
  28. in+-------||---+---|>|----+----|
  29.                |               |
  30.                -                \
  31.                ^                 |
  32.                -                 |
  33.                |                 |
  34.               GND               GND
  35.  
  36.  
  37.     The input capacitor pumps charge through the
  38. two diodes to turn the transistor ON, and the collector-to-base
  39. capacitor keeps the transistor from turning OFF quickly
  40. (until the next pulse comes in).  It is even possible to
  41. omit the diodes in some circumstances.  The input and output 
  42. voltages needn't be limited to logic levels, either.
  43.  
  44.     John Whitmore
  45.  
  46.  
  47.