Calculators
Free electronics calculators with the formula, a worked example and the schematic: Ohm's law, LED resistors, 555 timers, filters and more.
Essentials
- Component color codesOr enter a value to get the colors (4 bands):
Current dividerTwo parallel resistors split an incoming current in inverse proportion to their resistance — the smaller resistor carries the larger share.
LED series resistorNobody wires just one LED. Each string is its own series chain of LEDs behind its own resistor — stringing more LEDs raises the headroom the resistor has…
Ohm's LawFill in any two values — the third fills in automatically.
Series / parallelSeries:
Voltage dividerA load draws current from the divider's midpoint, which pulls Vout down from the unloaded (ideal) value — the classic "why did my sensor reading change…
Resistor networks
- Resistor combination solverYou need a value the E-series does not stock. This finds the nearest standard resistor, and the best two-resistor series and parallel combinations from…
Resistor ratio— or with "Invert" ticked.
Star / delta transformSome networks — a Wheatstone bridge with a load across the middle is the standard one — have no two resistors in plain series or plain parallel, so the…
Thévenin / NortonAny linear source, however many parts are inside it, behaves at one pair of terminals exactly like a voltage source behind a series resistance — or, just…
Wheatstone bridgeEach half is its own divider, and the output is the difference between their midpoints:
Components
BJT base resistorA transistor used as a switch is not an amplifier — you deliberately throw its gain away. The mistake is dividing the load current by the datasheet hFE:…
MOSFET gate driveA MOSFET draws no steady gate current, so holding it on is free — but every edge has to shovel the whole gate charge in and back out again, and is real…
NTC thermistorBeta-model resistance of an NTC at any temperature (T in kelvin), plus the divider voltage an ADC would read.
Pull-up resistorkeeps the driver inside its sink rating, and keeps the released line above the input threshold.- SMD marking decoderTwo or three characters on a package 2 mm long, and no reel to check them against. A resistor code is real arithmetic — three digits are two significant…
Filters & timing
LC resonanceAt that frequency the two reactances are equal — — and the branch currents cancel, so this parallel tank presents its highest impedance.
NE555 timerFully wired timing network — pins are grouped by function, not physical DIP order (see the 555 timer entry in Pinouts for the real pin layout). RST is…
RC filter— the −3 dB point, where the output has fallen to 70.7 % of the input. R in series, C to ground: C’s impedance falls as frequency rises, increasingly…
Reactance (capacitive / inductive)Capacitive: — reactance falls as frequency rises, so a capacitor passes high frequencies.
Sallen-Key filterA unity-gain second-order low-pass: sets where it turns, sets the shape. Q is peaking, not frequency — 0.707 is flat to the corner, below that the…
Time constantRC: — RL: — one time constant is how long the circuit takes to cover 63.2 % of the remaining gap to its final value.
Digital & MCU
- ADC step converterWhat one count is worth, and how counts and volts map onto each other, using the common Arduino-sketch convention where full scale is the top count,…
Crystal load capacitorsA crystal's datasheet states its load capacitance Cₗ. The two caps sit in series as seen by the crystal, and the board adds stray capacitance Cₛ (~3–5 pF…- Number bases & bitsOne value shown in binary, decimal, hex and octal at once, with every bit clickable. Reading 0x2A is arithmetic; seeing 00101010 with bits 5, 3 and 1 lit…
- PWM / timer resolutionA hardware timer counts clock ticks through a prescaler up to TOP, then wraps: . Those TOP + 1 counts are also the duty steps: . Bigger TOP = finer…
- UART baud-rate errorA UART divides the MCU clock by an integer: , where OSR is the oversampling factor chosen below. That rounding is the whole source of the error: under…
Amplifiers, audio & RF
AC voltage converterSine wave assumed. — —
Antenna length— the element is then a fraction of that wavelength, trimmed by a shortening factor k: for a quarter-wave. Two things trip people up. A λ/4 whip is…
Decibel converterVoltage: — Power: . Power goes as voltage squared, which is the whole reason for the 20 against the 10. A bench dB question is usually absolute, not…
Operational amplifierNon-inverting: — Inverting:
Slew rate & bandwidthFull-power bandwidth: — the highest frequency at which the op-amp can still swing the whole amplitude. Past it the output stops following and simply…
Speaker crossover (1st order)A series capacitor keeps the lows out of the tweeter; a series inductor keeps the highs out of the woofer. First order = 6 dB/octave each side, crossing…
Power supplies & batteries
Battery life— efficiency is entered as a percentage and applied as a fraction.
Current-sense shunt— but the sense voltage is the real decision, because it alone fixes the heat: , with R cancelling out. 100 mV at 10 A is a full watt no matter which…
DC-DC converterand . Continuous conduction with ideal switches — a real design lands a few percent higher on duty once the switch and diode drops are counted.
Decoupling capacitorA reservoir close to the chip supplies the current the rail cannot deliver fast enough: , and the capacitor's own must stay below or it drops the…
Energy storageCapacitor: — Inductor:
RC snubberTwo-measurement method. You are giving the ring as the scope sees it, then again with a known capacitor clipped across the switch. comes out of the two…
Voltage regulator— the zener must stay reverse-biased with Iz > 0 to regulate.
Zener shunt regulatorA zener holds the output at by swallowing whatever current the load leaves behind, so the series resistor is sized by the worst corner and never by the…
PCB, wiring & heat
AWG ↔ mm²The gauge is geometric, not a lookup table: mm, and . Resistance assumes copper at 20 °C.
Controlled impedanceA trace over a single reference plane, solved backwards from IPC-2141's own closed-form approximation: . Good to a few percent for 0.1 ≤ w/h ≤ 2 — the…
Heatsink / thermal— the heatsink only gets what is left of the budget after the package and the interface have taken theirs.
PCB trace ampacityEmpirical IPC-2221 formula — a design estimate, not a manufacturer guarantee.
Wire voltage drop, where for a round trip. The ohms per metre come from the AWG table.