The SPJ-1 (Sinclair Professional Joystick) was produced to accompany [[Amstrad]]'s unsuccessful Sinclair PC200 (an old PC with 8086 processor). The SPJ-1 was a fully-fledged analogue joystick, with potentiometers used to measure input (rather than switches as in its digital precessors). It was probably a rebadged PC-standard device. The SPJ-1 was, like its host machine, a flop, and today appears to be exceptionally rare.
== Analog PC Joysticks ==
Analog PC Joysticks have a very similar 15pin connector. Theoretically, a few PC joysticks might be CPC+ compatible. However, in practice, this is rather unlikely - the potentiometers must have correct value, the button common pin must be correct, and the joystick may not contain additional electronics like auto-fire circuits, electrolyte capacitors, or digital-switch to analog-signal converters.
'''Compared with PCs, the CPC+ outputs reversed polarity on the joysticks power supply pins, so any special electronics in PC joysticks may get destroyed when connecting it to a CPC Plus/GX4000.'''
If you want to use a PC joystick: Remove any electronics other than the potentiometers and buttons. Ensure that buttons are wired to correct common pin. And, probably you'd need to replace the potentiometers (if they aren't 180K) - however, this may be the complicated part: potentiometers in analog joysticks aren't necessarily matiching standard mechanic dimensions.
== CPC+ Software with Analog Joystick Support ==