PAGE 074 OF 150 · DRIVING WITH SWITCHES & TRANSISTORS
Flip-Flop LED Toggle
A flip-flop remembers one bit of state.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
One press on, next press off
A D flip-flop such as the 74HC74 can toggle an LED. Connect its inverted output back to the D input, and each rising clock edge flips the state.
Circuit
- Feed the button through a debounce circuit to the clock input.
- Q output through a resistor to the LED.
- Set and reset inputs tied high when unused.
Without debouncing, one press may produce several clock pulses and the LED will flicker unpredictably.
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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