Karnataka KSEAB SSLC NSQF Electronics and Hardware cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. A 12 V supply across 240 Ω draws 50 mA and dissipates 0.6 W.

Series circuit: same current through all elements, resistances add (R_total = R1 + R2 + …), voltages divide in proportion to resistance.

Parallel circuit: same voltage across all branches, conductances add (1/R_total = 1/R1 + 1/R2 + …); two resistors give R_total = R1R2/(R1+R2).

Kirchhoff's current law: sum of currents into a node equals sum out; Kirchhoff's voltage law: algebraic sum of voltages around any closed loop is zero.

Conductor resistance R = ρL/A: doubles with doubled length, halves with doubled cross-section; resistivity ρ rises with temperature in metals.

AC quantities: RMS = peak/√2 for a sine wave; frequency f = 1/T; reactance X_C = 1/(2πfC) falls with frequency, X_L = 2πfL rises with frequency.

Continuity and insulation checks use the ohmmeter on a de-energised circuit; a healthy insulation resistance to earth is in the megohm range, not ohms.

Power source rules: a voltage source ideally holds V constant regardless of load; a current source holds I constant; real sources have internal resistance that causes droop under load.

Resistor colour code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; gold band = ±5 %, silver = ±10 %.

Diode: conducts when anode is positive relative to cathode (forward, ~0.7 V drop for silicon, ~0.3 V for germanium); blocks in reverse until breakdown.

Bipolar transistor regions: cutoff (no base current, no collector current), active (I_C = β·I_B, used for amplification), saturation (V_CE ≈ 0.2 V, used as a switch).

LED needs a series current-limiting resistor: R = (V_supply − V_LED)/I_LED. Example: (5 − 2)/0.015 = 200 Ω for 15 mA.

Capacitor stores charge Q = CV; blocks DC and passes AC; electrolytic types are polarised and must be fitted with correct polarity or they vent.

Zener diode is operated in reverse breakdown to give a regulated reference voltage; a series resistor limits current through it.

Inductor opposes change in current (V = L·di/dt); its opposition to AC rises with frequency; core saturation reduces inductance.

Identify component polarity/orientation before fitting: diode band = cathode, electrolytic capacitor stripe = negative, IC notch or dot = pin 1.

Typical soldering iron tip temperature for tin-lead solder is 300–350 °C; for lead-free (SAC) solder 350–380 °C, with short contact times to protect components.

Good solder joint: smooth, shiny, concave fillet wetting both pad and lead; a dull, ball-shaped or grainy joint indicates cold joint or insufficient heat.

Sequence: clean tip, tin the tip, heat the pad and lead together for about 1–2 s, apply solder to the joint (not the iron), withdraw solder then iron, do not move the joint while cooling.

Solder flux removes oxides and improves wetting; excess or uncleaned corrosive flux causes leakage and corrosion, so clean the board after soldering.

Cold joint remedies: reheat with fresh flux and a little solder; a joint that is bridged between two pads is removed with desoldering braid or a solder sucker.

Do not use acid-core plumbing solder on electronics; use rosin or no-clean flux cored solder of appropriate diameter (0.5–1.0 mm for PCB work).

ESD-safe practice at the bench: earthed wrist strap, grounded iron tip, no live soldering of powered circuits.

Solder iron safety: rest in a stand, keep the cable clear of the hot tip, allow tips and joints to cool before handling; lead-bearing solder requires hand washing before eating.

PCB assembly order: lowest-profile parts first (resistors, diodes, ICs), then taller parts (capacitors, connectors, transformers); polarised parts checked before every solder.

Through-hole insertion: bend leads slightly to hold the part, solder on the copper side, trim leads flush after soldering; do not trim before soldering unless leads are pre-cut to length.

Wire colour and routing conventions: keep mains and low-voltage signal wiring physically separated, use grommets at chassis entries, and route signal wires away from transformer and switching-noise sources.

Crimp and screw terminals must be mechanically tight before soldering is considered; a soldered stranded wire must be tinned and not used in a screw clamp that requires bare strands.

Wire gauge selection follows current and voltage drop: thicker wire for higher current and longer runs; insulation rating must exceed circuit voltage.

Component marking and orientation: IC pin 1 indicated by notch, dot or chamfer; connector keying must match the mating part; keep a build drawing or BOM to verify values.

Inspection after assembly: check for solder bridges, missing parts, wrong polarity, tombstoned SMD parts, and flux residue before applying power.

Multimeter functions: V (voltage, always in parallel with the point measured), A (current, in series by breaking the circuit), Ω (resistance, only on a de-energised circuit).

Ammeter has very low internal resistance and must never be connected across a supply; voltmeter has very high internal resistance and must never be placed in series with a load.

Select the correct range or auto-range before measuring; start on the highest range for unknown values to avoid meter damage.

Continuity mode beeps below a few ohms; it is a go/no-go test, not a precision resistance measurement.

Oscilloscope: probe ground clip to circuit ground, set vertical scale (volts/div) and time base (time/div) before connecting to unknown signals; ×10 probe multiplies the displayed reading by 10.

Signal generator and logic probe uses: inject a known waveform to trace a signal path; logic probe shows high, low and pulse states in digital circuits.

Measurement error sources: meter loading, lead resistance, wrong range, and measuring while the circuit is powered; record units and tolerance with every reading.

Insulation tester (megger) is used on de-energised equipment only; never on a live circuit or on electronic assemblies that cannot withstand the test voltage.

Logic gates: AND outputs 1 only when all inputs are 1; OR outputs 1 when any input is 1; NOT inverts; NAND and NOR are the inverted forms and are universal gates.

Boolean identities: A + 0 = A, A·1 = A, A + A = A, A·A = A, A + A' = 1, A·A' = 0; De Morgan: (A·B)' = A' + B' and (A + B)' = A'·B'.

Binary and hexadecimal: 4 bits = 1 hex digit; 8 bits = 1 byte = 0–255 decimal; convert by grouping bits in fours from the LSB.

Flip-flops are 1-bit memory elements; a clock edge triggers the change; counters and shift registers are built from flip-flops.

Microcontroller minimum system: power supply with decoupling capacitors, clock source (crystal or internal oscillator), reset circuit, and programming/debug interface.

GPIO pins are configured as input or output; an input pin needs a defined level (pull-up or pull-down) and an output driving a load needs a driver stage or current limit.

ADC converts an analogue voltage to a digital number: resolution = V_ref/2^n, so a 10-bit ADC with 5 V reference resolves about 4.9 mV per step.

Firmware basics: setup runs once, loop repeats; use delay or timer for timing; debounce mechanical switches in software or with a capacitor.

Troubleshooting sequence: observe the fault, check power first, then fuses, connectors, and supply rails; divide the circuit in half to localise the fault (signal tracing).

Voltage-rail test: measure each supply rail at the load with the circuit powered; a rail that collapses under load indicates a short, a failing regulator, or an overloaded output.

Visual inspection before measurement: look for burnt parts, bulged electrolytic capacitors, cracked solder joints, loose connectors, and discoloured PCB.

Substitution and comparison: swap a suspect part with a known-good one, or compare readings against a working identical unit; change one variable at a time.

Preventive maintenance: clean dust from vents and fans, check and replace dried capacitors, tighten terminal screws, verify earth continuity, and record service history.

Intermittent faults are traced by flexing cables and tapping boards while monitoring the meter; note that the fault may be a cold solder joint or worn connector.

After repair: re-test all functions, check that no new shorts exist, confirm correct fuse rating, and record the root cause and parts replaced.

ESD damage threshold: many CMOS devices are damaged by discharges below 100 V, far below the ~3 kV level a human can feel; damage may be latent and appear later as failure.

ESD controls: earthed wrist strap with a 1 MΩ series resistor, conductive or dissipative work surface, grounded iron tip, and antistatic bags for storage and transport.

Never wear an earthed wrist strap when working on live mains equipment; the resistor protects against shock but the strap still creates a path to earth.

Mains safety: isolate and lock off before work, verify dead with a tester, discharge capacitors, use one hand where possible, and never bypass fuses or earth connections.

First aid for electric shock: isolate the supply before touching the casualty, call for help, and start CPR if trained and the casualty is unresponsive.

Soldering fume extraction is required; rosin flux fumes cause respiratory irritation, and lead solder requires hand washing and no eating or drinking at the bench.

Fire safety: use CO2 or dry powder on electrical fires, never water; know the location of the isolator and the nearest extinguisher.

Workstation ergonomics: adjust bench and chair height, keep the workpiece close, take breaks from repetitive soldering, and use magnification to reduce eye strain.

Resume structure for hardware roles: contact details, technical skills (soldering, multimeter, PCB assembly, microcontroller), projects with measurable outcomes, education, and references.

Interview preparation: be ready to explain a circuit you built, a fault you diagnosed, and the safety practice you followed; use the situation-task-action-result structure.

Teamwork and communication: report faults with specific measurements (rail voltage, current draw, part reference) rather than vague descriptions; keep a written handover log.

Entrepreneurship basics: fixed costs (rent, tools) do not vary with output; variable costs (components, consumables) do; break-even quantity = fixed costs / (price − variable cost per unit).

Pricing a repair or assembly job: cover material cost, labour time at an hourly rate, overhead, and a margin; quote after inspection and record the agreed scope.

GST and small-business registration in India: a business above the turnover threshold must register for GST; keep invoices, purchase bills and a simple ledger from day one.

Customer handling: give a written estimate, state the warranty terms, and record the device condition on receipt to avoid disputes.

Continuous learning: datasheets, application notes and manufacturer reference designs are the primary technical sources; keep a personal logbook of circuits built and faults solved.

Resistor colour code: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; gold band = ±5 %, silver = ±10 %.

Diode: conducts when anode is positive relative to cathode (forward, ~0.7 V drop for silicon, ~0.3 V for germanium); blocks in reverse until breakdown.

Bipolar transistor regions: cutoff (no base current, no collector current), active (I_C = β·I_B, used for amplification), saturation (V_CE ≈ 0.2 V, used as a switch).

LED needs a series current-limiting resistor: R = (V_supply − V_LED)/I_LED. Example: (5 − 2)/0.015 = 200 Ω for 15 mA.

Capacitor stores charge Q = CV; blocks DC and passes AC; electrolytic types are polarised and must be fitted with correct polarity or they vent.

Zener diode is operated in reverse breakdown to give a regulated reference voltage; a series resistor limits current through it.

Inductor opposes change in current (V = L·di/dt); its opposition to AC rises with frequency; core saturation reduces inductance.

Identify component polarity/orientation before fitting: diode band = cathode, electrolytic capacitor stripe = negative, IC notch or dot = pin 1.

Resume structure for hardware roles: contact details, technical skills (soldering, multimeter, PCB assembly, microcontroller), projects with measurable outcomes, education, and references.

Interview preparation: be ready to explain a circuit you built, a fault you diagnosed, and the safety practice you followed; use the situation-task-action-result structure.

Teamwork and communication: report faults with specific measurements (rail voltage, current draw, part reference) rather than vague descriptions; keep a written handover log.

Entrepreneurship basics: fixed costs (rent, tools) do not vary with output; variable costs (components, consumables) do; break-even quantity = fixed costs / (price − variable cost per unit).

Pricing a repair or assembly job: cover material cost, labour time at an hourly rate, overhead, and a margin; quote after inspection and record the agreed scope.

GST and small-business registration in India: a business above the turnover threshold must register for GST; keep invoices, purchase bills and a simple ledger from day one.

Customer handling: give a written estimate, state the warranty terms, and record the device condition on receipt to avoid disputes.

Continuous learning: datasheets, application notes and manufacturer reference designs are the primary technical sources; keep a personal logbook of circuits built and faults solved.