IanMetro wrote:

Resistor - Base to Earth
Although not needed in the theory, in practice the Base needs a Reference Point to overcome any Transistor Base to Collector leakage.
Without it the transistor will not switch off quickly or reliably.

Diode - Across Buzzer
The buzzer is an Inductive Load and without the Diode would be subject to Ringing when current suddenly stopped. This Back EMF could damage the transistor.
The circuit is very similar to that of Electronic Ignition.

768will
The electronics we studied at RAF Locking were pre-transistor, I still have my late 1960's Post Grad Notes.



We were taught by comparing valve ccts with the new-fangled solid state.
Example page below



I remember about providing the reference to earth very well after investigating failure of the telephone electronic test equipment that I was installing.
A circuit similar to that of Jules was used to provide the (now old fashioned) dialled pulses. As it was cutting edge in the early 70s the designers did not realise that just cutting the current to the base (as you can in theory) does not always immediately switch off the transistor.
I had get the factory to change the design.

PS Sorry Jules for pinching a post of your thread. FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 85k+ miles) (MY2015)
Metro in its 12th Year of (Extended) LR Warranty / Full LR Service History
(Expensive, but Trouble/Worry free - hopefully?)

No problem - Im enjoying reading it all. Every day is a school day.

Solid state electronics was completely absent from my school education - something I feel I missed out on really.
That said, it has not had much of an impact on the many hobbies Ive had; hill walking, cycling, sailing, SCUBA diving, building PCs (remember autoexec.bat and config.sys) and messing with Landrovers, plus the usual home DIY stuff.

I get frustrated with myself for not having a better understanding of things like this project - so this is a bit of self learning for my own benefit. Jules