555 Timer as an Astable Multivibrator

Posted on Feb 5, 2014

An astable multivibrator, often called a free-running multivibrator, is a rectan ­gular-wave generating cir ­cuit. Unlike the monostable multivibrator, this circuit does not require any ex ­ternal trigger to change the state of the output, hence the name free-running. Before going to make the circuit, make sure your 555 IC is working. For that go

555 Timer as an Astable Multivibrator
Click here to download the full size of the above Circuit.

through the article: How to test a 555 IC for working An astable multivibrator can be produced by adding resistors and a capacitor to the basic timer IC, as illustrated in figure. The timing during which the output is either high or low is determined by the externally connected two resistors and a capacitor. The details of the astable multivibrator circuit are given below. Pin 1 is grounded; pins 4 and 8 are shorted and then tied to supply +Vcc, output (VOUT is taken form pin 3; pin 2 and 6 are shorted and the connected to ground through capacitor C, pin 7 is connected to supply + VCC through a resistor RA; and between pin 6 and 7 a resistor RB is connected. At pin 5 either a bypass capacitor of 0. 01 F is connected or modulation input is applied. In figure, when Q is low or output VOUT is high, the discharging transistor is cut- ­off and the capacitor C begins charging toward VCC through resistances RA and RB. Because of this, the charging time constant is (RA + RB) C. Eventually, the threshold voltage exceeds +2/3 VCC, the comparator 1 has a high output and triggers the flip-flop so that its Q is high and the timer output is low. With Q high, the discharge transistor saturates and pin 7 grounds so that the capacitor C discharges through resistance RB with a discharging time constant RB C. With the discharging of capacitor, trigger voltage at inverting input of comparator 2 decreases. When it drops below 1/3VCC, the output...

Leave Comment

characters left:

New Circuits



Popular Circuits

Mobile Bug Detector
Fire Alarm Circuit using LDR (Light Depending Resistor)
Very Low Dropout Adjustable Breadboard Power Supply
Rear Light After Glow (For Bicycles)
Negative Voltage Hot Swap Controllers in SOT-23
Astable multivibrator in an 8 pin PIC
16 Step Sequencer
Thyristor - linking arm rectifier module three-phase half-controlled bridge rectifier circuit