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<div class="moz-cite-prefix">On 5/9/2015 1:49 PM, John wrote:<br>
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<blockquote
cite="mid:CALfzjYSwwkvm7nTHAUkQ1s+A+KVQ0VaZ1d-_-8PFbomm0dqUwA@mail.gmail.com"
type="cite">
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<div> <span
style="color:rgb(0,0,0);font-family:HelveticaNeue,'Helvetica
Neue',Helvetica,Arial,'Lucida
Grande',sans-serif;font-size:12px">Is the 0-5 Volt COS swing
on a Kenwood TKR-850 sufficient to switch the COS on a DMK
URI? Can't seem to get my system to repeat. I'm out in the
'boondocks' with very few parts and a long way from a Radio
Shack. Just hoping I don't need to buffer the COS somehow to
get it to switch. </span></div>
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</blockquote>
<br>
All FOB's, including the URI, but the exception of a RIM, require a
logic signal capable of sinking to near ground potential on the COS
and CTCSS inputs (active low). No matter if you choose a upright
logic or inverted (in software) the hardware needs to provide a
signal that goes nearly to ground. In other words, it needs to sink
to ground potential. Many radios provide a voltage when the logic
state is valid (active high), these don't interface to a FOB or URI
well. A pull-down resistor can help in these situations, but it's
best to use the right type of logic circuitry to feed these logic
inputs. <br>
<br>
Since the CM119 IC is internally pulled up, changing its logic state
requires a ground. This was originally done by mechanical switches
in a FOB, but must now be done with transistors or whatever logic is
available from the radio. Protection diodes on these inputs help to
complicate the situation because they have a voltage drop. The
hardware inputs of the CM119 require that the voltage drop to about
1V or less to become valid. While Schottky diodes keep this to a
minimum (because their voltage drop is about 2/3 that of a silicon
diode), you still need a solid pull to ground logic signal for a
valid COS or CTCSS signal for a URI or simple FOB.<br>
<br>
Some radios are difficult to properly interface the COS and CTCSS
logic to a FOB or URI. I have a board that I designed just for the
occasion:<br>
<a class="moz-txt-link-freetext"
href="http://www.repeater-builder.com/w3kkc/products/squelch/sc50.html">http://www.repeater-builder.com/w3kkc/products/squelch/sc50.html</a><br>
<br>
My suggestion is to use a NPN transistor or FET to invert the active
high TKR-850 logic for a solid pull to ground. A 2N2222 or 2N7000
can work well.<br>
<br>
Kevin<br>
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