Note:This project has been published in Elektor Electronics magazine, July/August 2007 issue

click here for a PDF version (with permission from Elecktor magazine)

What follows is an interesting project that produces a set of flashing LED's which might show chaotic fluctuations. Have a go and find out for yourself and let me know how you get on!

Coupled LED flashers
Here we couple LED oscillators together to produce some interesting effects. Instead of using the discharge pin on a 555 the timing capacitor can be charged and discharged using the output (via a resistor R1). If we assume the 555 output resistance is very low (use a bipolar rather than CMOS 555) this circuit provides a 50:50 mark-space ratio whose output frequency is independent of load. However if we deliberately increase the output resistance by using a series resistor (R2) the timing will now also be dependant on the current taken by the load (because R2 will effectively drop the available charging voltage to the R1C timing circuit).

The circuit for one of the oscillators and its corresponding symbol. In standard 555 oscillator circuits R2 is usually zero, but here 220R might be a good starting value to experiment with. SW1 allows R2 to be taken out of circuit if required (no coupling).

Now, imagine a number of such oscillators whose outputs are connected to each other via current limiting resistors and bi-colour LEDs. Each oscillators timing will then be dependant on the state of the other oscillators because these will determine the current that flows through the LEDs. For example if all the outputs are high (or all low) there will be no potential differences and so no current will flow through the LED circuits. In this case all the oscillators will be at maximum frequency. Other combinations of outputs will light some of the LEDs and these currents will thus effect each oscillators timing. The R2s therefore couple the oscillators to each other. Switch's across each R2 allows control of the coupling.

A bi-colour LED with added series resistor and its corresponding symbol.

Setting the oscillator frequencies to ca. 2Hz (R1 near max) shows the complex flashing of the LEDs switching between off, red and green. Sometimes the LEDs seem to settle down pulsing together. This is rather like an electronic version of what is observed in nature when a group of fireflies congregate in a bush - they pulse together and maybe our little circuit is a simple version of this rather complex natural feedback system. If the frequency is raised to ca. 100 Hz varying mixing (beating) of the flashing red and green colours cause a 'wave' of changing colour to go through the array of LEDs. Including light dependent resistors (LDR) in R2 might be a way of making each of our LED 'fireflies' 'see' each other. Even without the LDRs with three or more coupled oscillators there might also be the intriguing possibility of observing chaotic behaviour of the oscillators.

Schematic of 4 coupled oscillators and 6 bi-colour LED circuits.


Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ

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