HC-05 w/ ATTINY85 over 1 wire

I’m working on a project, where I’m turning a small RC car into an simple automous machine, which can avoid easy obstacles. It uses an ATTINY85, and it can use either a sonar, an infrared sensor or a HC-05 bluetooth module (for manual control).It recognises which sensor is attached and behaves accordingly.

I need 4 pins for the front steering coil and the DC motor (via H bridges), that leaves only one for the I/O (I don’t have, nor I don’t want to use a HV programmer). I managed to get the sonar and IR down to working on one pin, only thing left is the bluetooth module. I’ve switched from AVR-C to Wiring, and am using Arduino code to program the Tiny85, for educational purposes. I’m using the SoftwareSerial for the Bluetooth RX part, which works for the manual control purposes, but I’d like to have a TX capability as well.

You see, it’d be nice if the car gave some feedback to the serial terminal, of it’s current speed, etc. I’m also storing “constants” in the EEPROM (the car’s autonomous speed, which pin corresponds for “TURN LEFT” for example, etc…), and I can change these via bluetooth, but it’d be great if the car could tell us which value we are changing (“Enter new value for “speed”:…, etc.) so that it’d be more user friendly.

So in search for a solution, I came across a blog post, which is almost exactly what I need:

In this blog post he details, how you can separate RX/TX with a simple circuit. I tried out your code, but it doesn’t seem to work in my case with the HC-05. I might be doing something wrong, but I looked through it again and again, and I don’t see any error that I did.

Another thing I found was:

But I cannot seem to get this to work either. I’d really appreciate any help to get a single pin half-duplex serial working with the HC-05 and the ATTINY85.

I’m literally at the finish line with this project, and this is one of the final adjustments that I’d need to make this project a little bit more user friendly, and to help demonstrate the capabilities of this great small MCU.

For half duplex or similar solutions you need not only external hardware (multiplexers...), but also pins to control these multiplexers.

I'd distribute such a project onto multiple controllers. One could control the motors and movements, another one the additional sensors and communication. Each controller will add another serial port to your system, timers and more.