The Astable Circuit

Hi Alex,

Here is a circuit that you can use to make lights flash which you could use as it is, or plug it into your robot. This uses an NE555 timer chip to create an astable circuit. It is called this because the circuit is unstable, it keeps flipping from one state to another. There are many ways to create an astable and this is just one of them but I know you have a 555 example in the circuit software you showed me so you can see how it works in detail there.

At this stage, don't worry too much about how it works, just think of it as a building block that you apply power to and it's output flashes on and off. Here is block diagram of the parts.

The astable creates an output that turns on and off which is used to 'push' a switch that switches a larger current on and off (a bit like a relay). This is done because the astable output cannot drive large currents so would not be useful for turning on bright lights. The 'switch' in this case is actually a transistor that is controlled by the astable and switches a much larger current.

Here is the circuit diagram. Not all the components are shown, but the main ones of interest are shown. The 555 chip has 8 pins. The chip has a little dimple next to pin 1 and the pins are numbered as shown. The frequency the astable flashes is controlled by a resistor and capacitor marked as R and C on the diagram. The Resistor is variable so you can adjust the speed of flashing by using a small screwdriver. The output of the chip feeds a transistor which is used to switch a larger current if necessary.

This is the circuit. The individual parts that correspond to the circuit diagram are marked on the picture. You can see the variable resistor just to the right of the chip that you can turn with a small screwdriver to adjust the rate of flashing. Be careful where you put the circuit as bolt-heads protrude from the bottom - dont go putting it down on a polished table as it may scratch it.

Here is how you connect the circuit to make a light flash. Be careful to connect the battery terminals the right way round (+ve at the top) otherwise the chip may get damaged. You know how to stop that happening ? - use the diode in series with the battery, that way no damage can be done by connecting the battery incorrectly.

Note that the output is between +ve and the top of the transistor which will switch to ground. So it is important to have a real load (ie a lamp, LED or motor). If you connect the relay coil as the load, the relay will click in a regular pattern. It may sound familiar - just like the indicator clicking on a car - and this is exactly what they do on indicators. The astable turns on a relay so it clicks inside the car, and the contacts of the relay turn the indicator lamps on and off.

So that is it - hope you can make use of it.