Figure 1.
Astable Multivibrator Circuit Diagram Using the 555 Timer
This circuit
diagram shows how a 555 timer IC is configured to function as an astable
multivibrator. An astable multivibrator is a timing circuit whose
'low' and 'high' states are both unstable. As such, the output of
an astable multivibrator toggles between 'low' and 'high' continuously,
in effect generating a train of pulses. This circuit is therefore also
known as a 'pulse generator' circuit.
In this
circuit, capacitor C1 charges through R1 and R2, eventually building up
enough voltage to trigger an internal comparator to toggle the output
flip-flop. Once toggled, the flip-flop discharges C1 through R2
into pin 7, which is the discharge pin. When C1's voltage becomes
low enough, another internal comparator is triggered to toggle the
output flip-flop. This once again allows C1 to charge up through R1 and
R2 and the cycle starts all over again.
C1's
charge-up time t1 is given by: t1 = 0.693(R1+R2)C1. C1's discharge time
t2 is given by: t2 = 0.693(R2)C1. Thus, the total period of one
cycle is t1+t2 = 0.693 C1(R1+2R2). The frequency f of the output
wave is the reciprocal of this period, and is therefore given by:
f
= 1.44/(C1(R1+2R2)),
wherein f is in Hz if R1 and R2 are in megaohms and C1 is in
microfarads.
Learn more about the 555 Timer IC and
its applications...
See
also: Internal Block
Diagram of the 555; How a 555 Operates
Internally; 555 Timer Circuits
Flash Program
for 555; Astable Frequency Computation; Op Amp Square
Wave Generator
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