how to sink 9-15v under control of 5v or 3.3v pin to 'key-down' transceiver?

Question

I want to use an Arduino to control keying transmission on a simple CW-only transceiver (think: forty-9er or Pixie).

Looking at the schematics of various such transceivers, they basically 'key down' by connecting part of the circuit to ground through a button or morse key.

Somehow, I need to be able to use a GPIO pin with 5v or 3.3v logic to control whether or not 9-15v from the "radio" side of the metaphorical 'key' can flow to ground (triggering carrier transmission).

Any ideas how to accomplish this? I want to say "open-drain logic" and "MOSFET" come to mind, but I'm not sure. Up to now, the most challenging things I've ever had to interface with Arduinos were a 3.3v i2c sensor I had to connect to a 5v Arduino (using a MOSFET-based level shifter board), and a 5v string of WS2811 pixels to a ESP32 (using a SN74LVC1T45 dual-rail shifter, because the MOSFET level shifter boards didn't quite work for this). Switching 9-16v (potentially, up to 3-5 watts) to ground is completely new territory for me.

My knowledge of MOSFETs can be succinctly summarized as, "I know they exist, I own a bag of them, and I'm vaguely aware that they're used for implementing open-drain interface logic".

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For reference, here's the schematic to a "Forty-9er":

here's a "Pixie" schematic:

Answer 1

Keying to ground is simple enough with a single transistor and a resistor. This will work at 3.3 or 5 volts.

^{simulate this circuit – Schematic created using CircuitLab}

The Pixie seems to put all the PA current through the key, so you might use a smaller resistor, 220 ohms, to guarantee the transistor is saturated.

You could also use a MOSFET if you have a logic-level one in your junk box, the circuit is the same but no resistor required.

A small capacitor might reduce the amount of RF noise from the micro leaking into the radio circuit, but that will be a problem even just putting them anywhere near each other. Better might be to use an opto-isolator. For the 49-er you could probably use it with no external driver:

^{simulate this circuit}

Choose an isolator with a photo-transistor, not a photo-diode, and for the Pixie, you need to check whether its maximum current and "on" resistance will be good enough.

Answer 2

You could comfortably switch the Pixie directly, because the voltage on the base of Q3 when the key is open would be only 3.4 V when the supply voltage is 16 V. That voltage is set by the R5/R6 voltage divider. The current should be much less than the 20 mA (if my memory serves me correctly) that the microcontroller pin is capable of sourcing or sinking.

The Forty-9er has a full 12 V on the key, so you'd want a transistor for that. If it were my project I'd throw in a transistor for a Pixie anyway, in the hope that an accidental voltage transient would fry the transistor instead of the microcontroller.

Here's how to do it with an NPN. (This is my first time inserting a schematic, and I don't know how to make it smaller, sorry; if anyone wants to edit my question to make it smaller, please go ahead.) Any old generic switching transistor such as a 2N2222 or a 2N3904 would work. (I miss being able to buy them at Radio Shack...) R1 limits the current through the transistor. R2 limits the current through the base of the transistor.

^{simulate this circuit – Schematic created using CircuitLab}

Doing it with a MOSFET would also be simple:

^{simulate this circuit}