Practical Electronics/Plugins/Astable

This plugin consists of a single 555 Astable. This plugin is very versatile, and can be used for any number of things, but it often is used to give a clock to run a circuit. The plugin below gives a pulse of about 2Hz with a very low mark-space ratio. The circuit diagram is given below.



This circuit obeys the following formulae with regards to its frequency and mark space ratio.


 * $$f=\frac{1.44}{\left( R_1 + R_2 \right)\times C}$$.


 * This is different to the design equation for the standard 555 Astable. (R2 instead of 2R2)


 * $$\mbox{High time}=0.69 R_1 \times C$$
 * $$\mbox{High time}=0.69 R_2 \times C$$


 * $$\mbox{Mark-space ratio}=R_{ms}=\frac{R_1}{R_2}$$

The values below give a frequency of about 2Hz, with a mark-space ratio of about 1:50.

The stripboard layout is as shown below with a component list.

Enable pin
To give this plugin an enable pin, replace the wire link from Pin 4 to +V with a 10K resistor going from Pin 4 to ground (Pin 1). Then add a header pin to Pin 4. A high signal on this pin will cause the astable to function, a low will disable it, locking the output low.