Twelve bench tools on one page

These are the sums and look-ups that come up every time you build a circuit: what a resistor’s stripes mean, which resistor an LED needs, what two resistors make 3.3 volts from 5, how long a battery will last. Each tool updates as you type and remembers your last values. Type values the way they are written on parts and schematics: 4k7, 220R, 100n, 2.2u, 1M all work.

Reading a resistor

Hold the resistor with the gold or silver band on the right. On a four-band part the first two bands are digits, the third is the multiplier and the last is the tolerance: yellow, violet, red, gold is 4, 7, two zeros, so 4,700 ohms within 5 percent. Five-band parts have three digits before the multiplier. Tap the colours in the tool and it reads the value, the allowed range and the nearest standard value; or type a value and it shows the stripes to look for.

Choosing a resistor for an LED

Take the supply voltage, subtract the voltage the LED drops (about 2 V for red, 3.2 V for blue and white), and divide by the current you want, usually 10 to 20 mA. The tool then rounds up to a value you can buy and tells you how much heat the resistor has to shed.

Common questions

What is an E-series? Resistors are made in a fixed set of values per decade so that, with tolerance, the ranges just touch. E12 has twelve values (10, 12, 15, 18, 22…), E24 has twenty-four and E96 ninety-six. The tools round to these so the answer is a part that exists.

How do I read a tiny surface-mount resistor? Three digits work like colour bands: 472 is 47 followed by two zeros. A letter R marks a decimal point, so 4R7 is 4.7 ohms. A two-digit number with a letter, such as 01C, is the EIA-96 code used on 1 percent parts; the tool decodes all of these.

How accurate is the trace width tool? It uses the IPC-2221 curves, which are deliberately cautious. Treat the answer as a safe starting width, not a limit.