Maharashtra HSC Electronics Field Technician (NSQF F6) 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: R_total = R1 + R2 + …; same current through all elements. Parallel: 1/R_total = 1/R1 + 1/R2 + …; same voltage across all elements.

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

Capacitive reactance Xc = 1/(2πfC); inductive reactance XL = 2πfL; impedance Z = √(R² + (XL − Xc)²).

Resonance occurs when XL = Xc, giving f₀ = 1/(2π√(LC)); at resonance a series circuit is purely resistive and current is maximum.

Diode: conducts when anode is ~0.7 V (Si) or ~0.3 V (Ge) above cathode; Zener diode operates reverse-biased in breakdown to regulate voltage.

BJT terminals are emitter, base, collector; β = Ic/Ib. NPN conducts with base positive to emitter; PNP with base negative to emitter.

Transformer turns ratio: Vs/Vp = Ns/Np = Ip/Is; a step-down transformer raises current while lowering voltage, power roughly constant.

Moving-coil (PMMC) meter reads average value and suits DC only; moving-iron meter reads RMS and works on AC and DC.

Analog multimeter loading: a 20 kΩ/V meter on 10 V range has 200 kΩ input resistance, which loads high-impedance circuits and reads low.

Digital multimeter input impedance is typically 10 MΩ; use the highest range first when the value is unknown, then step down.

CRO: time period = horizontal divisions × time/div; voltage = vertical divisions × volts/div; probe ×10 divides the displayed voltage by 10.

Lissajous figures on a CRO give frequency ratio from the number of horizontal and vertical tangencies; a circle means equal frequencies.

Oscilloscope bandwidth rule: choose a scope with bandwidth ≥ 5 × the highest signal frequency to be measured.

Continuity test uses the low-ohms or buzzer mode with circuit power off; a good fuse or wire reads near 0 Ω.

Clamp meter measures current by magnetic field around a conductor, so the jaw must enclose one conductor only; enclosing both live and neutral reads zero.

Half-wave rectifier: one diode, output ripple frequency = supply frequency; full-wave centre-tap gives 2f, bridge gives 2f with four diodes.

Bridge rectifier output peak = Vm − 2 × 0.7 V (two diode drops); add a filter capacitor to reduce ripple.

Op-amp ideal rules: infinite input impedance, zero output impedance, infinite open-loop gain; inverting gain = −Rf/Rin, non-inverting gain = 1 + Rf/Rin.

Logic gates: AND outputs 1 only if all inputs 1; OR outputs 1 if any input 1; NAND and NOR are universal gates.

De Morgan: A·B (bar) = A(bar) + B(bar); A + B (bar) = A(bar) · B(bar).

Binary to decimal: multiply each bit by its power of 2. 1011₂ = 8 + 0 + 2 + 1 = 11₁₀.

Flip-flops: SR has an invalid state when both inputs are 1; JK removes it (toggles); D is data latch; T toggles on each clock.

555 timer astable frequency f ≈ 1.44 / ((R1 + 2R2) × C); monostable pulse width t ≈ 1.1 × R × C.

SMPS in TVs and set-top boxes first converts AC to high-voltage DC, then switches at high frequency; the primary side is live - discharge the bulk capacitor before servicing.

No power fault sequence: check mains plug and fuse, then SMPS output rails (e.g. 5 V, 12 V, 24 V), then the standby LED and power-on signal.

Microwave oven: the magnetron needs high voltage from a transformer plus doubler capacitor and diode; the high-voltage capacitor holds a lethal charge after unplugging.

Refrigerator: compressor does not start - check supply, thermostat, overload protector, start relay/PTC and winding resistance; a hot compressor with no cooling suggests gas leak.

Washing machine: inlet fault is usually the water-level sensor, inlet valve or filter; drain fault is the drain pump or blocked filter.

Induction cooktop requires ferromagnetic cookware; it will not heat aluminium or glass because no eddy currents are induced.

Electric iron and heater faults: open heating element (infinite ohms), faulty thermostat or bimetallic strip, or blown thermal fuse.

Ceiling fan: low speed with hum usually means a failed capacitor; winding continuity and capacitor value (typically 2.25–3.5 µF) should be checked.

AM: modulating signal changes carrier amplitude; bandwidth = 2 × modulating frequency; modulation index m = Vm/Vc, overmodulation when m > 1.

FM: modulating signal changes carrier frequency; deviation and bandwidth follow Carson's rule, BW ≈ 2(Δf + fm); FM resists amplitude noise better than AM.

Superheterodyne receiver: mixer converts incoming RF to a fixed intermediate frequency (455 kHz for AM, 10.7 MHz for FM) for selectivity and gain.

Antenna length for a half-wave dipole ≈ 143/f (MHz) metres; quarter-wave monopole ≈ 71.5/f (MHz) metres.

TV faults: no raster but sound present points to display backlight or panel; raster but no picture points to signal or video-processing stage.

Loudspeaker impedance and amplifier rating must match; connecting a 4 Ω load to an 8 Ω-rated output can overheat the output stage.

Audio distortion checks: clipping from overdrive, hum from poor earthing or filter capacitors, and hiss from noisy input stages.

Satellite/DTH dish alignment uses azimuth, elevation and LNB skew; a weak or absent signal with correct cabling indicates misalignment or LNB fault.

8051: 8-bit core, 4 KB on-chip ROM, 128 bytes RAM, four 8-bit ports, two timers, one serial port, 12 MHz crystal giving 1 µs machine cycle.

Arduino Uno uses ATmega328P: 14 digital pins (6 PWM), 6 analog inputs, 10-bit ADC (0–1023 for 0–5 V), 16 MHz clock.

Sensor types: thermistor (temperature, resistance changes), LDR (light, resistance falls with illumination), IR (reflective/obstacle), ultrasonic HC-SR04 (distance via echo time).

Ultrasonic distance = (echo time × speed of sound 340 m/s) / 2; the divide by two accounts for the out-and-back path.

Relay driver needs a transistor and a flyback diode across the coil; the microcontroller pin cannot supply coil current directly.

PIR sensor detects moving body heat (infrared) and gives a digital pulse; it does not measure exact temperature.

PWM controls motor speed and LED brightness by varying duty cycle; duty cycle = on-time / total period × 100%.

In PLC/automation terms, sensors are inputs to the controller, and relays, contactors and motors are outputs; ladder logic uses NO and NC contacts.

IS 732 governs wiring installation; domestic circuits use 1.5 mm² for lighting and 2.5–4 mm² for power sockets, with MCB ratings matched to cable size.

Earthing: green or green-yellow conductor; earth resistance should be low (typically under 1 Ω for a good installation, measured with an earth tester).

MCB trips on overload (thermal) and short circuit (magnetic); ELCB/RCCB trips on earth leakage, typically 30 mA for personnel protection.

Lockout/tagout before servicing: isolate supply, prove dead with a tester, discharge capacitors, then tag and lock the isolator.

First aid for electric shock: switch off supply or use dry insulating material to separate the person, then call for help and start CPR if unresponsive.

Soldering: tin the iron, heat the joint, apply solder to the joint not the iron; a good joint is shiny and concave; use flux sparingly.

Anti-static precautions: wrist strap and grounded mat when handling CMOS and microcontroller boards; store ICs in conductive foam.

Preventive maintenance schedule: check loose connections and insulation, clean filters and fans, tighten terminals, and record insulation resistance readings.

GST registration is generally required once turnover crosses the threshold (₹40 lakh for goods, ₹20 lakh for services in most states); composition scheme has lower rates.

A service quotation should state scope, parts cost, labour charge, taxes, warranty period and payment terms to avoid disputes.

Basic accounting: profit = revenue − (material cost + labour + overheads); maintain a cash book, invoice register and stock register.

Customer handling: confirm the fault in writing, give a written estimate before repair, and return replaced parts on request.

Workshop measurement: vernier calliper least count = 1 main-scale division − 1 vernier division (0.02 mm for a standard 50-division calliper).

Micrometer least count = pitch / number of circular-scale divisions (0.01 mm for a 0.5 mm pitch, 50-division micrometer).

Employability: a technician's résumé lists NSQF level, trade skills, tools handled, safety training and any apprenticeship or on-the-job experience.

Workplace 5S (sort, set in order, shine, standardise, sustain) keeps tools traceable and reduces repair turnaround time.