May 2012
I need a schematic for an LED chaser for an atomic model my class is building. I will be using about eight to 16 LEDs. They need to be adjustable in cycling and speed. Could this circuit be expandable from, say, four to 40 LEDs?
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First, I would recommend checking out the ‘Chaser Light Marquee’ in Smiley’s Workshop here on N&V.
If that doesn’t work for you, I would recommend building something with presettable shift registers, aka parallel-in parallel-out shift registers.
There are a number of different versions of those, the 74198 and 74199 are TTL versions, and there is the 4035 which is a CMOS version (4 bits per chip).
The idea is, string enough of those together to have the number of outputs you want, then use switches to set the input states (thus which LEDs are lit on startup). Tie the last used output bit to the serial data input of the first chip if you want the data to cycle around once loaded. If you would rather the pattern get reloaded once all bits are shifted out then you need a way to reload when terminal count is reached (a comparator or just an up or down counter).
Once the data is latched, let the clock run at your shift rate and you’re done.
You need a clock (555 for example) and a way to load the register(s) upon powerup.
By "LED chaser," I assume you mean a circuit that will light a series of LEDs one at a time in a sequential and cyclic progression. You can accomplish this using the MC14017 Decade Counter IC (download a datasheet from www.onsemi.com). A single MC14017 will provide 10 LED outputs. Figure 3 in the datasheet illustrates how to wire together several MC14017s (with the help of MC74HC08 AND gates) for obtaining more outputs (40 LEDs will require five MC14017s). You can drive the LEDs directly from the MC14017 outputs with a series resistor (~ 470 ohms).
You'll also need an oscillator source, and an LM555 is the easiest way to do this. A convenient calculator for determining component values is available at www.coolcircuit.com/tools/ne555_calculator/index.php. You can use a potentiometer to make the oscillator frequency variable. If you make the frequency 10 Hz, for example, each LED will light for 0.1 seconds, and it will take one second to cycle through all 10 LEDs.
All the components you will need can be ordered from Jameco (www.jameco.com). You can use the 74HC4017, 74HC08, and LM555, and any size/color LED you like. The figure below is a basic schematic diagram to get started.
