Electronics FAQ
Short, specific answers to common electronics questions — with the exact value and the tool that computes it, so you can change the numbers and get your own.
Resistors & LEDs
- What resistor do I need for a 5V red LED at 20 mA?
- Use 150 Ω (the nearest E24 value), rated 1/4 W. R = (5 V − 2 V) / 0.02 A = 150 Ω. Open the LED Resistor Calculator →
- What does a brown-black-red-gold resistor mean?
- 1 kΩ ±5%. Brown = 1, black = 0, red = ×100 → 10 × 100 = 1000 Ω, and gold is ±5% tolerance. Open the Resistor Color Code Decoder →
- What are the color bands for a 10 kΩ resistor?
- Brown–black–orange, with a gold tolerance band. Brown = 1, black = 0, orange = ×1k → 10 × 1000 = 10 kΩ; gold = ±5%. Open the Resistor Color Code Decoder →
- What is the nearest standard resistor value to 700 Ω?
- The nearest E24 value is 680 Ω — or 750 Ω if you round up for a safety margin. Open the Nearest E-Series Value →
- What do two 10 kΩ resistors in parallel make?
- 5 kΩ. Equal resistors in parallel halve: R = (10k × 10k) / (10k + 10k) = 5 kΩ. Open the Series / Parallel Resistor Calculator →
Timing & frequency
- What is the RC time constant of 10 kΩ and 1 µF?
- 10 ms. τ = R × C = 10 000 Ω × 1×10⁻⁶ F = 0.01 s; the capacitor reaches ~63% in one τ and ~99% after 5τ (50 ms). Open the RC Time Constant Calculator →
- What 555 values blink an LED at about 1 Hz?
- In astable mode, roughly R1 = 10 kΩ, R2 = 68 kΩ, C = 10 µF give ≈ 0.99 Hz — a classic ~1 Hz blinker. Open the 555 Timer Calculator →
- What is the resonant frequency of 100 µH and 100 nF?
- About 50.3 kHz. f₀ = 1 / (2π√(L·C)); the characteristic impedance Z₀ = √(L/C) ≈ 31.6 Ω. Open the LC Resonance Calculator →
Power, current & RF
- What current flows through a 1 kΩ resistor at 5 V?
- 5 mA, dissipating 25 mW. I = V / R = 5 / 1000 = 0.005 A; P = V² / R = 25 mW. Open the Ohm's Law Calculator →
- What is 0 dBm in watts?
- 1 mW (0.001 W). dBm is power referenced to 1 mW, so 0 dBm = 1 mW and +30 dBm = 1 W. Open the dBm / Watt Converter →
- What is the reactance of a 100 nF capacitor at 1 kHz?
- About 1.59 kΩ. Xc = 1 / (2π·f·C) = 1 / (2π × 1000 × 100×10⁻⁹) ≈ 1591 Ω; it falls as frequency rises. Open the Reactance Calculator →
Digital & numbers
- What is the two's complement of −5 in 8 bits?
- 11111011 (0xFB). Take 00000101, invert to 11111010, then add 1 → 11111011. Open the Two's Complement Calculator →
- How many values can an 8-bit number represent?
- 256 values: 0–255 unsigned, or −128 to 127 signed. In general n bits hold 2ⁿ values. Open the Bits to Range Calculator →
Connectors & ratings
- What does an IP67 rating mean?
- Dust-tight (first digit 6) and protected against immersion up to 1 m depth for 30 minutes (second digit 7). Open the IP Rating Decoder →
- Which Ethernet cable do I need for 10 Gbps over 100 m?
- Cat6a (500 MHz). Plain Cat6 only reaches 10 Gbps up to ~55 m; Cat6a carries 10 Gbps to the full 100 m. Open the Ethernet Cable Categories →
Looking for a specific tool? Browse the full tool overview, the IC pinout reference, or the guides.