1<!DOCTYPE html> 2<html lang="en"> 3<head> 4<title>555 Timer IC | Electronics Club</title> 5<link rel="stylesheet" type="text/css" href="zecstyle.css"> 6<link rel="stylesheet" href="fontawesome/css/all.css"> 7<link rel="icon" href="favicon.ico" type="image/x-icon" > 8<meta charset="UTF-8"> 9<meta name="description" content="Learn about the 555 Timer IC - its symbol, pin connections, power supply, inputs and output. Also the 556 dual timer IC."> 10<meta name="viewport" content="width=device-width"> 11 12</head> 13 14<body> 15 16<div id="header"><a id="menulink" href="menu.htm">Menu <i class="fas fa-bars"></i></a> 17<a id="homelink" href="https://electronicsclub.info/">Electronics Club</a> 18<span class="nav"> 19<a class="navlink" href="components.htm">Components</a> 20<a class="navlink" href="study.htm">Study</a> 21</span> 22<a class="navlink" href="555timer.htm">555</a> 23<a class="navlink" href="projects.htm">Projects</a> 24<a class="navlink" href="construction.htm">Construction</a> 25<a class="navlink" href="soldering.htm">Solder</a> 26<a class="navlink" href="books.htm">Books</a> 27<a class="navlink" href="faq.htm">FAQ</a> 28<a class="navlink" href="circuitsymbols.htm">Symbols</a> 29<a class="navlink" href="starterkit.htm">Starter kit</a> 30<a class="navlink" href="tools.htm">Tools</a> 31</div> 32<div id="topofpage"></div> 33 34<a href="https://electronicsclub.info/"><img class="logo" src="logos/eclogo.gif" alt="Electronics Club" width="125" height="62"></a> 35 36<h1>555 Timer IC</h1> 37<div class="clearboth"></div> 38<p><a href="#symbol">Symbol</a> | <a href="#supply">Supply</a> | <a href="#inputs">Inputs</a> | 39<a href="#output">Output</a> | <a href="#556">556</a></p> 40<p class="links">Also see these 555 timer pages: 41<a href="555astable.htm">Astable</a> | <a href="555monostable.htm">Monostable</a> | <a href="555bistable.htm">Bistable</a> | <a href="555buffer.htm">Buffer</a></p> 42 43 44<h2>Introduction</h2> 45 46 47<div class="wrap320"><div class="colflex"> 48<p>The 8-pin 555 timer must be one of the most useful ICs ever made and it is used in many 49projects. With just a few external components it can be used to build many circuits, not 50all of them involve timing!</p> 51</div> 52<div class="col320"> 53<p class="centered"><a target="_blank" href="https://www.rapidonline.com/texas-instruments-ne555p-bipolar-single-timer-82-0336"> 54<img src="images/555horizontal.gif" alt="555 timer IC" width="74" height="62"></a></p> 55</div></div> 56 57<div class="wrap320"><div class="colflex"> 58<p>A popular version is the NE555 and this is suitable in most cases where a '555 timer' 59is specified. Low power versions, such as the ICM7555, are available with the 60same pin arrangement but their maximum output current is much lower and 61they should only be used when specified (to increase battery life).</p> 62<p>The 555 can be used in several circuits:</p> 63<ul> 64<li><a href="555astable.htm">Astable</a> - producing a square wave for flashing LEDs, 65making sounds, driving counters etc.</li> 66<li><a href="555monostable.htm">Monostable</a> - producing a single pulse when triggered, this can used for timing.</li> 67<li><a href="555bistable.htm">Bistable</a> - a simple memory with two states.</li> 68<li><a href="555buffer.htm">Buffer</a> - an inverting buffer (NOT gate).</li> 69</ul> 70<h4>555 Timer ICs</h4> 71<p>Rapid Electronics: <a target="_blank" href="https://www.rapidonline.com/texas-instruments-ne555p-bipolar-single-timer-82-0336">NE555 timer (standard)</a></p> 72<p>Rapid Electronics: <a target="_blank" href="https://www.rapidonline.com/intersil-icm7555ipa-cmos-low-power-timer-82-0774">ICM7555 (low power)</a></p> 73</div> 74<div class="col320"> 75 76<!-- Derby Computer Museum 728x90/468x60/300x250 --> 77<div class="ad"><a target="_blank" href="https://www.derbycomputermuseum.co.uk"> 78<img src="ads/dcm300x250.jpg" alt="Derby Computer Museum" width="300" height="250"></a></div> 79 80</div></div> 81 82<div id="symbol" class="clearboth"></div><hr> 83 84<h2>555 circuit symbol</h2> 85<div class="wrap320"><div class="colflex"> 86<p>The circuit symbol pins are arranged to suit the circuit: for example pin 8 at the top 87for the +Vs supply, pin 3 output on the right.</p> 88<p>Usually just the pin numbers are used and they are not labelled with their function.</p> 89</div> 90<div class="col320"> 91<img class="centered" src="images/555symbol.gif" alt="555 timer circuit symbol" width="184" height="130"> 92</div></div> 93 94<div id="supply" class="clearboth"></div><hr> 95 96<h2>555 supply (pins 1 and 8)</h2> 97<div class="wrap320"><div class="colflex"> 98<p>The 555 timer can be used with a supply voltage (Vs) in the range 99<strong>4.5V to 15V</strong> (18V is the absolute maximum).</p> 100<p>Pin 1 connects to 0V.<br>Pin 8 connects to the positive supply +Vs.</p> 101<p><em>
101Beware that the 555 creates a significant 'glitch' on the supply when its output changes 102state. This is rarely a problem in simple circuits with no other ICs but in more complex circuits a 103<a href="powersupplies.htm#smoothing">smoothing capacitor</a> may be required.</em></p> 104</div> 105<div class="col320"> 106<p class="centered"><img src="images/555pins.gif" alt="555 timer pin arrangement" width="214" height="111"></p> 107<p class="centered">555 pin arrangement</p> 108</div></div> 109 110<div id="inputs" class="clearboth"></div><hr> 111 112<div class="wrapfull"> 113<div class="colhalf"> 114<h2>555 trigger input (pin 2)</h2> 115<p>When less than <sup>1</sup>/<sub>3</sub> Vs ('active low') this makes the output high (+Vs). 116It has a high input impedance of at least 2MΩ. 117It monitors the discharging of the timing capacitor in an astable circuit.</p> 118 119<h2>555 threshold input (pin 6)</h2> 120<p>When greater than <sup>2</sup>/<sub>3</sub> Vs ('active high') this makes the output low (0V)*. 121It has a high input impedance of about 10MΩ. 122It monitors the charging of the timing capacitor in astable and monostable circuits. 123<br>* providing the trigger input is greater than <sup>1</sup>/<sub>3</sub> Vs, 124otherwise the trigger input will override the threshold input and hold the output high (+Vs).</p> 125 126<h2>555 reset input (pin 4)</h2> 127<p>When less than about 0.7V ('active low') this makes the output low (0V), 128overriding the other inputs. When not required it should be connected to +Vs. 129It has an input impedance of about 10kΩ.</p> 130</div> 131<div class="colhalf"> 132<h2>555 control input (pin 5)</h2> 133<p>This can be used to adjust the threshold voltage (used by the threshold input, pin 6) which is set internally 134to be <sup>2</sup>/<sub>3</sub> Vs. Usually this function is not required and the control 135input is often left unconnected. If electrical noise is likely to be a problem a 0.01µF capacitor 136can be connected between the control input and 0V to provide some protection. 137 138<h2>555 discharge (pin 7)</h2> 139<p>When the 555 output (pin 3) is low the discharge pin is connected to 0V internally. 140Its function is to discharge the timing capacitor in astable and monostable circuits. 141<br> </p> 142<div class="eim"><p>There is more information about 555 timers and their circuits on the 143<a target="_blank" href="http://www.eleinmec.com/">Electronics in Meccano</a> website.</p></div> 144</div></div> 145 146<div id="output" class="clearboth"></div><hr> 147 148<h2>555 output (pin 3)</h2> 149<div class="wrap320"><div class="colflex"> 150<p>The output of a standard 555 can <a href="ics.htm#sinksource">sink and source</a> current. 151This means that two devices can be connected to the output so that one is on when the output is low and 152the other is on when the output is high, the diagram shows two LEDs connected in this way.</p> 153<p>The <strong>maximum output current is 200mA</strong>, this is more than most ICs and it is sufficient to 154supply many output transducers directly including LEDs (with a resistor in series), 155low current lamps, piezo transducers, loudspeakers (with a capacitor in series), 156relay coils (with diode protection) and some small motors (with diode protection). 157The output voltage does not quite reach 0V and +Vs, especially if a large 158current is flowing.</p> 159<p>To switch larger currents you can <a href="transistorcircuits.htm#ic">connect a transistor</a>.</p> 160<p>The maximum output current of low power versions of the 555 (such the ICM7555) is much 161lower: about 20mA with a 9V supply.</p> 162</div> 163<div class="col320"><img class="centered" src="images/555output.gif" alt="555 output sinking and sourcing" width="140" height="150"></div> 164</div> 165<div class="clearboth"></div> 166 167<h3>Connecting the 555 to a loudspeaker</h3> 168<div class="wrap320"><div class="colflex"> 169<p>A loudspeaker (minimum resistance 64Ω) 170may be connected to the output of a 555 astable circuit but a capacitor (about 171100µF) must be connected in series. The astable output is equivalent to a steady DC of about 172½Vs combined with a square wave AC (audio) signal. The capacitor blocks the DC, but 173allows the AC to pass as explained in <a href="capacitance.htm#coupling">capacitor coupling</a>.</p> 174<p>Piezo transducers may be connected directly to the output and do not require a capacitor in series.</p> 175</div> 176<div class="col320"><img class="centered" src="images/555speaker.gif" alt="connecting a loudspeaker to 555 output" width="128" height="101"></div> 177</div> 178<div class="clearboth"></div> 179 180<h3>Connecting the 555 to relay coils and other inductive loads</h3> 181<div class="wrap320"><div class="colflex"> 182<p>Like all ICs, the 555 must be protected from the brief high voltage 'spike' 183produced when an inductive load such as a relay coil is switched off. The standard 184<a href="relays.htm#protect">protection diode</a> must be connected 185'backwards' across the the relay coil as shown in the diagram.</p> 186<p>However, <strong>the 555 requires an extra diode</strong> connected 187in series with the coil to ensure that a small 'glitch' cannot be fed back into the IC. 188Without this extra diode monostable circuits may re-trigger themselves as the coil is 189switched off! The coil current passes through the extra diode so it must be a 1N4001 or 190similar rectifier diode capable of passing sufficient current, a signal diode such as a 1N4148 191is usually <strong>not</strong> suitable.</p> 192</div> 193<div class="col320"><img class="centered" src="images/555diodes.gif" alt="555 output protection diodes" width="160" height="275"></div> 194</div> 195 196<div id="556" class="clearboth"></div><hr> 197 198<h2>556 Dual Timer IC</h2> 199<div class="wrap320"><div class="colflex"> 200<p>The 556 is a dual version of the 555 timer housed in a 14-pin package, the 201two timers (A and B) share the same power supply pins. 202The circuit diagrams on this website show a 555, but they could all be adapted to use one half of a 556.</p> 203<p>The 556 is less popular and may cost more than two 555s so you may prefer to use two 555 timers.</p> 204<p>Rapid Electronics: <a target="_blank" href="https://www.rapidonline.com/texas-instruments-ne556n-dual-timer-82-0344">NE556 dual timer</a></p> 205</div> 206<div class="col320"><img class="centered" src="images/556pins.gif" alt="556 dual timer pin arrangement" width="248" height="188"></div> 207</div> 208<hr> 209 210<div class="wrap320"><div class="colflex"> 211<h4>Projects using 555 timer:</h4> 212<p>These are some examples:</p> 213<ul class="links"> 214<li><a href="p_flashingled.htm">Flashing LED</a> (Astable)</li> 215<li><a href="p_dummy.htm">Dummy Alarm (Astable)</a></li> 216<li><a href="p_timer.htm">1-10 Minute Timer</a> (Monostable)</li> 217<li><a href="p_quiz.htm">Quiz</a> (Bistable)</li> 218</ul> 219<p>Rapid Electronics: <a target="_blank" href="https://www.rapidonline.com/texas-instruments-ne555p-bipolar-single-timer-82-0336">NE555 timer</a></p> 220<p> </p> 221<p><strong>Next page:</strong> <a href="555astable.htm">555 Astable</a></p> 222<p><strong>Also see:</strong> <a href="555monostable.htm">Monostable</a> | <a href="555bistable.htm">Bistable</a> | 223<a href="555buffer.htm">Buffer</a></p> 224<p> </p> 225</div> 226<div class="col320"> 227 228<!-- Visit by Train 300 229<div class="ad"> 230<a target="_blank" href="https://visitbytrain.info/"><img src="ads/visitbytrain300.jpg" alt="visitbytrain.info" width="300" height="224"></a></div> 231--> 232 233</div></div> 234 235<footer> 236<h4 class="hide-print">Privacy Policy & Cookies</h4> 237<p class="hide-print"><small>This website does not collect personal information. 238If you send an email your email address and any personal information will be 239used only to respond to your message, it will not be given to anyone else. 240This website displays advertisements, if you click on
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