Simple-TR-Switch
This is a non-RF-sensing, MCU PTT-controlled Simple T/R Switch with RX
Grounding feature which can be built for less than 2 dollars!
It was designed, and tested for https://github.com/kholia/Easy-Transceiver project.
Various approaches
TX power is 10W from IC-705 operating in FM mode.
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With a 2-channel relay as a T/R switch, when both RX Ground and Center are disconnected and left floating, a voltage of around ~2.2v is induced across RX connections which is unsafe for typical SDRs.
~2.2 Vpp is 10.93 dBm!
https://www.random-science-tools.com/electronics/dBm-Watts-volts.htm
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With a 1-channel relay as a T/R switch which is switching the
Centerconnection only, and with all Grounds (TX, RX, Antenna) shorted, a voltage of around ~1.66v is induced across RX connections which is theoretically safe for common SDRs. -
With a 2-channel relay as a T/R switch, with all Grounds (TX, RX, Antenna) shorted, with
Centerswitching using Channel 1, and RX Center Grounding using Channel 2 during TX, a voltage of around ~500mV is induced across RX connections which should be fine for most common SDRs.
Comparisons
No external 12v power is needed unlike Easy TR Switch from https://www.qrpkits.com/ezseries.html.
The ~4000 INR SDR Transceiver and Receiver Switch Antenna Sharer TR Switch Box 160 MHz from Banggood doesn't work in Automatic Mode if there is
Broadcast/SW Interference in your area. The Banggood TR switch switches on
the RTX (red) and disconnects the SDR (RX) as soon as the antenna is connected
under BCI conditions, when operating in Automatic Mode.
BOM
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5V 2A DPDT OEN MICRO RELAY 46-05-2C
Added Protection
Here is another additional mechanism to protect sensitive SDRs.
This is OK for HF (shortwave).
Alternate (but riskier) Option
Solid switching using BS170 could be another option.
The info folder has more information on this.
Credits
Thanks to VU2ASH (Ashok Joshi), K.P.S. Kang, and LW1ECP (Daniel Alonso) for all the help.
https://circuits-diy.com/optocoupler-relay-driver-with-pc817-2n3904/