The 555 as an Astable Multivibrator

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> An Astable multivibrator, often a free-running  multivibrator, is a rectangular-wave-generating circuit. Unlike the monostable multivibrator, this circuit does not require an external trigger to change the state of the output is either high or low is determined by the two resistors and a capacitor, which are externally connected to the 555 timer.<br /> <br /> <div class="separator" style="clear: both; text-align: center;"> <img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhl-a31IH3y0oROe3fW7P1xM2pT6cS9ojR5iR6HhV5bNE8tfNh0Q_wOsjT2xYPgp2sdJ2CFBrTLA-t6xczLKZAczCso2heBXQNKHlCFf3LSBTp7SUGfi0cpNrMLNaWbVM8TkKDwtw6d-1Fr/s1600/555+as+an+astable+multivibrator.png" /></div> </div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> <h4> <b>Astable operation:</b></h4> </div> <div style="text-align: justify;"> <br /> Figure (a) shows the 555 timer configuration as an astable multivibrator. Initially, when the output is high, capacitor <i>C</i> starts charging toward <i><span style="font-size: 16px; line-height: 18.3999996185303px;">V</span><sub style="line-height: 18.3999996185303px;">CC</sub></i> through <i><span style="font-size: 16px; line-height: 18.3999996185303px;">R</span><sub style="line-height: 18.3999996185303px;">A</sub></i> and <i><span style="font-size: 16px; line-height: 18.3999996185303px;">R</span><sub style="line-height: 18.3999996185303px;">B</sub></i>. However  as soon as voltage across the capacitor equals 2/3 <i><span style="font-size: 16px; line-height: 18.3999996185303px;">V</span><sub style="line-height: 18.3999996185303px;">CC</sub></i>, comparator 1 triggers the flip-flop, and the output switches low. Now capacitor <i>C</i> starts discharging through  <i><span style="font-size: 16px; line-height: 18.3999996185303px;">R</span><sub style="line-height: 18.3999996185303px;">B</sub></i> and transistor <i>Q₁</i>. When the voltage across <i>C</i> equals 1/3 <i>Vcc</i>, comparator 2 output triggers the flip-flop, and the output goes high. Then the cycle repeats. The output voltage and capacitor voltage waveforms are shown in Figure (b).<br /> <br /></div> <div style="text-align: justify;"> <div class="separator" style="clear: both; text-align: center;"> <img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEirAgZC3X5RK_eGfmn-f5hRTG4lkdYX1wtquq13D7jOGEg0bbTD6pk0m-hJS1k43_yBeUrjB5QSgaPl0zFxynBS1losdWHUejUvNh2ezMiV0yo_tUOzs92EXfJs3O7-wm_WhicqZiP6LBDv/s1600/Voltage+across+capacitor+and+output+voltage+waveforms.png" /></div> <br /></div> <div style="text-align: justify;"> As shown in this figure, the capacitor is periodically charged and discharged between 2/3 <span style="font-size: 12pt; line-height: 115%;"><span style="font-family: inherit;"><i>V_CC</i></span></span> and 1/3 <i><span style="font-size: 16px; line-height: 18.3999996185303px;">V</span><sub style="line-height: 18.3999996185303px;">CC</sub></i>, respectively. The time during which the capacitor charges from 1/3 <i><span style="font-size: 16px; line-height: 18.3999996185303px;">V</span><sub style="line-height: 18.3999996185303px;">CC</sub></i> to 2/3 <i><span style="font-size: 16px; line-height: 18.3999996185303px;">V</span><sub style="line-height: 18.3999996185303px;">CC</sub></i> is equal to the time the output is high and is given by:</div> <div style="text-align: justify;"> <br /> <div class="MsoNormal"> <span style="font-family: inherit;"><b><span style="font-size: 12pt; line-height: 115%;">t</span>_c = 0.698*(R_A + R_B)*C</b></span>_{<span style="font-family: "Verdana","sans-serif"; font-size: 12.0pt; line-height: 115%;"><o:p></o:p></span>}</div> </div> <div style="text-align: justify;"> <br /> where <i>R_A</i> and <i>R_B</i> are in ohms and <i>C</i> is in farads. Similarly, the time during which the capacitor discharges from 2/3 <i><span style="font-size: 16px; line-height: 18.3999996185303px;">V</span><sub style="line-height: 18.3999996185303px;">CC</sub></i> to 1/3 <i><span style="font-size: 16px; line-height: 18.3999996185303px;">V</span><sub style="line-height: 18.3999996185303px;">CC</sub></i> is equal to the time the output is low and is given by:</div> <div style="text-align: justify;"> <br /> <div class="MsoNormal"> <span style="font-family: inherit;"><b><span style="font-size: 12pt; line-height: 115%;">t</span>_d = 0.69*(R_B)*C</b></span><span style="font-family: "Verdana","sans-serif"; font-size: 12.0pt; line-height: 115%;"><o:p></o:p></span></div> </div> <div style="text-align: justify;"> <br /> where <i><span style="font-size: 16px; line-height: 18.3999996185303px;">R</span><sub style="line-height: 18.3999996185303px;">B</sub></i> is in ohms and <i>C</i> is in farads. Thus the total period of the output waveform is</div> <div style="text-align: justify;"> <br /> <div class="MsoNormal"> <span style="font-family: inherit;"><b><span style="font-size: 12pt; line-height: 115%;">T = </span>t_c + t_d = 0.698*(R_A + 2R_B)*C</b></span><span style="font-family: "Verdana","sans-serif"; font-size: 12.0pt; line-height: 115%;"><o:p></o:p></span></div> </div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> This, in turn, gives the frequency of oscillation as:</div> <div style="text-align: justify;"> <br /> <div class="MsoNormal"> <span style="font-family: inherit;"><b><span style="font-size: 12pt; line-height: 115%;">f</span>_o = 1 / T = 1.45 / [(R_A + R_B)*C]</b></span><span style="font-family: "Verdana","sans-serif"; font-size: 12.0pt; line-height: 115%;"><o:p></o:p></span></div> </div> <div style="text-align: justify;"> <br /> The above equation indicates that the frequency <i><span style="font-size: 16px; line-height: 18.3999996185303px;">f</span><sub style="line-height: 18.3999996185303px;">o</sub></i> is independent of the supply voltage <i><span style="font-size: 16px; line-height: 18.3999996185303px;">V</span><sub style="line-height: 18.3999996185303px;">CC</sub></i>.</div> <div style="text-align: justify;"> <br /></div> <div style="text-align: justify;"> Often the term <i>duty cycle</i> is used in conjunction with the astable multivibrator. The duty cycle is the ratio of the time tc during which the output is high to the total time period <i>T</i>. It is generally expressed as a percentage. In equation form,</div> <div style="text-align: justify;"> <br /> <div style="margin: 0in 0in 0.0001pt;"> <span style="font-family: inherit;"><b>% duty cycle = (t_c / T)*100<o:p></o:p></b></span></div> <div style="margin: 0in 0in 0.0001pt;"> <span style="font-family: inherit;"><b>                      = [ (R_A + R_B) / (R_A + 2R_B) ] * 100</b></span></div> </div> </div>

The 555 as an Astable Multivibrator

An Astable multivibrator, often a free-running  multivibrator, is a rectangular-wave-generating circuit. Unlike the monostable multivibrat...

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The 555 as a Monostable Multivibrator

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> A monostable multivibrator, often called as a one-shot multivibrator, is a pulse generating circuit in which the duration of the pulse is determined by the RC network connected externally to the 555 timer. In a stable or standby state the output of the circuit is approximately zero or at logic-low level. When an external trigger pulse is applied, the output is forced to go high (<span style="font-size: 15px; line-height: 16.8666667938232px;">V</span><sub style="line-height: 16.8666667938232px;">CC</sub>). The time the output remains high is determined by the external RC network connected to the timer. At the end of the timing interval, the output automatically reverts back to its logic-low stable state. The output stays low until the trigger pulse is again applied. Then the cycle repeats.<br /> <br /> <div class="separator" style="clear: both; text-align: center;"> <img alt="555 Timer as a Monostable Multivibrator" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhqHyVDGPRHgMCKP5ErBmk6P-n2jYV5bc4Mtzbi1oXz0O-NjZJn6LkNdBaxa4CecD_W-u8HkVjtus6pR_BsRq8QjBVIGk_NjMY1dU6Q2k22tFYm_zAr3r07uddUeqpMgltu0FzpW3Iz1iKM/s1600/555-monostable-multivibrator.jpg" title="555 Timer as a Monostable Multivibrator" /></div> <br /> The monostable circuit has only one stable state (output low), hence the name monostable. Normally, the output of the monostable multivibrator is low.<br /> <br /></div> <div style="text-align: justify;"> <h4> <b>Monostable Operation:</b></h4> <br /> According to Figure (b), initially when the output is low, that is, the circuit is in stable  state, transistor Q1 is on and capacitor C is shorted out to ground.<br /> <br /> <div class="separator" style="clear: both; text-align: center;"> <img alt="555 Timer connected as a Monostable Multivibrator" border="0" height="537" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhDuhIlCFmKa2Sze7JGqOzFdJCtuqjSAMBpcSJJR-tTE63kucrxrfWf8V8MLwxnj6zjgAE7DhhajEZHIL3ZzvP3r8DyB5bhJY2S2Zj7WI9_JmYvuxTBPENP5Fj4hJGU3Ys7WwhWI06yVQRC/s1600/555+as+a+Minostable+Multivibrator.png" title="555 Timer connected as a Monostable Multivibrator" width="640" /></div> <br /> However, upon application of a negative trigger pulse to pin 2, transistor Q1 is turned off, which releases the short circuit across the external capacitor C and drives the output high. The capacitor C now starts charging up toward <span style="font-size: 11pt; line-height: 115%;"><span style="font-family: inherit;">V_CC</span></span> through Ra. However, when the voltage across the capacitor equals 2/3<span style="font-size: 15px; line-height: 16.8666667938232px;">V</span><sub style="line-height: 16.8666667938232px;">CC</sub>, comparator 1's output switches from low to high, which in turn drives the output to its low state via the output of the flip-flop. At the same time, the output of the flip-flop turns transistor Q1 on, and hence capacitor C rapidly discharges through the transistor. The output of the monostable remains low until a trigger pulse is again applied. Then the cycle repeats. Figure (c) shows the trigger input, output voltage, and capacitor voltage waveforms. As shown here, the pulse width of the trigger input must be smaller than the expected pulse width of the output waveform. Also, the trigger pulse must be a negative-going input signal with an amplitude larger than 1/3<span style="font-size: 15px; line-height: 16.8666667938232px;">V</span><sub style="line-height: 16.8666667938232px;">CC</sub>.<br /> <br /> <div class="separator" style="clear: both; text-align: center;"> <img alt="555 Timer Monostable Multivibrator input And output waveforms" border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjYwmUfnMlSGb4wU4mUCv9w0Km0dNVcy4OQUqU1lPwOCVgcWmil9HnnGhSoJD7u94DwgsCcDpboT0Yiq-2HrrJsGjTtiT2-T5i63PCjTM9mhMShw2x0PknpOTOsgonDORdDWpJsULdHWj0F/s1600/input+and+output+waveforms+of+555.png" title="555 Timer Monostable Multivibrator input And output waveforms" /></div> <span style="text-align: center;"><br /></span> <span style="text-align: center;">The time during which the output remains high is given by;</span><br /> <br /> <div style="text-align: center;"> <div style="text-align: left;"> <span style="font-size: 12pt; line-height: 115%;"><span style="font-family: inherit;"><b>t_p = 1.1<i>R_AC</i>        seconds</b></span></span></div> </div> <br /> where <span style="font-size: 12pt; line-height: 115%;"><span style="font-family: inherit;">R_A</span></span> is in ohms and <span style="font-size: 12pt; line-height: 115%;"><span style="font-family: inherit;">C</span></span> is in farads.<br /> <br /> Once triggered, the circuit's output will remain in the high state until the set time tp elapses. The output will not change its state even if an input trigger is applied again during this time interval tp. However, the circuit can be reset during the timing cycle by applying a negative pulse to the reset terminal. The output will then remain in the low state until a trigger is again applied.</div> </div>

The 555 as a Monostable Multivibrator

A monostable multivibrator, often called as a one-shot multivibrator, is a pulse generating circuit in which the duration of the pulse is ...

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The 555 Timer

<div dir="ltr" style="text-align: left;" trbidi="on"> <div style="text-align: justify;"> <span style="font-family: inherit;">One of the most versatile linear integrated circuits is the 555 Timer. Signetics Corporation first introduced this device as the SE/NE 555 in early 1970. Since its debut, the device has been used in a number of novel and useful applications. A sample of these applications includes monostable and astable multivibrators, dc-dc converters, digital logic probes, waveform generators, analog frequency meters and tachometers, temperature measurement and control, infrared transmitters, burglar and toxic alarms, voltage regulators, electric eyes, and many others. The 555 Timer is a monolithic timing circuit that can produce accurate and highly stable time delays or oscillation. The timer basically operates in one of the two modes: either as a monostable (one-shot) multivibrator or as an astable (free running) multivibrator. The device is available as an 8-pin metal can, an 8-pin mini DIP, or a 14-pin DIP. Figure below shows the connection diagram of the SE/NE 555 timer;</span><br /> <span style="font-family: inherit;"><br /></span> <br /> <div class="separator" style="clear: both; text-align: center;"> <span style="font-family: inherit;"><img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiah18ASXWC5bySSKuU5MbTj7xceKIK-TM4HhpQoo72qSY3UtGrYcCFcls_z88HE-LSSGpW69GYqxW6WQTeWpx3tOmw_PFZTCv7MF-s4Rg0EHwgHC-7bQj6okfgU6e-jp1AuylauCBYej7L/s1600/555_pinout.png" /></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><br /></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;">The SE555 is designed for the operating temperature range of -<span style="font-size: 11pt; line-height: 115%;">55⁰</span> to +12<span style="font-size: 11pt; line-height: 115%;">5⁰</span> C, while the NE555 operates over a temperature range of <span style="font-size: 11pt; line-height: 115%;">0⁰</span> to <span style="font-size: 11pt; line-height: 115%;">70⁰</span> C. The important features of the 555 timer are these: it operates on +5 to +18 V supply voltage in both free-running (astable) and one-shot (monostable) modes; it has an adjustable duty cycle; timing is from microseconds through hours; it has a high current output; it can source or sink 200 mA; the output can drive TTL and has a temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature, or equivalently 0.005%/^{<span style="font-size: 11pt; line-height: 115%;">0</span>}<span style="font-size: 11pt; line-height: 115%;">C. Like general-purpose op-amps, the 555 timer is reliable, easy to us, and low cost.</span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><br /></span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><b><u>Pin Configurations</u></b></span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><br /></span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><b>Pin 1: Ground:</b> All the voltages are measured with respect to its terminal.</span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><br /></span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><b>Pin 2: Trigger:</b> The output of the timer depends on the amplitude of the external trigger pulse applied to this pin. The output is low if the voltage at this pin is greater than 2/3Vcc. However, when a negative-going pulse of amplitude larger than 1/3Vcc is applied to this pin, the comparator 2 output goes low, which in turn switches the output of the timer high. The output remains high as long as the trigger terminal held at a low voltage.</span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><br /></span></span></div> <div class="separator" style="clear: both; text-align: center;"> <img border="0" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjuxM8GYFAKT5X_pcHTssItXuEyN_2XVFXlLMvtvaPcR-Rj_QPAhmna7q-adkoOy0fqe5rPiJk50juWgHG12sCxlx90hqXt-aO8qz7P3nB5Vphi6BQcMYFOM3KCMFiHvndc44lMu2M-xYb-/s1600/555+timer.png" /></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><br /></span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><b>Pin 3: Output:</b> There are two ways a load can be connected to the output terminal: either between pin 3 and ground (pin 1) or between pin 3 and supply voltage +Vcc (pin 8). When the output is low, the load current flows through the load connected between pin 3 and +Vcc into the output terminal and is called <i>sink</i> current. However, the current through the grounded load is zero when the output is low. For this reason, and that connected between pin 3 and +Vcc is called the <i>normally on load</i> and that connected between pin 3 and ground is called the <i>normally off load</i>. On the other hand, when the output is high, the current through the load connected between pin 3 and +Vcc (normally on load) is zero. However, the output terminal supplies current to the normally off load. This current is called the <i>source</i> current. The maximum value of sink or source current is 200 mA.</span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><br /></span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><b>Pin 4: Reset:</b> The 555 timer can be reset (disabled) by applying a negative pulse to this pin. When the reset function is not in use, the reset terminal should be connected to +Vcc to avoid any possibility of false triggering.</span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><br /></span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><b>Pin 5: Control voltage:</b> An external voltage applied to this terminal changes the threshold as well as the trigger voltage. In other words, by imposing a voltage on this pin or by connecting a pot between this pin and ground, the pulse width of the output waveform can be varied. When not used, the control pin should be bypassed to ground with a 0.01 uF capacitor to prevent any noise problems.</span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><br /></span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><b>Pin 6: Threshold:</b> This is the non-inverting input terminal of comparator 1, which monitors the voltage across the external capacitor. When the voltage at this pin is greater than or equal to threshold voltage 2/3Vcc, the output of comparator 1 goes high, which in turn switches the output of the timer low.</span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><br /></span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><b>Pin 7: Discharge:</b> This pin is connected internally to the collector of transistor Q1, as shown in above figure. When the output is high, Q1 is off and acts as an open circuit the external capacitor C connected across it. On the other hand, when the output is low, Q1 is saturated and acts as a short circuit, shorting out the external capacitor C to ground.</span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><br /></span></span></div> <div class="separator" style="clear: both; text-align: justify;"> <span style="font-family: inherit;"><span style="font-size: 11pt; line-height: 115%;"><b>Pin 8: +Vcc:</b> The supply voltage +5 V to +18 V is applied to this pin with respect to ground (pin 1).</span></span></div> </div> </div>

The 555 Timer

One of the most versatile linear integrated circuits is the 555 Timer. Signetics Corporation first introduced this device as the SE/NE 555...

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