olemus
(a) elektriahelate põhimõtteskeem
= a graphical representation of an electrical circuit
https://en.wikipedia.org/wiki/Circuit_diagram
näiteid:
http://www.sciencekids.co.nz/images/pictures/physics/electricitycircuit.jpg
https://i.pinimg.com/236x/7d/6e/df/7d6edf76851421c67b86f9434550b410--circuit.jpg
http://www.electroschematics.com/wp-content/uploads/2009/11/ultrasonic-sensor-circuit-diagram.gif
https://i0.wp.com/bestengineeringprojects.com/wp-content/uploads/2015/02/circuit-diagram-of-digital-timer-with-clock.jpg

(b) muu analoogiline põhimõtteskeem
a graphical representation of pneumatic or hydraulic flow circuits
http://learnchannel-tv.com/wp-content/uploads/2019/05/Pneumatic-circuit-diagram-how-to-read.gif

tingmärgid
https://electronicsclub.info/circuitsymbols.htm

https://www.edrawsoft.com/electrical-symbols.html

https://symbols.radicasoftware.com/

http://www.rapidtables.com/electric/electrical_symbols.htm

standardid
IEC 60617
http://std.iec.ch/iec60617