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Step 6: 555 Timer: Astable Mode Circuit

As I described in the last step, setting the 555 timer up in astable mode causes it to output a continuous series of pulses. In this circuit, I'll set up the 555 timer to output a pulse wave with a frequency inside the audible range, this way I can connect the output to a speaker and hear the results.

Parts List:
555 timer Digikey LM555CNFS-ND
0.01uF capacitor Digikey 445-5297-ND
100kOhm linear taper potentiometer Digikey 987-1300-ND
10kOhm 1/4watt resistor Digikey CF14JT10K0CT-ND
0.47uF capacitor (or anything between 10uF and 0.1uF should be fine) Digikey P5173-ND
speaker
22 gauge jumper wire
breadboard Amazon
5-15V power supply- if you don't have a bench power supply, try using a 9V battery and battery snap or use the 5V output from an Arduino

Wiring Info
The schematic is shown in fig 6. Connect power and ground to pins 8 and 1 of the 555 timer (red and black wires). I used a 9V supply and battery snap for my circuit. As indicated in the schematic in fig 6, connect a 0.01uF capacitor between pins 5 and 1. Connect a 0.47uF capacitor between pins 1 and 6, make sure that the negative lead of the capacitor is connected to pin 1. Wire a 10kohm resistor between pin 6 and 7. Wire a 100K potentiometer wired as a variable resistor between pins 7 and 8. Use jumper wire to connect pins 4 and 8 to each other (red) and pins 2 and 6 to each other (yellow).

Attach the positive lead of a speaker to pin 3 of the 555 and connect the negative lead to ground (pin 1).

Operation
When you power this circuit you should begin to hear the pulse waveform coming from the 555. Turn the potentiometer to change the frequency of this pulse wave. If you want to generate a particular frequency, try changing the values of RA, RB, and C according to the following equation (derived in the last step):

Frequency of Output = 1/[0.7*(RA+2*RB)*C]
where RA, RB, and C are shown in fig 7

Based on the components I used in this sample circuit, we can calculate the range of possible output frequencies as follows:

assuming the potentiometer is turned all the way to one side and the resistance = 100kohms
Frequency of Output = 1/[0.7*(100,000+2*10,000)*0.00000047]
Frequency of Output =~ 25Hz

this output is shown on an oscilloscope in fig 8

Low values of RA should be avoided because they prevent the 555 timer from discharging the capacitor C normally. When I turned the pot all the way to the other side (for a resistance of 0ohms) the 555 timer stopped working as expected (fig 10). So let's calculate the output frequency from the timer when the pot is turned to its halfway point, for a resistance of 50Kohms.
Frequency of Output = 1/[0.7*(50,000+2*10,000)*0.00000047]
Frequency of Output =~ 43Hz

this output is shown in fig 9 (note- there is some error bc I was guessing at where halfway was)

Also notice that although the frequency of the output changes between figs 8 and 9, the duration of the low output phase does not change significantly. This is because the duration of the low output phase is not dependent on RA (the variable resistor). As calculated in the last step:
t = 0.7*RB*C
substituting RB = 10kOhms and C = 0.47uF you get:
t =~ 3ms
which can be verified in figs 8 and 9.

<p>Can any of the pin be used to give an output logic 1 unless I open the switch??</p>
<p>Can any of the pin be used to give an output logic 1 unless I open the switch??</p>
Can any one help me to calculate the resistance of PWM to make astable pulse 50% duty cycle time on/off 5 second.. <br>The formula T=0.693 x R x C or T= 1.1 x R x C ?
<p>so if say a triangular input is applied at the trigger input of monostable 555 timer is it still possible to get rectangular waveforms from the output of the timer ? and how does the period of trigger input affect the output? thank you</p>
<p>The output on pin 3 is always a switched ON/OFF output. Pin 7 is the same except that it is open collector (no internal or active pull-up). That is, the rise and fall time are fast. The output is from a comparitor which produces an instant ON and OFF. The trigger level is when the voltage on pin 2 and pin 6 reach their respective trigger voltages (1/3 and 2/3 of the voltage that is on pin 5). The dead-band or hysteresis is the voltage difference between 1/3 and 2/3 of the voltage on pin5 and the difference in voltage of pins 2 and 6. Most simple timers/oscillators have pins 2 and 6 tied together. </p><p>The 555 is a very versatile device and can be operated in many nonstandard ways. The 7555 is a little better equivalent because it is 100% CMOS and can operate far more symmetrically than the 555 at lower operating currents. </p><p>The input on pins 2 &amp; 6 can have any shape from square to sine depending on what you are designing.</p><p>good Luck </p><p>Pixeltamer</p>
<p>like in this picture ???</p>
<p>Hi,</p><p>Thanks for the detailed instructable!</p><p>I was wondering if anyone knows the minimum input pulse length in monostable mode (for catching a <em>very</em> short sensor input pulse in a ballistic chronograph, for example).</p><p>Thanks</p>
<p>For what I have found in the datasheets, the minimum triggertime should be about 25nS. So... quite fast I think ;-)</p>
Thanks!
<p>hi . If i'm gonna change the momentary switch to a electret mic to make it a clap switch circuit. is it possible? what should be change on the connections?</p>
<p>Johanna, you have to amplifie the output of that micro first. Then you can connect the amplified output to the trigger input .</p>
Thank you for this instructable! Question: if I set up the resistors and capacitors right, would I be able to create a 5 minute pulse? Or would I need a microcontroller? Probably a stupid question but I am very new to electronics
<p>You can do that, Brad. Although you have to take care about your capacitators. When using ELCO's, high values give high leakage-currents. So the pulses might not be stable. For very long timing, it is possible to use a 10x divider (or more than one).</p>
<p>Great information, you put a lot of work into this to help people learn useful info.</p>
<p>Very detailed instruction set, Thank You for the explanation, I was struggling with trying to figure out how to get a 10% duty cycle on a &quot;blinker&quot; and this worked great. Again Thank You very much</p>
<p>Thanks sir, very nice and very clear.</p>
<p>Hi!<br>Is there any way to mix the mono and bistable mode? I mean pressing 1 button to turn it on the led, and keep it on untill press the other button (or the same button, this is not a critical issue) then after a time, the led goes off?</p>
<p>In the diagram pin 7 is connected directly to the pot and then to Vcc, but in your breadboard the pot goes to pin 6. Can you explain this discrepancy?</p>
<p>Nice article. One mistake though. You use a non standard layout for your astable circuit diagram. This is very amateurish.<br><br>http://www.555-timer-circuits.com/common-mistakes.html</p>
<p>put the mic at the trigger but light will only stay on for 5 seconds lol, i suppose this circuit is to trigger something for only 5 seconds after that its an off circuit</p>
<p>Thank you so much for the information you provided and used simple words.Can you please tell what is the use of capacitor in this circuit? Can anyon please...............answer?</p>
<p>Well, you could specify a little bit better what circuit you mean, but in general the use of the capacitors in these examples is because they need a very stable Voltage. The capicator insures this, ( just because of how a capacitor is build and where it is placed). Hope this helps.</p>
<p>You have an error in one of your equations. In point 3 you have:<br><br><strong>2/3*Vcc/</strong><strong>(Vcc - V0)</strong><strong> = 1- e^(-t / [(RA+RB)*C])</strong></p><p><strong>1/3*</strong><strong>Vcc/(Vcc - V0)</strong><strong> = e^(-t / [(RA+RB)*C])<br><br></strong>But this is not correct, it just happens to work for V0 = 0. This becomes a much larger problem in point 5 when you reuse the result to determine the amount of time the output is HIGH. <br><br>The way to calculate the time the output is HIGH is to subtract the time it takes to repeat the calculation from point 3, except solve for 1/3*VCC. Then subtract this result from the solution to point 3.<br><br>Your answer coming out correct is actually quite surprising. </p>
<p>does anyone know how to configure the 555 timer as a monostable multivibrator with a delay of about 100-500ms?? thx</p>
<p>T = 1.1 RC select C= 1uf @T= .1 sec R= T/1.1xC,,Do the same with T= .5 sec</p>
<p>This is exactly what i was looking for! I have little to no base in electronics and i'm trying to conceive a led dimmer in order to control a rgb led color. Your article mad me understand how a 555 timer worked and your equations will come in very handy! Thank you!</p>
<p>I'm planning on creating an astable multivibrator with as close to 50% duty cycle as possible. Thank you for the equations!</p>
<p>I followed your Astable guide to make a 200 Hz tone using Ra=2700, Rb=2200, and C=1uF. Was going crazy at first because I had two broken 555 chips in a row and didn't know. I wanted to say that this instructable is such good documentation to have, it is both thorough and informative -- thank you!</p>
<p>thanks for the post... the bistable circuit was exactly what i needed to toggle 2 hall effect sensors (A3144E) when all i had laying around were some 555 chips.</p>
<p>Im working on a model train layout, what i want is for a kid to press a button the train goes for 2 <strong>min</strong> then stops. the child does this 4 more times. after the train stops for the 5th time it activates a cool down clock for 10 min. it would have to work with electricity from an out lit. Any ideas? </p>
<p>I want this circuit, but I have 741 opamps in stead of the momentary switches. Can I connect the 741 output to the bottom of the pull-up resistors to set or reset the 555? or is some sort of buffering or isolation necessary between the 741 and the 555?</p>
question looking a building a 555 monostable mode can I use a switch that when trigger high, it will delay off after x seconds and stay off without the trigger input going back to low?
<p>Nice! Well written.</p>
<p>Hey everyone, I need to run a motor for a 3 second pulse, but only once, not a continuous on-off situation. and i need it to run in both directions. Is it possible to use this circuit for that or will i need something else? </p>
<p>sounds like you will need a microcontroller</p>
<p>Hi.. I have tried to make the monostable mode of 555 timer exactly as it is shown here for 5.17s pulse width. But i m not getting the output.The pulse becomes high when the push button is on but it comes to low only when the push button is switched off. I have checked the circuit connections numerous times but there is no error. Can you please tell me what might be the problem?</p>
<p>i had the same problem on my first trial,try changing your breadboard,and see it is works. :)</p>
<p>Thank you :)</p>
hi,pls am new in d electronic world but understand the basis.how can i increase or amplifer dc 2v to 12v dc to charge 12v battery.pls state all d component require and the diagram to build dis circult,and i want the 12v dc to constant. <br>thanks
For astable mode,when I added a load to output, the frequency increase. But it seems like the frequency is independent of load. Anyway to overcome it?
I'm not sure why that's happening, try putting a buffer between the load and the 555, use an op amp or transistor wired up like this: <br />http://en.wikipedia.org/wiki/Buffer_amplifier <br />
Thanks a million :)
Is there a way to adjust the low pulse so it's not 0?
nope sorry, these are digital circuits so 0 and 5V is all you can get. Do you have something specific in mind that you'd like to do?
I was looking for a way to make a pulse feature for my TIG welder. It's an inexpensive one and does not have a feature that helps control the amount of heat input to a weld. For my welder, it takes an input between 0 and 10v and, based on your amp settings, applies a percentage of the current to the weld. I'm looking for a way to take the voltage input and pulse it with the ability to adjust frequency, duty cycle and background (what % of the max voltage the low should be). I thought I might be able to use an op amp on the output of an arduino, but I am not well versed in electronic circuitry.
you can do this, but you will need to learn about biasing (also called dc offset) and amplification
plz, if i want to find the water level by using 2 plates capacitor, then i will connect the output of the timer to a f/v converter and display the voltage <br>which capacitor i should change with the 2 plates??
do you have any idea how to install into a XBOX360 Controller? can it be a two or three wire hookup?
no idea, can you give me any more info? what are you trying to do?

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Bio: I'm a grad student at the Center for Bits and Atoms at MIT Media Lab. Before that I worked at Instructables, writing code for ... More »
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