PAGE 066 OF 150 · DRIVING WITH SWITCHES & TRANSISTORS
Base Resistor Calculation
The base resistor limits current into the transistor.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
The idea
The base resistor limits base current. Rb = (Vin − 0.7) ÷ Ib. To saturate the transistor, make Ib about one tenth of the load current, which assumes a gain of 10 and is safe even for low-gain parts.
Example
An LED uses 20 mA, so Ib = 2 mA. With a 5 V signal, Rb = (5 − 0.7) ÷ 0.002 = 2,150 ohms, so choose 2.2 kilohms.
Values from 1 to 4.7 kilohms work for most small LED switches. A base resistor that is too large leaves the transistor partly on and warm.
REMEMBER
A transistor lets a tiny signal control a much larger LED current.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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