Final answer:
Yes, a signal circuit is any electrical circuit that controls another through a relay or similar device. It is true and is crucial in various digital signal applications in electronics (A).
Step-by-step explanation:
A signal circuit is correctly defined as any electrical circuit that controls another circuit through a relay or an equivalent device. This is true. Signal circuits can use digital signals, as found in computers, where integrated circuits contain transistors acting as on-off switches, representing binary code.
Such applications of signal circuits are fundamental to the operation of various electronic devices and systems. In a more broad perspective, electrical circuits include components like capacitors, resistors, and switches that are interconnected to perform specific functions. Circuit diagrams represent these components and connections symbolically.