Thank you so much for taking the time to reply to my questions, the varying suggestions are very helpful and have given me quite a lot to consider (still reading up on some background hence the delay in this reply).
I have segmented this post to make it a little simpler to read.
The lighting effect I am looking to achieve is this (video).
To shed some light this is the size of the model we’re talking about.
There are two areas where electronics can fit. Just under the roof there is a small space, not fully uniform due to a curvature but it is approx. 30mm L x 16mm W. Depth is no more than 3mm.
The roof will however need to accommodate the LEDs legs and wiring so I suspect that may be all that can go there. I may be able to squeeze in the resistors there too.
The other available area is to the back of the car, it’s a larger area but would need to fit the battery and everything else (microprocessor, switch, maybe resistors).
This area is about 3cm in total length and has varying depths (2cm L x 0.6cm D and 1cm L x 1.2cm D)
From reading all the suggestions as size constraints is the determining factor here it seems that a standalone Attiny processor with no PCB and any other non-essential components on a typical Arduino might be the best approach.
The DIP socket suggestion seems a good way to avoid soldering issues – I am not sure what the risk is but I am a beginner with the soldering too so I suppose it could be high. Does anyone know what the smallest DIP socket-Attiny combo is, are the Attiny24/45/85 all the same physical size? (I couldn’t find their dimensions listed online but the only difference seems to be memory)
Robin2’s suggestion of the Attiny 1634 with SOIC format is of interest although if the soldering poses too high a risk perhaps I should avoid. Does anyone know if there is some sort of socket housing for SOIC formats which might remove this issue?
Do I need to use ICSP to upload to the Attiny – this looks a little bit of a challenge. Is @Robin2 ’s suggested link as simple a way as any
I read that one can buy Attinys with the Arduino bootloader already pre-loaded. Does this make sense to simplify the task or will I have had to do the difficult bit already for the ICSP above?
A few other questions on my mind..
Battery model – any idea of particularly small Lipo batteries. I see the Attiny works with 2.7-5.5V.
LEDs – I found reference to nano and pico LEDs for blue light. Does anyone know where these can be purchased online? I didn’t see them on the HobbyKing site.
What are the symbols connected to pins 5/6/7 representing are in @CrossRoads schematic can someone confirm?
Again thanks for all the help. Really appreciated.