LED flasher

This simple circuit pulses an LED to maximum brightness approximately once every 0.5 second. Runs off only 3 volts so you can use two 1.5v cells of any type. A pair of "AA" cells will run this flasher for 6 to 12 months continuously.

USES

Put it in your car to discourage theft and make it easier to find in a dark car park. Hang it from the bathroom light switch pull-cord so you can find it in the dark. Fit it inside a model aircraft, train or car.

• Order LEDFLASHER kit price £2.95
  Battery not supplied
  PCB not supplied

CONSTRUCTION

Build it on a tiny piece of "Veroboard", stripboard or make a printed circuit board like this one.

Double the value of the capacitor to make it flash only once per second (battery cells last even longer).

In your text on the 555 LED driver, you imply that the LED is a more efficient illuminator when pulsed.

I always assumed that the eye integrated fairly perfectly and that it will smooth (or average the brightness) and therefore no efficiency is gained by pulsing.

Do you know something that I do not? I ask seriously.

BTW as an opto engineer I found your website interesting.

Roy.

Thanks. You misunderstand the way the human eye works. It perceives short-duration bright pulses (faster than 25 per second) as continuously bright whereas a cat or bird allegedly doesn't see short duration pulses at all. This is why TV sets use a scan rate of 25 frames per second (later models and monitors use a higher speed to eliminate the "flickering-at-the-corner-of-the-eye" effect completely.

The reason it works is the fact that the human eye suffers from a "persistence-of-vision" effect, rather like the cathode ray tube used in radar systems. Once the optic rods in the retina are stimulated, they fire nerve pulses for a significant fraction of a second. Just imagine what happens when someone points a camera flash at you. The flash persists in your eye for much longer than the actual light duration. Now imagine a strobe light - it's bright. Now imagine a strobe light at 50 flashes per second; all you see is a continuously dazzling light.

I learned all this at school in my early 'teens, back in the 1960s. I despair of the "modern" educations sytem. What the heck do they teach?

Martin