K2 Muting

[Bob Whelan]

Hi
 
I am putting together a second K2 so I have a 2nd RX set up.  Does anyone know how to mute a K2?
 
Thanks for any ideas
 
73,
 
Bob G3PJT
 
 
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[Don Wilhelm-3]

----- Original Message -----
>
> I am putting together a second K2 so I have a 2nd RX set up.  Does anyone
> know how to mute a K2?
>
---------------------------------------
Bob,

A sledgehammer would do a permanent job of it <G>

For something more temporary, an open collector transistor connected to pin
17 of the MCU (Control Board U6) would do the job for you - drive the base
of the transistor with 3 to 5 volts to turn it on when you want the K2
muted.  A 2N2222 or similar should work just fine.

73,
Don W3FPR



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[Jack Brindle]

Ack! You sure you want to do this, Don? Pin 17 is bipolar drive, so you
would be shorting a relatively high-current output (25 mA per spec)  to
ground. That has a good potential for letting smoke out of the MCU.

IF you do this, I HIGHLY recommend a current limiting protection
resistor in the circuit. A few hundred ohms should do, but might also
affect muting operation...

On Dec 9, 2004, at 10:41 AM, W3FPR - Don Wilhelm wrote:

> For something more temporary, an open collector transistor connected
> to pin 17 of the MCU (Control Board U6) would do the job for you -
> drive the base of the transistor with 3 to 5 volts to turn it on when
> you want the K2 muted.  A 2N2222 or similar should work just fine.

-Jack Brindle, W6FB
=======================================================================

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[Don Wilhelm-3]

Jack,

It is not uncommon to 'wire-OR' bipolar devices.  We commonly do that with
other CMOS family devices which also spec a source/sink capability of 25 ma.
or more.

Yes, that is equivalent to shorting to ground with one of the devices at a
logic high level.

The sourcing spec for a bi-polar device tells me that is the max it can
drive.  I have not seen a limit on the 18C452 cumulative drive limit (number
of outputs sourcing current simultaneously), so I don't believe that is any
different than one output driving several loads for which the total current
sourced would be a total of 25 ma. in the logic high state.  The current
comes from the equivalent of a collector resistor within the device itself.

OTOH, Where overload usually occurs in bipolar devices is at Vol (when the
device itself is holding a logic low level), when all attached devices (in
sum) could cause greater than the I-sink rating to be conducted through the
saturated output transistor - this usually results in a quick death.  In a
'wire-or' application where only one of the driving devices contains the
collector resistor, the sink current is not increased by adding another open
collector driver, and for that reason, I did state that it should be an open
collector driver.

I admit I could not find detailed drive/sink information specific to the
18C452, and I am basing my conclusions on the specs for the remainder of the
family.  If you have information that differs, please let me know where to
find it.

73,
Don W3FPR

----- Original Message -----

> Ack! You sure you want to do this, Don? Pin 17 is bipolar drive, so you
> would be shorting a relatively high-current output (25 mA per spec)  to
> ground. That has a good potential for letting smoke out of the MCU.
>
> IF you do this, I HIGHLY recommend a current limiting protection resistor
> in the circuit. A few hundred ohms should do, but might also affect muting
> operation...
>
> On Dec 9, 2004, at 10:41 AM, W3FPR - Don Wilhelm wrote:
>
>> For something more temporary, an open collector transistor connected to
>> pin 17 of the MCU (Control Board U6) would do the job for you - drive the
>> base of the transistor with 3 to 5 volts to turn it on when you want the
>> K2 muted.  A 2N2222 or similar should work just fine.
>

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[Don Wilhelm-3]

Folks,

I had previously suggested a 'wire-OR' off pin 17 of the MCU to accomplish
external muting for the K2.
Jack Brindle pointed out that CMOS logic devices can be overloaded with such
a 'wire-OR' configuration without a current limiting resistor.  I have used
many 'wire-OR' circuits with TTL logic, and since CMOS is "TTL compatible"
(with a 5 volt Vdd supply), I erroneously extended the wire-OR solution to
CMOS logic (in error).

It turns out that in this particular case, a current limiting resistor is
easy to add, and in fact, for this particular circuit, Wayne Burdick has
previously suggested adding a series resistor of about 100k to slow down the
K2 mute timing in order to reduce the audio pops that some have seen with
the KDSP2 added.

So if you wish to mute the K2 from an external source, you may still use the
'wire-OR' technique of attaching an open collector transistor to the gates
of Q6 and Q7 on the Control Board (the 'hot' end of C23 is a good attachment
point), but in addition, one should cut the trace from U6 pin 17 on the
solder side of the Control Board and add a resistor between U6 pin 17 and
C23.  A value as low as 1k ohms will limit the current from the MCU to 5 ma
or less (a safe value), while a value of 100k has been suggested by Wayne
Burdick as a 'mute slow-down' for reducing the KDSP2 pop.  So take your pick
of values between these two to protect the CMOS MCU output from being
overloaded when in its logic high state.

Sorry for any confusion that I may have caused.

73,
Don W3FPR


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