Good place to tap for PTT-OUT #v5


Rob French (KC4UPR)
 

Hi,

I'm looking to add a PTT-OUT to the back panel of my uBITX, to control e.g. an amplifier or a switch to isolate an external SDR during transmit.

My thought is to grab C154/D11 or one of the other filter-switching relay circuits.  Those look like easy places to tack a short wire to without having to completely disassembly my uBITX.

Two quick questions:
(1) Dumb question... but that junction does provide what I want, right?  i.e. +12V on transmit.
(2) Any downsides to tapping the +12V on transmit, from that location?  e.g. some RFI effect

Thanks,
Rob KC4UPR


Jerry Gaffke
 

Yes, the TX rail has 12v on it whenever the rig is transmitting, 0v when it is not.
Probably your best choice, and easy to tack a wire on it.

Does your amplifier really want a 12v signal, or something more like 5v?
How much current will the amplifier draw from this signal?

I would think about limiting the current somehow, perhaps a series resistor inside the uBitx.
Otherwise a miss-wire in the cable could cause 10 Amps to flow, and lots of smoke.

Jerry, KE7ER


On Tue, Mar 16, 2021 at 10:38 AM, Rob French (KC4UPR) wrote:

I'm looking to add a PTT-OUT to the back panel of my uBITX, to control e.g. an amplifier or a switch to isolate an external SDR during transmit.

My thought is to grab C154/D11 or one of the other filter-switching relay circuits.  Those look like easy places to tack a short wire to without having to completely disassembly my uBITX.

Two quick questions:
(1) Dumb question... but that junction does provide what I want, right?  i.e. +12V on transmit.
(2) Any downsides to tapping the +12V on transmit, from that location?  e.g. some RFI effect

Thanks,
Rob KC4UPR


_Dave_ AD0B
 

An easy place is  the the TX/rx relay driver transistor I went from the base of that transistor and added in a second driver transistor and relay. You might just need the transistor and drive the output to your amp or what ever. 
--
73
Dave
ADOB
Raduino bracket and Ham_Made_Keys


Dean Souleles
 

Hi Rob - look here...

BITX Hacks: Adding an External Linear Amplifier Control circuit to your uBitx

Use PTT to trigger a relay - then you can switch to whatever you need.  Important thing is to keep the amp isolated from the transceiver.

Dean
KK4DAS


Zvi Segal
 

I added additional relay and drove it from the Tx (+12V).   Use its contacts to deliver ground PTT to PA and isolate the uBitx.

Good practice is adding a reverse diode on the relay's coil to protect from surge voltage on disconnection.


Zvika 
4Z1ZV


Jerry Gaffke
 

Rob,

As I indicated in my response, I really don't know what kind of signal your amp might expect.
From responses give by others, sounds like an amp typically wants that wire shorted to ground
when transmitting, otherwise no connection.

This could be done by driving a relay from the TX rail, which is 12v when transmitting.
Or it could be done by driving an NPN transistor or an NFET with the 5v T/R signal
in the same way that Q15 is driven through R150, except the collector of Q15 would
go to a wire out to your amp.
Since that wire must be low when transmitting, it might be possible to drive it from 
the RX rail through a resistive divider, the resistor values would depend on how the
amp treats that signal.

Jerry, KE7ER.


On Tue, Mar 16, 2021 at 05:06 PM, Jerry Gaffke wrote:

Yes, the TX rail has 12v on it whenever the rig is transmitting, 0v when it is not.
Probably your best choice, and easy to tack a wire on it.

Does your amplifier really want a 12v signal, or something more like 5v?
How much current will the amplifier draw from this signal?

I would think about limiting the current somehow, perhaps a series resistor inside the uBitx.
Otherwise a miss-wire in the cable could cause 10 Amps to flow, and lots of smoke.

Jerry, KE7ER

 

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On Tue, Mar 16, 2021 at 10:38 AM, Rob French (KC4UPR) wrote:

 

I'm looking to add a PTT-OUT to the back panel of my uBITX, to control e.g. an amplifier or a switch to isolate an external SDR during transmit.

My thought is to grab C154/D11 or one of the other filter-switching relay circuits.  Those look like easy places to tack a short wire to without having to completely disassembly my uBITX.

Two quick questions:
(1) Dumb question... but that junction does provide what I want, right?  i.e. +12V on transmit.
(2) Any downsides to tapping the +12V on transmit, from that location?  e.g. some RFI effect

Thanks,
Rob KC4UPR