Imagine a circuit that causes a little light to flash on and off. Imagine that the frequency of that flashing is itself dependent on the light at a particular sensor. Imagine that such a circuit is placed next to another identical circuit, such that the light from each circuit is directed at the sensor on the other circuit. What do you expect to see? Find out after the break…
The two lights flash in unison! Note that this is *NOT* because they are driven by the same signal – the circuits themselves are quite separate. Each circuit modifies its own flashing until it synchronises with the other one. To prove they are synchronising, I put a piece of paper in between one LDR-LED pair, and you see that the synchronisation is lost. But when I take it away, they sync up again.
To build the circuit, you just need some 555 timer chips, a breadboard, a power supply (or 9v battery would do), LEDs, LDRs and some capacitors and resistors. You can find the basic circuit here - just replace the output with the LED and the potentiometer with two LDRs (one regulates the time the output is high, the other regulates the time the output is low). I’ll post a full circuit diagram when I get time to draw it up. Note that you’ll need to make sure there’s not too much ambient light interfering with the sensors.
You can play around with different values of capacitors, resistors etc. For example, I found that by replacing one of the LDRs with a fixed resistor, such that you fix the “on” time of the circuit (i.e. the LEDs always stay lit for a fixed period, the only thing that changes is the time they go off for), then you can get the circuits to actually synchronise in anti-phase. This is hours of fun!