Ohm's law, voltage dividers, LED resistors, RC timing and UART baud rates
Values accept SI suffixes — 4.7k, 4k7, 100n, 20mA, 1M.
Enter any two of voltage, current, resistance and power.
Vout = Vin × R2 / (R1 + R2). Or work backwards from the Vout you need — e.g. scaling a 12 V battery into a 3.3 V ADC — to the nearest standard resistor.
R = (Vsupply − n × Vf) / I, rounded up to the next E12 value, with the resistor's power and wattage rating.
τ = R × C, the low-pass / high-pass cut-off frequency, the charge curve, and how long until the capacitor reaches a given voltage — for debounce, reset delays and filters.
The divider register and real baud rate a given clock produces, and whether the error is small enough for reliable serial links.
Voltage, current and resistance are tied by V = I × R, and power by P = V × I. Knowing any two of the four values gives the other two. The calculator also suggests a resistor wattage with a 2× safety margin — a resistor run at its full rating gets hot enough to drift and shortens its life.
Two resistors in series scale a voltage down: Vout = Vin × R2 / (R1 + R2). The usual job is bringing a battery or 12 V rail into a 3.3 V microcontroller ADC. Switch to "R1 for a target Vout" to start from the voltage you need. The tool then picks the nearest standard E24 and E12 resistors and shows the Vout each one really gives.
An LED needs a resistor to set its current: R = (Vsupply − Vf) / I, where Vf is the LED's forward voltage. The result is rounded up to the next standard value, so the current stays at or below the target. Several LEDs in series share one resistor, as long as their combined Vf stays below the supply.
Typical forward voltages: infrared 1.2 V, red 2.0 V, yellow 2.1 V, standard green 2.2 V, blue/white/bright green 3.0–3.4 V. Check the datasheet for high-power LEDs, and drive those with a constant-current driver rather than a resistor.
A capacitor charging through a resistor reaches 63.2 % of the supply after one time constant τ = R × C, and is considered fully charged after 5 τ (99.3 %). The same pair forms a first-order filter with cut-off frequency fc = 1 / (2π R C).
Give a supply and threshold voltage to get the exact time the capacitor takes to cross it, t = −RC × ln(1 − Vt / Vs). That is the delay for a reset circuit, or the settle time for a button debounce filter feeding a Schmitt-trigger input.
A UART derives its baud rate by dividing its clock. The division rarely comes out exact, so the real baud rate is a little off. Both ends sample each bit against their own clock, and once the combined error passes about 2 % the last bits of a byte are misread. You get garbage characters, or errors that only show up with long messages.
The table lists every standard rate for the chosen clock, so you can choose the fastest reliable one.