

Article 10629 of rec.ham-radio:
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>From: [email-masked] (Bob Parnass, AJ9S)
Newsgroups: rec.ham-radio
Subject: A carrier operated light for the PRO-2004 [repost]
Message-ID: <[email-masked]>
Date: 25 Mar 89 12:33:46 GMT
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 91

Several PRO-2004 owners have asked for a repost of this article:

x-------------- CUT HERE --------------------------------------x

       A CARRIER OPERATED LIGHT FOR THE PRO-2004 SCANNER

                     by Bob Parnass, AJ9S

   The July 1988 issue of Monitoring Times suggested that it
   may  be  easy  to add an S-meter to the Radio ShackO PRO-
   2004 scanner:

    "...But adding an "S" meter is even easier  than  previ-
   ously  thought.   Pin  10 on the IF amplifier (IC1) is an
   "S" meter output.   With  the   proper  bridge  or  meter
   buffer/amplifier an "S" meter is reality...."

   Brace yourself for disappointment - the  integrated  cir-
   cuit  mentioned  in  the  article  is used for WBFM only.
   Despite the internal IC  block  diagram  in  the  service
   manual,  pin  10  on  my PRO-2004 is useful as an S-meter
   output only when the radio is in the WBFM mode.

   Between pin 10 and ground, I placed a 10,000 ohm resistor
   in  series  with  a  250 microamp meter for a simple test
   setup.  The meter read  full  scale  on  strong  signals.
   With  no  signal at all, the meter read about 70% of full
   scale.  When the mode is set to AM or NBFM, the meter was
   always at zero.

   One could add a bridge circuit here,  but  this  metering
   point is of limited utility.

                      Add a Light Instead

   With a room full of functioning scanners, it's  difficult
   to  determine  quickly  which  radio is "talking."  I use
   separate external speakers on each radio, and the spatial
   separation helps.

   In addition to "hearing" which radio is active, I like to
   "see"  which radio is active, and carrier operated lights
   are effective at providing such visual cues.  The idea is
   to  illuminate a lamp when a signal opens the squelch.  A
   small yellow light emitting diode (LED, another Bell Labs
   invention) is well suited to this purpose.  The following
   modification works well on all PRO-2004 modes.

   To add a COR light to the PRO-2004, make use of the "scan
   control" pin (pin 13) on IC2, the TK10420 IC.  Pin 13 has
   voltage present only when a  signal  is  detected.   This
   chip contains the IF, detector, limiter, and squelch cir-
   cuitry for NBFM.

   If you tremble with an electric drill in your hands, read
   no  further.   The  LED  can  be  mounted in a small hole
   drilled through the plastic  front  panel,  just  to  the
   right of the headphone jack.

   Electronically, the circuit is simple.   The  voltage  at
   pin 13 is not enough to drive the LED directly, so a gen-
   eral purpose NPN transistor (e.g., a 2N2222) can be  used
   as a solid state switch.

      - Pin 13 of IC2 is connected to  the  transistor  base
        through a 10,000 ohm resistor.

      - The emitter is grounded.

      - The collector is connected through a 1000 ohm resis-
        tor  to one end of an LED.  This resistor limits the
        LED current to about 13 milliamps.

      - The other end of the LED is connected to one contact
        on  the  rear  of the PRO-2004's on/off, volume con-
        trol.  This furnishes about 14  VDC  unregulated  to
        the  circuit.  The back of the on/off switch has two
        contacts.  Use the one with the brown wire connected
        to  it,  as  this  contact  is  only "live" when the
        scanner is turned on.

   I mounted the 2 resistors and transistor on  a  small  PC
   board,  which  I fastened to the PRO-2004 chassis using a
   metal standoff.

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Bob Parnass AJ9S,  AT&T Bell Laboratories  -  att!ihuxz!parnass - (312)979-5414