A circuit to flash multiple LEDs

Since Radio Shack (as well as others) sells a nice LED that flashes — or, to use the company’s term, “blinks” — why would one want to build a circuit devoted to flashing LEDs?

There are multiple answers:

• The RS device is relatively small — 5mm and only 880 mcds, which is fairly tiny and fairly dim.

• The RS device’s blinking rate can’t be controlled.

• It’s only one LED — what if you want to control more than one?

I wanted to have a flashing sign on one of my diners (amazing the number of birdhouse diners there are), using two LEDs like they were flood lights illuminating the wooden sign at night.

In the various crossing light experiments, I had acquired a number of LM555 and LM556 integrated circuits and had built up a couple of crossing-light circuits, so I was familiar with the way that family of timer ICs worked: when a certain voltage threshold (a percentage of the operating voltage) was reached on one pin, a signal was fired off on another pin. A series of resistors and capacitors controlled the overall span of the cycle as well as the amount of time the pin was hot, or turned on. In this instruction, I have a 22-ohm resistor, a 47-ohm resistor and a 47-microfarad capacitor, which works out to a four-second cycle, with the pin hot for three of those four seconds (and cold — or off — for one second). (See my thoughts on buying components.)

You can Google “LM555” and get all the gory details, but suffice it to say, personally, I found the way the on-line calculators worked out the resistor-capacitor combos for cycle and hotness mystifying. It’s clear to me that only voodoo and/or black magic can give you what you want.

Or, perhaps a clearer understanding of electronics, but I’m still partial to the voodoo theory.

Nonetheless, my ham-handed methods got me the four-second cycle, three-seconds on, one second off, formula, so I’m happy. I arrived at this arrangement of resistors and capacitors with my normal, brute-force methods: I have a junk box full of components; I dumped it out on the table, put the components I knew I needed into a solderless breadboard and started switching resistors and capacitors out until I got a cycle and hot time I liked.

I’ve built two of these circuits, one out of the LM555 and one out of the LM556 (had a couple of those left over) and both have worked wonderfully. The circuits I’ve built incorporate my standard 5-volt DC circuit, and all you need to do is build that circuit on one end of the Radio Shack PC board (276-149) and put the flashing circuit on the other end. You can see from the photos how this works. The photos also provide you with a component layout.

There are no real tricks to this circuit — unless you want to have a different cycle and different hot time, in which case I’m not going to be much help anyway — so I’ll dispense with the BOM, tools or instructions.

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