555 Astable Multivibrator

555 astable output, a square wave (Tm and Ts may be different)

555 astable circuit

An astable circuit using 555 produces a square wave. Figure shows a digital waveform with sharp transitions between low 0V and high. The durations of the low and high states may be different. The time period (T) of the square wave is the time for one complete cycle, but it is usually better to consider frequency(f) which is the number of cycles per second.

T = 0.7 × (R1 + 2R2) × C1 and f =	1.4
	(R1 + 2R2) × C1

T = time period in seconds (s)
f = frequency in hertz (Hz)
R1 = resistance in ohms ()
R2 = resistance in ohms ()
C1 = capacitance in farads (F)

The time period can be split into two parts: T = Tm + Ts
Mark time (output high): Tm = 0.7 × (R1 + R2) × C1
Space time (output low): Ts = 0.7 × R2 × C1

Choosing R1, R2 and C1

555 astable frequencies
C1	R2 = 10k R1 = 1k	R2 = 100k R1 = 10k	R2 = 1M R1 = 100k
0.001µF	68kHz	6.8kHz	680Hz
0.01µF	6.8kHz	680Hz	68Hz
0.1µF	680Hz	68Hz	6.8Hz
1µF	68Hz	6.8Hz	0.68Hz
10µF	6.8Hz	0.68Hz (41 per min.)	0.068Hz (4 per min.)

R1 and R2 should be in the range 1k to 1M. It is best to choose C1 first because capacitors are available in just a few values.

• Choose C1 to suit the frequency range you require (use the table as a guide).
• Choose R2 to give the frequency (f) you require. Assume that R1 is much smaller than R2 (so that Tm and Ts are almost equal), then you can use:
  

  R2 =	0.7
  	f × C1

• Choose R1 to be about a tenth of R2 (1k min.) unless you want the mark time Tm to be significantly longer than the space time Ts.
• If you wish to use a variable resistor it is best to make it R2.
• If R1 is variable it must have a fixed resistor of at least 1k in series
  (this is not required for R2 if it is variable).