Circuit Design Basics

Circuit Design Workflow: From Ohm's Law to Complete Circuits

Every electronic circuit starts with Ohm's Law: V = I × R. From this single formula, you can calculate any missing value and derive power (P = V × I = I²R = V²/R). Getting these basics right prevents component failures, blown LEDs, and overheated resistors — common mistakes that waste time and components.

Start with Ohm's Law

The Ohm's Law Calculator solves for voltage, current, resistance, or power given any two variables. For a 5V supply powering a 470-ohm resistor: I = V/R = 5/470 = 10.6mA; P = V²/R = 25/470 = 53mW. This matters because a 1/4W (250mW) resistor handles that load fine, but if you accidentally use a 47-ohm resistor instead (I = 106mA, P = 530mW), a 1/4W resistor will fail almost immediately.

LED Current-Limiting Resistors

LEDs have a fixed forward voltage drop (Vf) and require current control. A red LED has Vf ≈ 2.0V; blue/white LEDs have Vf ≈ 3.2V. Desired current for a standard LED is 10-20mA. The formula: R = (Vsupply - Vf) / Iled. For a red LED on 5V at 20mA: R = (5 - 2.0) / 0.02 = 150 ohms. Use the next higher standard value (150 ohms is standard). The LED Resistor Calculator handles single LEDs, series strings, and parallel configurations.

Resistor Color Code Decoding

The Resistor Color Code Calculator decodes 4-band, 5-band, and 6-band resistors from color selections, and encodes a resistance value to its color bands. The color sequence: black=0, brown=1, red=2, orange=3, yellow=4, green=5, blue=6, violet=7, gray=8, white=9. For a 4-band resistor with brown-black-orange-gold bands: 1-0-×10³-±5% = 10,000 ohms ± 5% (10kΩ).

Voltage Dividers

A voltage divider uses two resistors to produce a lower output voltage from a higher supply. Output voltage: Vout = Vin × R2 / (R1 + R2). To level-shift a 5V sensor output to 3.3V for an ESP32 input using R1=10kΩ: Vout = 5 × R2 / (10k + R2). Solving for 3.3V output: R2 = 3.3 × 10k / (5 - 3.3) = 19.4kΩ — use a 20kΩ resistor. The Voltage Divider Calculator solves for either output voltage or resistor values.

Capacitor Codes, Inductors, and 555 Timer

The Capacitor Code Calculator decodes 3-digit ceramic capacitor codes: 104 means 10 × 10⁴ pF = 100nF = 0.1µF. The Inductor Calculator handles air-core and toroidal inductors for RF and switching supply applications. The 555 Timer Calculator finds R1, R2, and C values for astable (oscillator) and monostable (pulse) configurations — enter your target frequency and duty cycle and it outputs component values to use.