Assam ASSEB HS Skill Subject Electronics and Hardware cheat sheet

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

Series: R_total = R1 + R2 + …; same current through all elements; voltages divide in proportion to resistance.

Parallel: 1/R_total = 1/R1 + 1/R2 + …; same voltage across all branches; currents divide inversely with resistance.

Voltage divider (unloaded): V_out = V_in × R2/(R1 + R2). Loading the output with R_L reduces V_out; account for R_L ∥ R2.

Kirchhoff's current law: algebraic sum of currents at a node = 0. Kirchhoff's voltage law: algebraic sum of voltages around a closed loop = 0.

Capacitor: Q = CV, I = C·dV/dt, energy = ½CV². Time constant τ = RC; after 1τ charge reaches 63.2%, after 5τ essentially complete.

Inductor: V = L·di/dt, energy = ½LI². Time constant τ = L/R; inductor current cannot change instantaneously.

AC: V_rms = V_peak/√2 ≈ 0.707·V_peak; V_peak = 1.414·V_rms. For a sine wave, average of a full cycle is zero.

Impedance: Z_R = R, Z_C = 1/(jωC), Z_L = jωL; resonance when X_L = X_C, f₀ = 1/(2π√(LC)).

Diode: forward biased when anode is more positive than cathode by ~0.7 V (Si) or ~0.3 V (Ge); reverse biased diodes block current until breakdown.

Zener diode operates in reverse breakdown to regulate voltage; a series resistor must limit current: R = (V_in − V_Z)/(I_Z + I_load).

LED needs a series resistor: R = (V_supply − V_f)/I_f. Typical V_f: red ~2.0 V, blue/white ~3.0–3.4 V.

BJT: NPN conducts when base-emitter is forward biased (V_BE ≈ 0.7 V); I_C = β·I_B; operates in cutoff, active, and saturation regions.

MOSFET: voltage-controlled; gate draws negligible DC current; enhancement mode needs V_GS above threshold to conduct. Handle with ESD care.

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

Capacitor types: ceramic (small, non-polar, stable), electrolytic (large values, polarized - reversed polarity can rupture), film (stable, non-polar).

Photodiode and phototransistor respond to light; LDR resistance falls as illumination rises; thermistor resistance changes with temperature (NTC falls when heated).

SCR and TRIAC are latching thyristors; SCR conducts one way after a gate pulse, TRIAC conducts both ways for AC control.

Logic gates: AND outputs 1 only if all inputs 1; OR outputs 1 if any input 1; NOT inverts; NAND and NOR are universal gates.

XOR outputs 1 when inputs differ; XNOR outputs 1 when inputs match. XOR is the core of half-adder sum: S = A ⊕ B, carry = A·B.

De Morgan: (A·B)' = A' + B'; (A + B)' = A'·B'. NAND = inverted AND; NOR = inverted OR.

Binary to decimal: positional weights 2ⁿ. 1011₂ = 8 + 0 + 2 + 1 = 11₁₀. Hex digits map to 4 bits each.

Flip-flops: SR (set/reset), D (data, Q follows D on clock edge), JK (toggle when J = K = 1), T (toggles on each clock).

Registers store n bits; shift registers move bits left/right on each clock; counters are flip-flop chains dividing clock frequency by 2 per stage.

Multiplexer selects one of many inputs to one output; demultiplexer routes one input to one of many outputs; decoder converts binary code to one-hot line.

Karnaugh map groups adjacent 1s in powers of two (1, 2, 4, 8) to simplify Boolean expressions; wrap-around edges are adjacent.

TTL logic levels: 0 ≈ 0–0.8 V, 1 ≈ 2.0–5 V; CMOS is more voltage-flexible and lower power but more ESD-sensitive.

Microcontroller integrates CPU, RAM, ROM/flash, timers, and I/O peripherals on one chip; a microprocessor needs external memory and peripherals.

Harvard architecture separates program and data memory buses (common in microcontrollers); von Neumann uses a shared bus.

GPIO pins are configured as input, output, or alternate function; input pins need defined levels (pull-up/pull-down) to avoid floating reads.

ADC converts analog voltage to digital code: code = (V_in/V_ref) × (2ⁿ − 1) for an n-bit converter; resolution = V_ref/2ⁿ.

PWM duty cycle controls average output voltage: V_avg = duty × V_supply; used for LED dimming and motor speed.

UART is asynchronous serial (start/stop bits, baud rate must match); I²C uses two wires (SDA, SCL) with device addresses; SPI uses four wires and is faster.

Interrupts suspend main code to service an event; ISRs should be short, and shared variables with main code should be volatile.

Watchdog timer resets the MCU if firmware fails to service it; used to recover from hangs in embedded systems.

Embedded firmware is usually written in C, compiled to a hex file, and flashed to on-chip memory via ISP/JTAG.

Ammeter connects in series and has very low resistance; voltmeter connects in parallel and has very high resistance.

Analog multimeter (VOM) reading: choose range so the needle falls in the upper two-thirds of the scale for best accuracy.

Digital multimeter accuracy is specified as ±(% reading + counts); e.g. ±(0.5% + 2) on a 200 V range.

Oscilloscope: vertical axis is voltage (volts/div), horizontal is time (time/div); probe ×10 divides signal by 10 at the scope.

CRO measures amplitude, frequency, period, and phase; frequency = 1/period. Lissajous patterns compare two frequencies.

True-RMS meters read non-sinusoidal waveforms correctly; average-responding meters are calibrated for sine waves only.

Bridge circuits measure unknown resistance (Wheatstone), inductance (Maxwell), and capacitance (Schering) by balancing.

Continuity mode checks for near-zero resistance; diode test mode shows forward drop and reverse blocking.

Signal generator produces test waveforms (sine, square, triangle); frequency counter counts cycles per unit time.

Solder alloy: 63/37 Sn-Pb eutectic melts at 183 °C; lead-free (SAC305) melts around 217–220 °C and needs a higher iron tip temperature.

Soldering iron tip should be tinned before use; a clean, tinned tip transfers heat efficiently. Wipe on damp sponge or brass wool.

Good solder joint: shiny, concave fillet, smooth surface. Cold joint: dull, grainy, ball-shaped - reheats and reflows.

Flux removes oxides and improves wetting; rosin flux is common; clean residue after soldering where required.

PCB layers: copper traces on FR-4 substrate; silkscreen shows component outlines; solder mask protects copper from bridging.

Through-hole components are inserted and soldered on the opposite side; SMD components are placed on pads and reflowed or hand-soldered.

ESD protection: use grounded wrist strap, anti-static mat, and store ICs in conductive foam; never touch pins directly.

Polarized parts (electrolytic capacitors, diodes, ICs) must be oriented per silkscreen; check before soldering.

Desoldering uses solder sucker or desoldering braid; avoid overheating pads, which can lift copper traces.

CPU socket types differ (LGA vs PGA); LGA pins are on the socket, PGA pins on the CPU - bent pins cause boot failure.

RAM: DDR generations are not interchangeable; DIMM slots are keyed. Dual-channel needs matched pairs in correct slots.

Storage: HDD uses spinning platters; SSD uses flash, faster and shock-resistant; NVMe uses PCIe, SATA uses AHCI.

Motherboard headers: 24-pin ATX main power, 4/8-pin CPU power, SATA data, front-panel USB and audio headers.

PSU wattage must exceed total system draw; 80 PLUS rating indicates efficiency; modular PSUs reduce cable clutter.

Peripherals connect via USB, HDMI/DisplayPort (video), Ethernet (RJ45), and audio jacks; USB-C can carry power, data, and video.

BIOS/UEFI initializes hardware and selects boot device; CMOS battery retains settings when unplugged.

Thermal paste improves heat transfer between CPU and heatsink; a thin, even layer is ideal - excess can insulate.

Troubleshooting order: verify power, inspect for physical damage, check connections, then measure voltages and signals.

Divide-and-conquer: split the circuit at a midpoint and test each half to localize the fault quickly.

Continuity and voltage checks: a dead node indicates open circuit; a shorted node shows near-zero resistance to ground.

Capacitors can hold charge after power-off; discharge them through a resistor before servicing power supplies.

Safety: unplug before servicing, use one-hand rule for live measurements, and never bypass fuses or earth ground.

Isolation transformer protects the technician when servicing live mains-powered equipment; variac limits inrush.

Preventive maintenance: clean dust from fans and heatsinks, check thermal paste, verify fan operation, and inspect for swollen capacitors.

Solder iron safety: rest in stand, never leave unattended while hot, keep away from flammable material, and use eye protection.

Static-sensitive devices should be handled at a grounded workstation; store ICs in anti-static bags or foam.