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)

${\mbox{High time}}=0.69R_{1}\times C$

${\mbox{High time}}=0.69R_{2}\times C$

${\mbox{Mark-space ratio}}=R_{ms}={\frac {R_{1}}{R_{2}}}$

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.

Ref. Component IC1 555 Timer IC R1 10K Resistor R2 330K Resistor C1 2.2μF Capacitor D1 1N4148 Signal Diode

Variations

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.