PAGE 072 OF 150 · DRIVING WITH SWITCHES & TRANSISTORS
Shift Register for LED Control
A shift register turns a few pins into many outputs.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
What it does
A 74HC595 shift register converts serial data from three microcontroller pins into eight parallel outputs, so you can control eight LEDs with only three pins.
The three signals
- Data: the bit to shift in.
- Clock: shifts each bit along.
- Latch: copies the stored bits to the outputs.
Practical notes
Each output can supply only a few milliamps to about 20 mA, and the whole chip has a total limit, so use resistors of 330 ohms or more and keep the total modest. Chips can be chained for 16, 24 or more outputs.
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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