This website is great for wiring projects and how to’s:
www.the12volt.com
A common one that we use is a 3 relay latch – which allows you to pulse 12V to a relay, to let it latch a constant 12V on the output. Another pulse disengages. This is great for hooking up boom sense wiring to Hardi or other sprayers which do not supply constant 12V. It is also useful for using momentary footswitches for sprayer/spreader on/off situations.
Here is the example: