As promised, we corrected the error in the 555 oscillator circuit that failed to work last time, and made a modification to allow the frequency to be changed.
We then discussed the difference between relaxation oscillators and harmonic oscillators. We looked at a simple 1-transistor harmonic oscillator that uses a 3-stage phase-shift network to determine the frequency of oscillation. We obtained a very respectable sine wave output at about 22kHz.
Finally, we followed up on Mike's question about interfacing the 12V output of a central-heating valve to the 3V input of a microprocessor. He showed us the working circuit, which opened and closed the valve very nicely. We were particularly impressed with the Bluetooth interface on the processor, which allowed Mike to control the device from his phone.