PCB Design

[vaelen]

Hi,

I'm creating a PCB for this that I plan to make available, but I have a question.

When you don't have a variable resistor, what values do you use for R7 and R8? The same as R6? (Based on the radio?)

Thanks,
Andrew (JJ1OKA)

[vaelen]

Here is my version of that piece of the circuit. R4 and R5 on this diagram are the same as R7 and R8 in your original schematic.

[n0nas]

Hi Vaelen.
You have one error in your schematic. R9 is in the wrong place. It should be between base of Q1 and ground. The emitter of Q1 must go directly to ground, zero ohms. Q1 is the PTT transistor and must pull the PTT line directly to ground. The purpose of R8 is to limit the current into Q1 to a safe value for the Pico CPU pin. The purpose of R9 is to make sure Q1 turns off when the Pico CPU pin goes low. The purpose of R6 is to make the audio out pin compatible with handie-talkie keying circuits. The value changes with radio brand. If connecting the circuit to a mobile radio that needs a separate PTT line, then R6 should be removed. It is really better if R4+R5 is a pot instead of fixed values. It is far easier to turn a pot than it is to change resistors to set the audio level going into the radio transmitter. You can always replace R5 with a pot, probably anything in the 1K to 10K range, using one outside leg plus the center leg to replace R5. But I think I'd recommend the circuit that K4APR used instead of R3/C3.
Doug.

[n0nas]

Nope, I was thinking of some other schematic. K4APR didn't include a circuit and probably used the existing design. I was concerned that R3/C3 makes a very "stiff" pull-down on the Pico CPU PWM output and I'm not sure what the output level and signal harmonics will look like at the audio OUT pin. I was thinking of a different RC filter configuration, but if this works, use it.....
Doug.

[vaelen]

I'm pretty sure you were correct that I made a mistake on the schematic. I never could get it working quite right and I ended up giving up on it and going with a different solution.

Here is what I had: https://oshwlab.com/andrew_3060/picotnc-tnc-board

If someone wants to collaborate on this using EasyEDA, please let me know and I will add you as a contributor on the project.