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    If ground can be shared, a simple resistor voltage divider (4:1) in parallel with the light bulb will work fine for detecting 12V signals . But this question has probably been asked and answered here and on electronics.se before, if you search . There are many other approaches with varying degrees of isolation, safety, and complexity. The next step up is an open collector buffer like the trusty 7407 or an opto-isolator. Commented Mar 15, 2018 at 14:49
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    A Zener diode would help to protect the circuit. Commented Mar 15, 2018 at 15:07
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    Please note that a 12V source may be quite a lot more than 12V if it is not regulated. You need to take this into account if you choose the resistor divider option. Commented Mar 15, 2018 at 15:56
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    @joan Yes, thank you for mentioning that. The 12V source is regulated on this particular circuit, along with fuse protection. Commented Mar 15, 2018 at 15:58
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    It is a bad idea to implement either of these solution in an automotive circuit. This question is beyond the scope of rpi.se Commented Mar 15, 2018 at 17:24