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  1. Path: sparky!uunet!cs.utexas.edu!sun-barr!ames!elroy.jpl.nasa.gov!sdd.hp.com!uakari.primate.wisc.edu!zazen!doug.cae.wisc.edu!umn.edu!csus.edu!netcom.com!netcom!mjohnson
  2. From: mjohnson@netcom.Netcom.COM (Mark Johnson)
  3. Newsgroups: sci.electronics
  4. Subject: Re: Simple transistor project (Q)
  5. Message-ID: <MJOHNSON.92Aug13162014@netcom.Netcom.COM>
  6. Date: 14 Aug 92 00:20:14 GMT
  7. References: <1992Aug13.062418.999@cs.uri.edu>
  8. Organization: Netcom Online Communications Service
  9. Lines: 36
  10. In-Reply-To: reynhout@cs.uri.edu's message of 13 Aug 92 06:24:18 GMT
  11.  
  12. In the spirit of the poster's first article, the following
  13. discrete circuit will get the job done:
  14.  
  15.                                +5.0V      +5.0V    +5.0V
  16.                                  |          |       |
  17.                                  |          |       |  minus
  18.             C1                 C |          |      D1
  19.            10uF                 /         relay   1N4148
  20. logic     + | | -         B  | /          coil      |
  21.  in  >------| |-----+--------|  Q1          |       |  plus
  22.             | |     |        | \            +--------
  23.                     |         E \           |
  24.                     R1          |         C /
  25.                     50K         |      B | /
  26.                     |           +--------|  Q2
  27.  Q1 and Q2          |           |        | \
  28.  are NPN            |           R2        E \
  29.  general purpose    |          3.3K         |
  30.  (like 2N2222)      |           |           |
  31.                     |           |           |
  32.                    GND         GND         GND
  33.  
  34. When the logic signal makes a low-to-high transition,
  35. the base of Q1 rises.  This turns on Q1.  The emitter
  36. of Q1 rises, and that turns on Q2 (energizing the relay).
  37.  
  38. After a delay equal to (R1 * C1) = 0.5 seconds, the base
  39. of Q1 falls low again, turning off Q1.  This turns off
  40. Q2 and the relay.
  41.  
  42. Protection diode D1 clamps the inductive back-EMF
  43. induced when the current in the relay coil cuts off
  44. to zero.
  45.  
  46. TTL compatibility is achieved by setting the turnon
  47. point to 2*VBE (same as TTL).
  48.