*** REsound (Sound Sweep) Microphone Connections ***

Date: Mon, 14 Apr 2003 22:14:30 -0700
From: Whistler SAR <whis_sar@whistler.net>
Reply-To: whis_sar@whistler.net
Organization: Whistler Search & Rescue

To: Martin Colwell <sarinfo@mindlink.bc.ca>
Subject: GM300 mic pinout

Hi Martin - we received the RESOUND unit from Craig Hollinger
the other day. Will try and get it wired in tomorrow. The pinout for
the Motorola GM300 radio is:

PIN 1	OPTIONS
PIN 2	   "
PIN 3	HOOK SWITCH (ground to enable CTCSS [PL] or open for monitor)
PIN 4	GROUND
PIN 5	MIC AUDIO
PIN 6	PTT
PIN 7 	PROGRAM
PIN 8	Handset Audio (low level receive audio)

The resound will be making connections to pins 3, 4, 5, and 6.
Could use the blue wire for the HOOK SWITCH.  Looks like the White
wire is currently not wired through.  Could strip the cables back a
bit and wire it straight through, like the blue wire, to allow
it to be used for receive audio in case a headset is used instead
of a hand mic. 

I am contemplating wiring up two RJ45 jacks - one on each of the
cables coming from the Resound.  I'll then use a standard RJ45 cable
from one connector to the radio, and the microphone will plug into the
other connector.  I'll let you know how I make out.



Hi Martin...wired the RESOUND yesterday, but didn't get it tested
until today. I had to redo the connections on the jack on the radio
side of the resound.  The standard (telephone) RJ45 cahles that I have
are "crossed over."  Pin 1 goes to Pin 8, Pin 2 goes to Pin 7, etc.
Could be that computer network cables are Pin 1 to Pin 1 etc., but keep
this in mind when putting it together.

The audio level wasn't overly loud, but did adjust it downwards.  Found
that the level dropped very little until the last few turns...or at
least that when the audio was noticibly lower, it was "on the hairy
edge" of being too low.  Will get opinions from others later as to
whether they think the audio is too low or too high, but sounded
OK to me.

Pin 8 on the connector (radio end) is low level handset audio.
I wired the white wire to this pin.  The white wire isn't connected
in the Resound, so if receive audio was needed the other side
of the Resound, the white wires would have to be joined.

I think the piezo-electric transducer built into the Resound is
too quiet.  It appears to be switched by a 2N3904 with a 100 ohm
resistor in series with it.  There is 1.56 volts across the resistor
when the "speaker" is sounding.  This would work out to about 16 ma
through the transducer.  Tried shorting the 100 ohm resistor with
a milliamp-meter and read 27 ma.  Am pretty sure this is within
the limits of the 2N3904.  Will check the specs tonight.  I might
replace the 100 ohm resistor with 10 ohms.  I think if there was
much background noise, the radio operator would have trouble
hearing the audio prompt from the transducer with the present level.

PS - you might want to check the supplied 12V cable.

1) Unscrew connector and make sure not shorted.  Probably best
   to seperate the wires and put some take around one of the
   conductors...unless there's already a plastic sleeve in there.

2) Be very careful when the cable is unplugged.  There is 12 volts
   on the exposed part of the barrel connector - assuming the other
   end is wired in to a 12v source.

Nit picking aside, it seems to do a fine job!


Bye for now - Keith Powell.

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