Electronic Design

Low-cost LED sequencer

Cost is always an important consideration in circuit design, but even a couple of pennies often makes a critical difference in electronic toy designs. The circuits presented here for blinking LEDs are based on the 74HC04, one of the most common and least expensive chips available. Its six inverters can easily drive LEDs, and it operates over a wide voltage range, which is ideal for battery-operated circuits.

An odd number of inverters chained together in a loop is an inherently unstable circuit . Two adjacent gates will always have the same output, so the inverters constantly change state in sequence. LEDs, connected between every other output, light only when the gate between the outputs is switching. Resistors with capacitors slow and control the rate at which the LEDs blink. A single resistor to VCC limits current for all of the LEDs, and the voltage drop across this resistor extends the voltage range of the circuit.

The two circuits shown operate from about 2 to 9 V, and cycle at around 1 to 2 Hz. The first circuit lights six LEDs, one at a time, in the order indicated by the number in the reference designator (Fig. 1). The second circuit cycles through 10 LEDs (Fig. 2). Two are lit at a time, with one LED turning off as the next is turning on. Simple variations of these circuit also can cycle through odd numbers of LEDs. Inverters can be fun building blocks.

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