Assam ASSEB HSLC NSQF Electronics and Hardware cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. In a series circuit current is common and voltages add; in a parallel circuit voltage is common and currents add.

Series resistance: R_total = R1 + R2 + …; parallel resistance: 1/R_total = 1/R1 + 1/R2 + …, and two resistors in parallel give R1R2/(R1+R2).

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

A capacitor stores charge Q = C × V and blocks DC while passing AC; reactance X_C = 1/(2πfC) falls as frequency rises.

An inductor stores energy in a magnetic field, opposes changes in current, and has reactance X_L = 2πfL that rises with frequency.

AC RMS and peak: V_rms = V_peak/√2 ≈ 0.707 V_peak; V_peak = 1.414 V_rms. Indian domestic supply is 230 V RMS, 50 Hz.

A transformer changes AC voltage by turns ratio V_s/V_p = N_s/N_p; it does not work on steady DC and does not change frequency or power (ideal).

Conductor resistance rises with length and temperature and falls with cross-sectional area: R = ρL/A.

Semiconductor diode: forward biased above ~0.7 V (silicon) it conducts; reverse biased it blocks until breakdown. A PN junction is the basis of rectifiers, LEDs and Zeners.

Resistor colour code (B-B-ROY-G-BV-G-S): black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9; third band is multiplier, gold ±5%, silver ±10%.

Four-band resistor: first two bands are digits, third is the number of zeros, fourth is tolerance. A 4.7 kΩ ±5% reads yellow-violet-red-gold.

SMD resistors print a three-digit code: first two digits are significant, third is the multiplier (number of zeros); '472' = 47 × 10² = 4.7 kΩ.

Ceramic capacitor codes are in picofarads: '104' = 10 × 10⁴ pF = 100 nF = 0.1 µF; '223' = 22 nF.

Electrolytic capacitors are polarized: the striped side with minus signs is the negative lead; reversing polarity can cause venting or explosion.

Diode identification: the band on the body marks the cathode (bar side of the symbol). LED longer lead is anode; flat spot on the package marks cathode.

Transistor pinouts differ by type: for common small-signal BJTs (BC547, 2N2222) the flat face with leads down is usually C-B-E or E-B-C - always confirm with datasheet or meter test.

A BJT has three terminals (base, collector, emitter) and is current-controlled; a MOSFET has gate, drain, source and is voltage-controlled with very high gate impedance.

IC packages: pin 1 is marked by a dot, notch or chamfer; count anticlockwise from pin 1 when viewed from the top for DIP packages.

Correct solder for electronics hand work is rosin-core Sn-Pb 60/40 or 63/37 eutectic (melts at 183 °C); lead-free SAC305 needs about 217–220 °C and a hotter tip.

Typical iron tip temperature: 300–350 °C for leaded solder, 350–380 °C for lead-free; a shiny concave fillet indicates a good joint, a dull ball or spike indicates a cold joint.

Heat the pad and lead together for about 1–2 seconds, feed solder into the junction (not the iron), then remove solder first and iron second; total contact should not exceed ~3–4 seconds.

Use flux to improve wetting and remove oxide; clean residue with isopropyl alcohol (IPA) after soldering.

Desoldering methods: solder sucker (vacuum pump), desoldering braid/wick, and hot-air rework for SMD; add fresh solder to a dry joint before wicking to improve heat transfer.

ESD-safe practice: use a grounded wrist strap and grounded iron tip; never touch IC pins or board traces with bare hands during assembly.

Component order on a PCB: fit and solder lowest-profile parts first (resistors, diodes, ICs), then taller parts (electrolytics, connectors); check polarity before soldering.

Solder bridges between adjacent pads are removed with braid and flux; cold joints are reflowed by re-heating pad and lead with a little fresh solder.

A voltmeter is connected in parallel with the component and has high input impedance; an ammeter is connected in series and has very low resistance.

To measure current, break the circuit and insert the meter in series; never connect a meter set to current range across a voltage source - it will short the supply.

Continuity mode (beeper) tests for a low-resistance path; isolate the component or the reading is affected by parallel paths.

A digital multimeter on the diode range shows forward drop (~0.5–0.7 V silicon, ~0.2 V germanium, 1.8–3.3 V LED); OL in both directions indicates an open device.

An analogue multimeter's pointer must be zeroed on the ohms range before use; the meter movement is a moving-coil (D'Arsonval) type that reads average and responds to DC.

An oscilloscope displays voltage against time; check probe attenuation (×1 or ×10) and set the timebase and volts/div before interpreting the trace.

Clamp meters measure current by sensing the magnetic field around a conductor and need only the single conductor inside the jaw.

Insulation resistance is measured with a megger at high test voltage (500 V or 1000 V DC) with the circuit de-energized and isolated.

Half-wave rectifier uses one diode and gives 50 Hz ripple; full-wave centre-tap uses two diodes; bridge uses four diodes and gives 100 Hz ripple on 50 Hz mains.

Filter capacitor ripple: larger C and larger load resistance reduce ripple; ripple frequency for full-wave is twice the mains frequency.

A linear regulator such as 7805 gives fixed +5 V (7812 = +12 V, 7905 = −5 V) and needs input at least ~2–3 V above output plus a heatsink for high current.

SMPS converts AC to high-frequency switching, is more efficient than a linear supply, and its output must be checked for ripple and regulation under load.

Lead-acid battery: each cell is nominally 2 V; a 12 V battery has six cells. Specific gravity of a fully charged cell is about 1.26–1.28, discharged about 1.10–1.15.

Lead-acid charging: use constant-voltage/current-limited charging; never overcharge a sealed battery, and top up only flooded types with distilled water.

Li-ion cells are 3.6–3.7 V nominal, charge to 4.2 V per cell with CC/CV charging, and must never be shorted, punctured, or charged below 0 °C.

Ni-Cd and Ni-MH cells are 1.2 V nominal; Ni-Cd suffers memory effect and needs full discharge cycles; Ni-MH has higher capacity but higher self-discharge.

Battery safety: check polarity before connecting, use a fuse, and never test a battery by shorting the terminals.

Binary place values are powers of 2: 1011₂ = 8+0+2+1 = 11₁₀; hexadecimal digits 0–9, A–F map to 4-bit groups.

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

XOR gives 1 when inputs differ; XNOR gives 1 when inputs are equal. Half-adder sum = A XOR B, carry = A AND B.

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

A flip-flop stores one bit; the D flip-flop output follows D at the clock edge, the JK flip-flop toggles when J = K = 1.

A multiplexer selects one of many inputs to one output; a decoder converts n binary inputs into 2ⁿ outputs; a register stores n bits.

Logic families: TTL uses 5 V and logic 0 ≤ 0.8 V, logic 1 ≥ 2.0 V; CMOS is high-impedance, low-power, and unused CMOS inputs must be tied high or low, never left floating.

A 7-segment display is driven by a decoder such as the 7447 (common-anode) or 7448 (common-cathode); each segment is an LED.

A microcontroller integrates CPU, RAM, ROM/flash and peripherals (GPIO, timers, ADC, UART, SPI, I²C) on one chip; a microprocessor needs external memory and I/O.

Harvard architecture uses separate program and data memory buses (e.g., AVR); von Neumann uses a shared bus.

GPIO pins are configured as input or output; input pins need a defined level (pull-up or pull-down) to avoid floating reads.

The 10-bit ADC on a 5 V reference gives resolution 5 V/1024 ≈ 4.9 mV per step; ADC value = (Vin/Vref) × 1023.

UART is asynchronous and needs matched baud rate plus start/stop bits; SPI uses four wires (MOSI, MISO, SCK, SS) and is full-duplex; I²C uses two wires (SDA, SCL) with device addresses.

A PWM signal's average voltage = duty cycle × supply voltage; PWM controls LED brightness and motor speed.

Interrupts let a peripheral suspend the main program and run a service routine; the watchdog timer resets the MCU if the program hangs.

Arduino Uno uses the ATmega328P with 32 KB flash, 2 KB SRAM, 14 digital I/O and 6 analogue inputs; digital pin 13 has an onboard LED.

Power off and unplug before opening a case; discharge yourself on a grounded surface and keep the PSU switch off during assembly.

PC assembly order: CPU and cooler on the motherboard, RAM in matched slots, motherboard into the case, then drives, PSU and front-panel connectors.

A POST beep code or no-display fault is first checked by reseating RAM, checking the CPU power connector and the monitor cable.

SMPS colour code (ATX): yellow = +12 V, red = +5 V, orange = +3.3 V, black = ground, purple = +5 V standby, green = power-on (PS-ON).

SATA data cable connects drive to motherboard; SATA power comes from the PSU; M.2 NVMe drives fit directly into the motherboard slot.

Troubleshooting sequence: observe the symptom, form a hypothesis, test from the simplest/cheapest cause, swap one part at a time, and verify the fix.

Overheating faults: check fan direction, thermal paste application, dust blockage and BIOS fan curves before replacing the cooler.

No-boot with spinning fans but no display most often indicates RAM, GPU or CPU power, not a dead PSU; verify with a known-good part.

Before servicing any mains appliance, switch off, unplug, and discharge large filter capacitors in SMPS sections - they can hold hundreds of volts.

A TV/display with no backlight but sound usually indicates failed LED strips, inverter or backlight driver, not the main board.

In a CRT television the anode cap and flyback area retain very high voltage; discharge through a proper probe before handling.

A microwave oven's high-voltage capacitor and magnetron circuit are dangerous; the capacitor must be discharged and the door interlock switches tested for continuity.

Refrigerator and air-conditioner compressors are checked for winding resistance and insulation; a humming compressor that trips overload may have a seized or shorted winding.

Inverter air-conditioners and washing machines use a control board with an error code display; read the code before replacing parts.

Mobile phone faults: no charge is checked at the charging port, battery and charging IC; water damage requires ultrasonic/IPA cleaning and corrosion inspection.

After repair, verify insulation resistance, earth continuity and correct current draw with a clamp meter before returning the appliance.

Electric shock severity depends on current through the body (about 30 mA can be dangerous, 50 mA potentially fatal); 230 V mains must be treated as lethal.

Use the lockout/tagout rule: isolate the supply, lock the breaker, tag it, and test for dead before working.

First aid for electric shock: switch off the supply before touching the casualty, then call emergency services and start CPR if breathing has stopped.

ESD damage is cumulative and may cause latent failures; the human body can carry several kilovolts, so a grounded wrist strap and ESD mat are required at the bench.

Store ICs and boards in anti-static (dissipative) bags, not ordinary plastic; keep soldering irons in a stand and never leave them unattended.

Use fume extraction or work in a ventilated area when soldering; wash hands after handling solder (lead) and keep food away from the bench.

E-waste is separated from general waste: PCBs, batteries, CRT glass and toner cartridges go to authorized recyclers; never burn cable or dump in drains.

Fire safety: use CO₂ or dry powder on electrical fires, never water; keep a Class C extinguisher in the lab.

A resume should list contact details, skills, education and experience in reverse chronological order, with measurable achievements rather than vague duties.

A job interview answer to 'tell me about yourself' should be 60–90 seconds covering present role, relevant skills and why the employer.

Communication in the workplace uses the 7 Cs: clear, concise, concrete, correct, coherent, complete and courteous.

Teamwork and time management: prioritize tasks by urgency and importance, and report delays early to the supervisor.

Entrepreneurship basics: a business plan covers product/service, market, competition, operations, cost, pricing, break-even and funding.

Break-even quantity = fixed costs / (selling price − variable cost per unit).

Small electronics service businesses need GST registration above the turnover threshold, a current account, invoices and basic bookkeeping.

Occupational safety and customer service are employability skills: document the repair, explain the fault to the customer, and honour the warranty terms.

Self-employment sources of finance include own savings, bank loans under schemes such as MUDRA, and government skill-development grants.