The IC 555 is well known for its features which become readily suitable for an unlimited range of electronic circuit applications. Though the chip is fundamentally designed for generating time delays and oscillations periods, it can be modified or manipulated to do a number of other functions. Some examples of this IC being used for other applications are voltage monitors, automatic battery chargers, comparators and PWM and ramp generators.
The present article deals with the chip configured for pulse dimensioning purposes. Primarily here we are discussing how the IC 555 can be used as Pulse Width
The following discussion is especially made easy, keeping the newbies in mind. These circuits can be easily built by them for testing and analysis purposes.
What is a Pulse or a Clock Signal
All electronic circuits engaged in generating or shaping voltages require a pulse signal for the necessary operations. These pulses are mostly in the form of square wave signals also called clock signals.
The above square wave signal is normally symbolized as indicated in the diagram. In this type of pulse, the voltage rises suddenly from zero (shown by the red section), then holds for a moment (shown by the green section), and then suddenly falls back to zero (shown by the blue line). The cycle continues generating a string of these waveforms, also termed oscillations or frequency. The spacing between these pulses is normally equal or simply, the waveform is uniform throughout.
The green section is the period for which the output of the IC remains high; the black section is the time period for which the output of the IC remains OFF or at zero voltage level. These time delays are called mark and space ratio respectively, which can be varied through external resistors with a 555 IC. The ratio of this mark and space delay is identified as the duty cycle of the IC.
Though we can easily vary the frequency or the number of pulses per second using an astable multivibrator and its associated resistors and capacitor network, varying the width of the pulses can be complex.
Due to the great versatility of the above IC, it can be wired up as an astable as well as a pulse modifier and the two stages can be integrated to form an excellent PWM circuit.
IC 555 as an Astable Multivibrator
The diagram shows how a 555 IC can be used as an astable multivibrator circuit for generating square wave clock pulses. In this mode the pin 2 and 6 of the IC joined which forces the IC to go on a self oscillating mode, hence it’s also called free running multivibrator. The ratio of the resistors R1 + R2 determines the duty cycle of the oscillator which is the percentage of period for which the output of the IC may stay ON or OFF. The capacitor alternately charges through R1 + R2 and discharges R2. Varying its value directly affects the output frequency, hence it can be suitably adjusted for acquiring a particular specified frequency rate.
IC 555 as a Pulse Width Modulator
As the definition suggests, the circuit is intended for modulating the width of the input applied pulses at its output.
Referring to the diagram, the IC can be seen wired up in the usual monostable mode, however if the trigger input pin #2 is now fed with a train of pulses, the width of these pulses available at its output can be simply modified through a control voltage applied at pin #5 of the IC. This transforms the design into a standard IC 555 PWM circuit stage.
The above two modes of operation can be staged together for obtaining an ideal PWM controlled output, which can be implemented for many different practical applications, here it’s used as a DC motor speed controller circuit.
The figure shows the entire details of the wiring connections between the stages. The circuit functioning can be understood with the following points:
The AMV circuit wired up around IC1 generates a constant 100 Hz frequency, which becomes triggering pulses for the next PWM stage configured around another 555 IC2. This frequency also becomes the frequency of the PWM signals.
A continuously varying voltage is applied to the pin #5 of IC2 whose level directly corresponds to the width of the acquired PWM signals at its output.
The single PNP transistor and its associated components act as a current source, also forming sawtooth pulses across C3. This sawtooth waveform is compared against the applied control voltage at pin #5 for generating the PWM output signals.
The above PWM output is applied to a Darlington transistor which responds to these pulses and translates the information into varying speed levels of the connected motor.
LM 555 Datasheet - http://www.national.com/ds/LM/LM555.pdf
Authors Own Experience
Images Drawn by the Author