ASVAB Electronics Information Subtest: Decoding Electrical Circuit Codes

By Rod Powers

Electronic circuits can be combined to create complex systems, such as those required to operate a stereo system. Block diagrams are used to show the various combined circuits that form a complex system.

Many of the questions on the Electronics Information subtest require you to identify an electronic component symbol and know what that component does in an electronic circuit.

  • Wires: Wires are used to pass current from one part of the component to another. Wires that are connected to each other are indicated by a dark circle and are called joined wires. Sometimes in complex circuit diagrams, it’s necessary to draw wires crossing even though they aren’t connected. In this case, the dark circle is omitted, or a hump symbol is drawn to make it clear the wires aren’t connected — this is called unjoined wires.

  • Cell: A cell supplies electrical current. Some call this a battery, but technically a battery is more than one cell. The large terminal is positive.

  • Battery: A battery is two or more cells. The large terminal is positive.

  • DC power supply: A DC power supply provides direct current. Direct current always flows in one direction.

  • AC power supply: An AC power supply provides alternating current. Alternating current constantly changes direction at a specific frequency.

  • Fuse: A fuse is a safety device that blows if the current flowing through it exceeds a specified value.

  • Transformer: A transformer consists of two coils of wire linked by an iron core. Transformers are used to step up and step down AC voltages. No electrical connection exists between the coils. Energy is transferred between the coils by the magnetic field in the core.

  • Ground: A ground is a connection to the earth.

  • Transducer: A transducer is a device that converts energy from one form to another. Here are various types of transducers:

    • Lighting lamp: Converts electrical energy to light, such as in a light bulb or automobile headlight

    • Indicator lamp: Converts electrical energy to light for such uses as a warning light on a car’s dashboard

    • Motor: Converts electrical energy to kinetic energy

    • Heater: Converts electrical energy to heat

    • Bells and buzzers: Convert electrical energy to sound

    • Microphone: Converts sound to electrical energy

    • Earphones and speakers: Convert electrical energy to sound

      Symbols in electronic circuit diagrams.
      Symbols in electronic circuit diagrams.
  • Inductor: An inductor is a coil of wire that creates a magnetic field when current passes through it.

  • Switch: Here are several types of switches:

    • Push switch: A push switch allows current to flow only when the button is pressed, such as in a doorbell.

    • Push-to-break switch: With this switch, the circuit is normally closed; the circuit is open only when the button is pressed.

    • On/off switch: An on/off switch allows current to flow only when it’s in the closed (on) position.

    • Two-way switch: A two-way switch directs the flow of current to one of two routes, according to its position.

    • Dual on/off switch: This type is often used to switch main electricity because it can isolate both the live and neutral connections.

    • Relay (relay switch): A relay is an electrically operated switch that may operate multiple switches at one time. Current flowing through a coil sets up a magnetic field, which causes the lever(s) to move, effectively changing the (relay) switch’s position(s).

  • Resistor (nonvariable): Resistors restrict the flow of electric current. Resistors are rated in ohms and have a color code on them to indicate their value, tolerance, and sometimes quality. The band code is as follows:

    • Black is 0.

    • Brown is 1.

    • Red is 2.

    • Orange is 3.

    • Yellow is 4.

    • Green is 5.

    • Blue is 6.

    • Violet is 7.

    • Gray is 8.

    • White is 9.

    The first and second bands on the resistor are the first two digits in the resistor’s value. The next band indicates the multiplier. A gold or silver band after the first bands indicates tolerance, and a quality band may follow the tolerance band.

  • Variable resistor: Variable resistors also restrict the flow of electric current. There are several symbols in use in circuit diagrams for standard variable and preset variable resistors. Types of variable resistors include the following:

    • Rheostat: A type of variable resistor with two contacts, usually used to control current; examples of controlling current would be adjusting lamp brightness or adjusting motor speed

    • Potentiometer: A type of variable resistor with three contacts that’s used to control voltage

    • Preset variable resistor: A device that operates with a small screwdriver or similar tool; it’s designed to be set when the circuit is made and then left without further adjustment

  • Capacitor: Capacitors store electric charge. They’re used with resistors in timing circuits because it takes time for a capacitor to fill with charge. They’re also used in filter circuits because capacitors easily pass AC signals but they block DC signals. Two types of capacitors include the following:

    • Polarized capacitors must be connected the correct way in circuit.

    • Variable capacitors are used most often in radio tuning circuits.

  • Diode: Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve. Light-emitting diodes (LEDs) emit light when an electric current passes through them. Specialized diodes, called Zener diodes, do allow current in the opposite direction after a threshold is met.

  • Transistor: Transistors amplify current.

  • Amplifier: An amplifier isn’t actually an electronic component but instead is a complex circuit. Amplifier circuits are used to magnify power, current, or voltage.

  • Antenna: An antenna is a device designed to receive and/or transmit radio signals.

Here’s a diagram of an adjustable timer circuit. See how many components you can identify.

An adjustable timer circuit.
An adjustable timer circuit.