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555 Timer IC in Robotics

By Sonakshi Singh on January 02, 2012
In this article we will learn about the 555 Timer IC which is used as a PWM generator, more commonly in robots.
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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 input
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
detection.

Bistable mode

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
to temperature.
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:

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