A 555 is an integrated circuit; it is used as a timer,
pulse generator and oscillator. A 555 provides time delays in the form of
oscillators. It is named 555 because of a 3-5k internal resistor divider which
are used to set comparator levels. Sinking and Sourcing in 555 timer output.
This is to be noted that 555 is non-programmable. The two 555 timers in a single
IC makes 556.
As Source when the output at pin 3 is low, its current path through to ground
through 555 makes the LED turned on.
As sink, when output at pin 3 is high, it provides a current source from 555 and
makes the LED turned on. This concept of sinking and sourcing is used in
applications of Robots.
555 for controlling motor
555 has three operating modes
Monostable mode, astable and bistable mode
Monostable mode: In this mode, the 555 functions
as a one-shot pulse generator. The 555 timer receives a signal from trigger
and then the pulse begins. The width of the output pulse is determined by time
constant of RC network. Pulse is determined by
the time constant of an RC network; this RC network consists of resistors and
capacitors. The output pulse ends when the
charge equivalent to 2/3 of the voltage is accumulated on the capacitor. This
mode is used in timers, frequency divider and pulse width modulation. This is
also used as a detector for missing pulse
In bistable mode, the 555 timer acts as a basic
flip-flop. The trigger (pin2) and reset (pin4) inputs are kept high with the
help of pull-up resistors. Tthe threshold pin is kept grounded. Pulling the
reset input to ground acts as a reset. It is to be noted that in bi-stable mode,
no capacitor is needed.
Astable mode: This mode is also known as free
running mode; the 555 in astable mode is used as a temperature sensor as
well. The period of the output is determined by the temperature. This can be
done by selecting a thermistor as timing
resistor. Period of output pulse is determined by the temperature, in this
application, microprocessor converts pulse period
In astable mode, the 555 timer puts out a continuous stream of rectangular
pulses having a specified frequency. Resistor R1 is connected between VCC and
the discharge pin and another resistor (R2) is connected between the discharge
pin, and the trigger (pin 2) and threshold (pin 6) pins that share a common
node. Thus, the capacitor is charged through R1 and R2, and discharged only
through R2, since pin 7 has low impedance to ground during output low intervals
of the cycle, therefore discharging the capacitor.
In the astable mode, the frequency of the pulse stream depends on the values of
R1, R2 and C.
Where R1 and R2 are the values of the resistors in ohms and C is the value of
the capacitor in farads.
The power capability of R1 must be greater than.
This 555 timer is used in robotic field in building robots like wall following,
light tracker and servo ball smacker etc. The servo is controlled by the PWM
signals which is generated by these timers. One of the most fundamental problems
in robotics is DC motor speed control. The most common method of speed control
is PWM or pulse width modulation. Pulse width modulation is a process of
switching the power to a device on and off at a given frequency, with varying on
and off times.
Although we can build robots with the help of a
microcontroller, which is used to generate the PWM signals, but using a 555
timer we can do it very easily. This timer in a PWM circuit is used as an
astable oscillator. It oscillates without any external trigger. All we need is
to provide power once for the application.
The 555 timer in the PWM circuit is configured
as an astable oscillator. This means that once power is applied, the 555 will
oscillate without any external trigger. Before the technical explanation of the
circuit, let's look at the 555 timer IC itself.
These on and off times are whenever we use two
servo motors, we need two of these timers hence we can use 556 which is a
combination of two 555 timers. One or two applications of these timers are being
shown in diagrams: