Designing 555 Astables

Posted on Feb 6, 2014

If an oscillator of a particular frequency and mark to space ratio is required, (see Fig. 4. 4. 1) the method would be to calculate periodic time from the required frequency and the discharge time and charge time using the formulae for tD and tC described in Oscillators Module 4. 3. To do this, some component details will be needed. Starting with C1

, a suitable value can be assumed from the diagram in Fig. 4. 4. 2, which shows that for an astable having a frequency of 1kHz, and so for a periodic time of 1ms, capacitors of 1nF to 1 F would suffice, depending on which of the total resistances (indicated by the red lines) was chosen. Manufacturers specify the maximum total resistance that may be used with their particular variant of the 555, and these maximum values are usually around 10 to 20M ©, however using such high values can increase the error between calculated and actual frequencies, so for many uses a 1M © maximum can be recommended. The minimum total resistance value for the combination of R1 and R2 depends largely on the value of R1. The junction of R1/R2 is connected to pin 6 and to the trigger input pin 2. If the value of R1 is less than about 1K ©, there is a danger that the trigger input may not be able reach a low enough voltage to trigger comparator 1, and so oscillations cannot take place. From this it can be assumed that if R1 must be 1K © or above and R1 + R2 should be kept below 1M ©; a 10nF capacitor would allow a suitable total resistance of around 100K © to be calculated. The basic 555 astable design described above uses two timing resistors when producing square waves. In timing the high (charging) period the timing capacitor (C1 in Fig. 4. 4. 3) is charged via R1 and R2, but only R2 is used whilst discharging C1. In this basic configuration,...

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