LED SEQUENCER

This circuit is the essence of many "chaser" schemes that provide animation in advertising displays, lighting ten LEDs sequentially.

Referring to the first schematic, U1 is a six-section inverting Schmitt trigger, with one section used as a relaxation oscillator. The other five sections of U1 aren't used, and their inputs are grounded. U1 drives the clock input of U2, a decimal counter. Each of the counter's ten outputs drive Darlington current buffers in U3 and U4, which, in turn, drive ten LEDs. The display's advancement rate is adjustable from about 5 to 140Hz with RV1.

The quantity of LEDs may be increased, most conveniently in multiples of ten, as illustrated in the second schematic. The arrangements of twenty and thirty LEDs utilize one common-anode resistor as in the first schematic. The forty-LED arrangement utilizes a series-parallel scheme, with two common-anode resistors, R4 and R5. In all cases, one-in-ten LEDs will be illuminated. (E.g., in the forty LED arrangement, DS1, DS11, DS21, and DS31 all illuminate together.)

The specified LEDs are average-efficiency types that provide ample brightness for indoor applications. Given their typical forward-voltage characteristic of 2 volts, the anode resistors have been selected for a LED current of about 20 milliamperes. Other LEDs with a forward-voltage characteristic of 1.8 to 2.5 volts may be used.

Rectifier CR1 protects the circuit from accidental battery reversal. A 9-volt battery eliminator, such as Mouser stock number 41AC109, is suggested for economical continuous operation.

Parts Table

May 11, 2004

Text and images ©2004 by Arthur Harrison

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