TEJ3M · Unit 4 test review

Electronics and Digital Logic: Test Review

Everything for the Unit 4 test on one page: the key facts from all 5 lessons, the mistakes that cost the most marks, and a practice test that points you back to the lessons you need.

Key facts

Voltage, Current, Resistance and Power

$V = I R$ , and its rearrangements $I = V / R$ and $R = V / I$ .
$P = V I = I^{2} R = V^{2} / R$ .
Convert to base units (V, A, Ω) before substituting: 20 mA = 0.020 A, 4.7 kΩ = 4700 Ω.
Voltmeter in parallel; ammeter in series; ohmmeter with the power off.
Mains in Canada is 120 V AC at 60 Hz. Everything in this course is DC.
About 30 mA through the chest can be fatal — current does the harm, voltage drives it.

Components and Schematic Diagrams

Resistor colour code: digit, digit, multiplier, tolerance. Black=0 through White=9.
Gold tolerance band = ±5%; silver = ±10%.
Silicon diode forward drop ≈ 0.7 V; LED forward voltage is 1.8–3.4 V depending on colour.
An LED cannot limit its own current and always needs a series resistor.
Electrolytic capacitors are polarized; ceramic capacitors are not.
A transistor lets a small current control a large one — the basis of every digital switch.
On a schematic, a dot at a crossing means connected; no dot means the wires cross without joining.

Series, Parallel and Voltage Dividers

Series: current is shared, voltages add, resistances add. Total > largest resistor.
Parallel: voltage is shared, currents add, reciprocals add. Total < smallest branch.
Two in parallel: $R_{T} = \frac{R _{1} R _{2}}{R _{1} + R _{2}}$ .
n equal resistors in parallel give $R / n$ .
Voltage divider: $V_{o u t} = V_{in} \times \frac{R _{2}}{R _{1} + R _{2}}$ .
Kirchhoff: voltages around a loop sum to zero; currents into a junction equal currents out.

Logic Gates and Truth Tables

In 5 V logic, above ~2.0 V is logic 1 and below ~0.8 V is logic 0.
AND needs every input true; OR needs at least one; XOR needs the inputs to differ.
A bubble on a gate's output means invert: NAND is AND then NOT.
n inputs give $2^{n}$ truth-table rows.
Half adder: SUM = A ⊕ B, CARRY = A · B.
NAND and NOR are universal — any logic function can be built from either alone.
Never leave a logic input floating; tie unused inputs high or low.

Boolean Algebra and Simplification

$A + A = A$ and $A \cdot A = A$ — there is no 2 in Boolean algebra.
$A + 1 = 1$ , $A \cdot 0 = 0$ , $A + A^{'} = 1$ , $A \cdot A^{'} = 0$ .
Absorption: $A + A \cdot B = A$ .
De Morgan: $(A \cdot B)^{'} = A^{'} + B^{'}$ and $(A + B)^{'} = A^{'} \cdot B^{'}$ .
Inverting an expression inverts each variable AND swaps the operator.
AND binds more tightly than OR, exactly as × binds more tightly than +.
Always verify a simplification with a truth table.

Where marks get lost

Substituting milliamps or kilohms straight into $V = I R$ without converting to amps and ohms. (Voltage, Current, Resistance and Power)
Connecting an ammeter across a supply instead of in series — it is nearly a short circuit and blows the fuse. (Voltage, Current, Resistance and Power)
Measuring resistance in a live circuit. The ohmmeter supplies its own current and the reading is meaningless. (Voltage, Current, Resistance and Power)
Choosing a resistor by value alone and ignoring its power rating. (Voltage, Current, Resistance and Power)
Saying "voltage kills". Current does the damage; voltage is what drives it through the body's resistance. (Voltage, Current, Resistance and Power)
Forgetting that $P = I^{2} R$ squares the current — doubling current quadruples the heat. (Voltage, Current, Resistance and Power)
Reading resistor bands from the wrong end. Start where the bands are crowded; the lone tolerance band is at the far end. (Components and Schematic Diagrams)
Connecting an LED with no series resistor. (Components and Schematic Diagrams)
Fitting an electrolytic capacitor backwards — it will overheat and can burst. (Components and Schematic Diagrams)
Confusing an LED's anode and cathode. The longer lead is the anode (positive); the flat side of the rim marks the cathode. (Components and Schematic Diagrams)
Reading a schematic crossing without checking for a junction dot. (Components and Schematic Diagrams)
Placing components along a breadboard row instead of across the column groups. (Components and Schematic Diagrams)
Adding reciprocals and forgetting the final flip — $1/ R_{T}$ is not $R_{T}$ . (Series, Parallel and Voltage Dividers)
Getting a parallel total larger than one of the branches and not noticing it is impossible. (Series, Parallel and Voltage Dividers)
Assuming voltage is shared equally between series resistors. It divides in proportion to resistance. (Series, Parallel and Voltage Dividers)
Using product-over-sum with three resistors. That shortcut is only valid for exactly two. (Series, Parallel and Voltage Dividers)
Forgetting that a real load connected to a divider's output sits in parallel with R₂ and changes the answer. (Series, Parallel and Voltage Dividers)
Mixing up which resistor is R₂ in the divider formula. R₂ is the one between the output node and ground. (Series, Parallel and Voltage Dividers)
Mixing up NAND and NOR. NAND is 0 only when both inputs are 1; NOR is 1 only when both are 0. (Logic Gates and Truth Tables)
Treating XOR as OR. XOR gives 0 when both inputs are 1. (Logic Gates and Truth Tables)
Missing rows in a truth table. n inputs always need exactly $2^{n}$ rows — list them in counting order. (Logic Gates and Truth Tables)
Leaving unused logic inputs floating. A floating CMOS input oscillates and can destabilize the whole chip. (Logic Gates and Truth Tables)
Reading the Boolean + as ordinary addition. $1 + 1 = 1$ in Boolean algebra, because it means OR. (Logic Gates and Truth Tables)
Forgetting that AND binds tighter than OR, so $A + B \cdot C$ means $A + (B \cdot C)$ . (Logic Gates and Truth Tables)
Writing $(A \cdot B)^{'} = A^{'} \cdot B^{'}$ . De Morgan requires swapping the operator too. (Boolean Algebra and Simplification)
Writing $A + A = 2 A$ . There is no 2 — OR of a value with itself is the value. (Boolean Algebra and Simplification)
Forgetting $A + 1 = 1$ and treating it like arithmetic addition. (Boolean Algebra and Simplification)
Ignoring precedence: $A + B \cdot C$ means $A + (B \cdot C)$ , not $(A + B) \cdot C$ . (Boolean Algebra and Simplification)
Applying De Morgan to only part of an expression while a bar still spans the rest. (Boolean Algebra and Simplification)
Declaring a simplification finished without verifying it against a truth table. (Boolean Algebra and Simplification)

Practice test

25 questions from across the unit. Each one shows the lesson it comes from.

Voltage, Current, Resistance and Power
1
A 330 Ω resistor has 5 V across it. What current flows, in milliamps?
mA
Voltage, Current, Resistance and Power
2
What resistance draws 40 mA from a 9 V supply? Answer in ohms.
Ω
Voltage, Current, Resistance and Power
3
How much power does a 1 kΩ resistor dissipate with 10 V across it, in milliwatts?
mW
Voltage, Current, Resistance and Power
4
Express 2.2 kΩ in ohms.
Voltage, Current, Resistance and Power
5
A ½ W resistor of 100 Ω is used. What is the maximum current it can safely carry, in milliamps?
mA
Components and Schematic Diagrams
6
A resistor is banded brown, black, orange. What is its resistance in ohms?
Ω
Components and Schematic Diagrams
7
Why must an LED never be connected directly across a supply?
Components and Schematic Diagrams
8
What is the approximate forward voltage drop of a silicon diode?
Components and Schematic Diagrams
9
Match each component to its function.
Resistor
Capacitor
Diode
Transistor
Components and Schematic Diagrams
10
A circuit on a breadboard does nothing. The resistor is inserted with both legs in the same column of five holes. What is wrong?
Series, Parallel and Voltage Dividers
11
Find the total resistance of 470 Ω and 220 Ω in series, in ohms.
Ω
Series, Parallel and Voltage Dividers
12
A divider has V_in = 12 V, R₁ = 4.7 kΩ and R₂ = 10 kΩ. What is V_out in volts?
V
Series, Parallel and Voltage Dividers
13
In a series circuit with a 100 Ω and a 900 Ω resistor across 10 V, how is the voltage shared?
Series, Parallel and Voltage Dividers
14
Why is household wiring parallel rather than series?
Series, Parallel and Voltage Dividers
15
Order these steps for analyzing a combination circuit.
1. 1Repeat until a single total resistance remains
2. 2Work back outward to find each individual voltage and current
3. 3Use Ohm's law on the total to find total current
4. 4Reduce the innermost series or parallel group to one equivalent resistance
Logic Gates and Truth Tables
16
A NAND gate has inputs A = 1 and B = 0. What is the output?
Logic Gates and Truth Tables
17
How many rows does the truth table of a circuit with 5 inputs have?
rows
Logic Gates and Truth Tables
18
Evaluate $Y = A \cdot B + C$ when A = 1, B = 0, C = 1.
Logic Gates and Truth Tables
19
Why must an unused input on a CMOS logic chip be tied high or low?
Logic Gates and Truth Tables
20
A safety circuit must run only when $G \cdot S \cdot E^{'} = 1$ . The emergency stop E is pressed (E = 1). What is the output?
Boolean Algebra and Simplification
21
Which is the correct application of De Morgan's theorem to $(A + B)^{'}$ ?
Boolean Algebra and Simplification
22
Simplify $A + A \cdot B$ .
Boolean Algebra and Simplification
23
Simplify $A + 1$ .
Boolean Algebra and Simplification
24
Match each expression to its simplified form.
1. A. $0$
2. B. $1$
3. C. $A$
$A \cdot A$
$A \cdot A^{'}$
$A + A^{'}$
$A \cdot (A + B)$
Boolean Algebra and Simplification
25
Order these simplification steps for $A \cdot B + A \cdot B^{'}$ .
1. 1Apply the complement law to $B + B^{'}$
2. 2Verify the result with a truth table
3. 3Apply the identity law to $A \cdot 1$
4. 4Factor out the common variable A

Endless drill

Fresh numbers every time, for the calculation questions in this unit.

Endless drillnew numbers every time

Generating a problem…