Maharashtra SSC Electronics Technology Pre-Voc/NSQF cheat sheet

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. Units: volt, ampere, ohm, watt.

Series circuit: same current through all elements, R_total = R1 + R2 + R3, voltage divides in proportion to resistance.

Parallel circuit: same voltage across all branches, 1/R_total = 1/R1 + 1/R2 + ..., current divides inversely with resistance.

Kirchhoff's current law: sum of currents entering a node equals sum leaving. Kirchhoff's voltage law: algebraic sum of voltages around a closed loop is zero.

Conductor vs insulator: copper and silver are low-resistivity conductors; rubber, PVC and glass are insulators; semiconductors (Si, Ge) sit between and are doped to control conduction.

AC quantities: RMS value = peak/√2 for a sine wave; frequency f = 1/T; mains supply in India is 230 V RMS, 50 Hz, single phase.

Resistance of a wire R = ρL/A: doubles with length, halves with double cross-section area, rises with temperature for metals.

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

Resistor colour code: Black 0, Brown 1, Red 2, Orange 3, Yellow 4, Green 5, Blue 6, Violet 7, Grey 8, White 9; Gold ±5%, Silver ±10%, no band ±20%.

Diode: conducts when anode is positive w.r.t. cathode (forward bias, ~0.7 V drop for silicon, 0.3 V for germanium); blocks in reverse bias up to its peak inverse voltage.

Zener diode works in reverse breakdown at a specified Zener voltage and is used as a shunt voltage regulator.

LED: forward-biased diode emitting light; needs a series resistor R = (Vsupply − Vled)/Iled to limit current.

BJT has three terminals base, emitter, collector; NPN turns on with base-emitter positive; β (hFE) = Ic/Ib, and Ie = Ic + Ib.

Transistor as a switch: cut-off when base current is zero (no collector current), saturation when base current is large enough to drive Ic below Vcc/Rc.

MOSFET and JFET are voltage-controlled; gate draws almost no steady current; terminals are gate, drain, source.

Capacitor stores charge Q = CV; energy = ½CV²; electrolytic capacitors are polarised and must never be reverse-connected.

Inductor opposes change of current, stores energy ½LI²; transformer works only on AC and steps voltage by turns ratio Ns/Np.

Rectifier output: half-wave gives ripple frequency equal to supply (50 Hz), full-wave and bridge give 100 Hz; bridge needs four diodes and no centre tap.

Filter capacitor value rule: larger C or larger load resistance gives smaller ripple; ripple factor of half-wave ≈1.21, full-wave ≈0.48 before filtering.

Voltmeter is connected in parallel, ammeter in series; ideal voltmeter has infinite resistance, ideal ammeter zero resistance.

Multimeter use: select correct function and range above the expected value, observe polarity on DC, and never measure resistance on a live circuit.

Cathode ray oscilloscope: measures amplitude (vertical, volts/div) and time (horizontal, time/div); frequency = 1/period.

Continuity test uses the low-resistance range or buzzer mode; a reading near 0 Ω indicates a closed path, open circuit shows over-range.

Voltage divider: Vout = Vin × R2/(R1 + R2); loaded divider output drops because the load shunts R2.

Oscillator needs positive feedback with loop gain ≥1 and 360° total phase shift; a 555 timer can be wired as astable (oscillator) or monostable (one-shot).

Good solder joint is shiny, smooth and concave (fillet); a dull, grainy or ball-shaped joint is a cold joint and must be reworked.

Typical soldering iron tip temperature for 60/40 tin-lead solder is about 300–350 °C; lead-free solder needs a higher temperature, around 350–380 °C.

Flux removes oxide and improves wetting; do not use acid (plumbing) flux on electronics - use rosin or no-clean flux.

Heat the joint (pad and lead) for 1–2 seconds, apply solder, remove solder then iron; avoid moving the joint until it solidifies.

ESD control: wear a grounded wrist strap, use an earthed iron tip, and store MOS devices in conductive foam.

PCB terms: pad is the copper land for a component lead, trace is the copper conductor, via connects layers, silkscreen shows component outlines.

Through-hole components are inserted and soldered on the solder side; SMD parts are placed on the pad and reflowed or hand-soldered.

Desoldering uses a solder sucker or desoldering wick; do not overheat pads - lifted pads and delamination are caused by excessive heat or time.

Binary to decimal: each bit position has weight 2^n; e.g. 1011₂ = 8+0+2+1 = 11₁₀.

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

XOR outputs 1 when inputs differ; it is the core of half-adder sum, and half-adder carry = A·B.

Universal gates NAND and NOR can each implement all other logic functions.

De Morgan's theorems: (A·B)′ = A′ + B′ and (A + B)′ = A′·B′.

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

Combinational circuits (adders, multiplexers, decoders) have no memory; sequential circuits (counters, registers, flip-flops) do.

Hexadecimal digit represents exactly 4 binary bits; 1 byte = 8 bits = 2 hex digits.

Linear supply chain: transformer → rectifier → filter → regulator (e.g. 7805 for +5 V, 7812 for +12 V, 7905 for −5 V).

78xx regulators need input at least ~2–3 V above output and a decoupling capacitor on input and output to prevent oscillation.

SMPS switches at high frequency for efficiency (often 70–90%); it is smaller and lighter but generates switching noise.

Lead-acid cell: nominal 2 V per cell, 12 V battery = 6 cells; never discharge below about 1.75 V per cell; top up with distilled water only.

Li-ion cell nominal 3.6–3.7 V, charged to 4.2 V with CC-CV method; requires protection against overcharge and over-discharge.

NiCd memory effect is reduced by full discharge before recharge; NiMH has higher capacity and less memory effect.

Battery capacity in Ah: runtime hours ≈ capacity (Ah) / load current (A), allowing for derating.

Full-wave bridge output without filter has average DC ≈ 0.637 × peak; with a large filter capacitor the DC approaches the peak value.

SMPS in TV and computer supplies produces standby 5 V and main rails; a blown fuse or shorted switching device is a common first check.

Microwave oven: magnetron generates 2.45 GHz; the high-voltage capacitor and transformer retain lethal charge - discharge before servicing.

Electric iron and heater use a nichrome element; a thermostat bimetallic strip cycles the element to hold temperature.

Refrigerator cycle: compressor → condenser (releases heat) → capillary/expansion → evaporator (absorbs heat); a warm condenser or iced evaporator signals a fault.

Ceiling fan speed control uses a regulator (triac or capacitor type); a humming fan that does not turn often has a bad capacitor or bearing.

Inverter and stabiliser protect appliances from voltage fluctuation; UPS additionally supplies backup power during outage.

Remote control uses infrared (about 38 kHz carrier); test with a phone camera to see the IR LED blink.

Wiring colour convention in India: live/phase red or brown, neutral black or blue, earth green or green-yellow.

8051 has 40 pins, 4 KB on-chip ROM (classic variant), 128 bytes RAM, four 8-bit ports P0–P3, and 8-bit architecture.

Arduino Uno uses ATmega328P: 14 digital I/O, 6 analog inputs (10-bit ADC, values 0–1023), 16 MHz clock.

ADC resolution: step size = Vref / 2^n; for 5 V reference and 10 bits, each count ≈ 4.9 mV.

Sensor types: LDR (light, resistance falls with light), thermistor (temperature, NTC resistance falls with heat), IR sensor, ultrasonic HC-SR04 (distance by echo time), DHT11 (temperature and humidity).

Microcontroller needs a clock source (crystal plus capacitors), reset circuit, and decoupling capacitors to work reliably.

GPIO pin current is limited (tens of mA); drive relays, motors and lamps through a transistor or driver IC, and use a flyback diode for inductive loads.

UART is asynchronous serial with baud rate and TX/RX lines; I2C uses two lines SDA and SCL with device addresses; SPI uses MOSI, MISO, SCK and SS.

Embedded program flow: initialise ports and peripherals, then loop reading inputs, processing and driving outputs.

First rule of electrical safety: isolate the supply, lock/tag it, verify dead with a tester before touching conductors.

Earth wire protects the user by giving fault current a low-resistance path so the fuse or MCB trips; never bypass an earth connection.

Fuse rating is chosen just above normal load current; it protects wiring from overload and short circuit, not the user directly.

Hand tools: insulated pliers and screwdrivers for live-adjacent work, wire stripper sized to the gauge, soldering stand and fume extractor for soldering.

E-waste handling: separate PCBs, batteries, CRTs and capacitors; do not burn cable or dump in general waste; route to authorised recyclers.

Capacitors in power supplies and CRTs can hold charge after switch-off; discharge through a resistor before handling.

Never use water on an electrical fire; use CO₂ or dry powder extinguisher, and switch off supply first if safe.

5S workplace discipline: sort, set in order, shine, standardise, sustain - keeps tools traceable and reduces accidents.

Business plan essentials: product/service description, target customer, cost and pricing, break-even, marketing route, and working capital need.

Break-even quantity = fixed cost / (selling price − variable cost per unit); below this level the venture makes a loss.

GST registration threshold for most goods in India is ₹40 lakh turnover (₹20 lakh for services); small traders may opt for the composition scheme.

MSME/Udyam registration gives access to government schemes, subsidised credit and delayed-payment protection.

Interview basics: research the employer, carry a resume with clear skills and projects, answer with concrete examples, and confirm availability.

Resume must be truthful, one to two pages, with contact details, qualifications, practical skills (soldering, multimeter, Arduino), and references.

Communication in the workplace: listen fully, confirm instructions, report problems early, and keep written records of work done.

Customer service rule: acknowledge the complaint, explain the fault in plain language, quote the cost before starting repair, and hand over a written warranty/service note.