This is the schematic for an old DC fan control circuit (automotive), and I don't recognize the symbol highlighted in yellow below. Can anyone say for sure? I'm guessing a transistor, but there's no arrow indicating NPN or PNP.

Thanks!

 

The fact it is enclosed in a square box with multi I/O, I would say it indicates Solid State Device.
IOW, Black Box.!
Max.

 

Thanks for the reply, you could be correct. After seeing a picture of the real device that this schematic supposedly describes, there are a lot more discrete components on the PCB than are shown in the schematic. So either the author of the schematic didn't think those parts were important, or they didn't understand it.. or both!

 

It is an IC package or solid state device. A lot of OEMs do not put the full schematic in there for three reasons. One is they take up too much space. The second is most rechnicians wouldnt understand them and the third is that they simply dont want you to know how it works. A transistor without an arrow just suggests it is a switching device of sorts. That is all we need to know in their eyes. This diagram actually has more info than most.

 

there are a lot more discrete components on the PCB than are shown in the schematic. So either the author of the schematic didn't think those parts were important, or they didn't understand it.. or both!

Also maybe the circuit for this function may be made proprietary by the OEM so the schematic cannot be shown for this section.
Max.