555 Timer Basics




About: I've always loved to figure out how things work, so hacking and making just fits for me. I'm a husband, a father, an EOD technician with the WA Army National Guard, an automation engineer at Schweitzer Engin...

The 555 timer IC is without doubt one of the most important and widely used single ICs in history. The design has remained unchanged for over 40 years, which makes it one of the longest running IC designs. It's been used in everything from toys to spacecraft.

The 555 was originally designed in 1971 by Hans Camenzind, a Swiss electronics engineer employed by Signetics in California. Camenzind spent months working on the final design, building several different test iterations by hand on a breadboard with discrete components. When the design was finalized, Camenzind sat at a drafting table and used a razor to cut the circuit design into a sheet of plastic. In total 23 BJTs, 15 resistors, and 2 diodes were cut into the plastic. This was then reduced to produce the etching mask for etching onto the silicon wafers. That kind of beginning-to-end design work by one man is now done by large teams of engineers with complex design, simulation, routing, and etching software to handle the difficult task of modern IC design. (For a good read on how Camenzind designed the 555, check out an article at semiconductormuseum.com)

The 555 is ridiculously simple to use, dead reliable in an extremely wide array of applications, and remarkably robust in what it can handle and do. And in all of it's applications, everything comes down to one of it's three main operation modes: monostable or "one-shot", astable or oscillator, and bistable or flip-flop. EVERY circuit out there that uses a 555, and there are countless iterations, comes down to one of these three modes.

To be clear, I'm not going to go into lots of neat 555 circuits, though I will include a short list of references where I've found some pretty cool ideas. My goal is to only go over the basics and what the internals are doing in each mode, not show you how to flash an LED or build a siren synthesizer. With that knowledge, it then becomes easier to design your own 555 circuits to do what you want instead of having to scour Google search results and decipher somebody else's thoughts.

Step 1: You'll Need Some Parts...

As mentioned, the circuits won't be complex, but will require a number of various components, all of which are easily available.

You will need:
  • 555 timers. Don't worry about the manufacturer or all the letters included in the name, like NE, LM, NA, SE, or SA, just look for the 555. Cheap ones work as well as or better than the more expensive ones will. Also, you will want to use the more common bi-polar type, which are made with BJTs instead of FETs. CMOS chips made with FETs are functionally identical, but are much easier to damage with electro-static discharge and generally cannot output as much current as the BJT type. The most reliable way to distinguish between BJT and FET style 555s is to check the datasheet, like this one for the LM555 BJT type, or this one for the TLC555 FET CMOS type, both from Texas Instruments. This info is usually found on the first page in the bullet point description or the first couple of paragraphs.
  • various resistors (including potentiometers) and capacitors. Nominal values will be given as needed.
  • other various components: LEDs, a buzzer, an 8Ω speaker, switches, jumper wires, batteries and battery holders (or some other power supply), breadboard, etc.

There are a lot of companies making 555 ICs today, but the core design remains unchanged from Camnzind's original 40 years ago, so it's not like one 555 is significantly better than any other. (But everybody has their favorite. Frankly, I don't care. They will all work for what we are doing here.) Below is an image of the 8-pin DIP that is the most common package type for the 555, though others are available. Next to it is an image of the pin assignments and the internal blocks to which the pins are connected. For a good look at the full internal schematic, check out this image.

8-pin DIPPin assignments and
internal function blocks

Step 2: The One-Shot or Monostable Mode

The first mode is called the one-shot, or monostable, because pin 3 (the output) will go high for as long as you want, but only one time. When the timer runs out, the output resets to low and waits for another trigger event to start again, stabilizing in only one state (off). A good example of this concept is a motion sensing light.

First let's look at the circuit schematic below, and then we can decipher what is going on later.

Push SW1 and the LED lights for a short time, then goes off. The time it stays on is found by multiplying the values of R4 and C2 and is expressed in seconds. The time is not exact, and as either value gets very large or very small, the error increases. A potentiometer of a similar magnitude to but in place of R4 will give you better control over the time. In the end, the only way to know the exact time is to actually time it with a clock.

Larger values for R4 and C2 will increase the time the LED stays on. Why? Well let's take a closer look at what is going on. (Now would be a good time to review the function block diagram from the previous step). Before we press SW1, output pin 3 is low and R3 pulls the signal at pin 2 (trigger) high, so the LED is off and stays that way. We press SW1 and it shorts the signal from pin 2 to GND, which triggers the comparator inside. If the voltage at pin 2 is less than 1/3 of the source voltage, the comparator activates the flip-flop which drives output pin 3 high. Since our source is +9V, we only need pin 2 to sense less than +3V, so the 0V at GND is more than enough. So now our LED is lit. Now what?

Capacitor C2 is initially empty before we press SW1 because it is connected to the discharge pin 7, which essentially connects C2 directly to ground internally and drains it. When we press SW1 and the flip-flop is triggered, the internal connection to discharge pin 7 is cut and C2 is allowed to charge through R4. This is where we get our timer. If the container (large C2) is big or the flow we use to fill it is small (large R4), it takes longer to fill C2. When the voltage across C2 reaches 2/3 the source voltage (so +6V here), the second comparator connected to threshold pin 6 is triggered, switching the flip-flop back to the original state, shutting everything off. C2 is again connected internally to discharge pin 7 and drains back to 0V, ready for the next trigger.

At any time between pressing SW1 and C2 reaching 2/3 source voltage, if we press SW2, we short the connection to reset pin 4, which until now has been forced high because of R2. The reset pin does exactly that, effectively switching the flip-flop back to the original state, turning off the LED and draining C2.

Step 3: The Flip-Flop or Bistable Mode

A flip-flop is like a switch, holding it's state indefinitely until something forces it to change. This is critical in digital logic and computing, but here we'll keep it simple and just use it like a switch. We now have two completely stable states that will not change on their own, hence bistable mode.

Look at the schematic below and then we'll discuss what's going on.

Press SW1 and the LED lights up. R2 forces trigger pin 2 high until the button is pressed, preventing the circuit from activating. Once SW1 is pressed, the internal flip-flop turns on the output pin 3 and waits for the signal from the second comparator to say that threshold pin 6 has reached 2/3 the source voltage. But we connected pin 6 to GND, so the comparator will never trip, keeping the circuit on and stable indefinitely. The only way to stop it is to press SW2, which connects reset pin 4 to GND, which until now has been forced high because of R3. This forces the flip-flop to revert back to it's original state, turning off the LED. The circuit is once again stable in this state, waiting for the user to press SW1.

Step 4: The Oscillator or Astable Mode

We've seen the 555 have a single stable output state (monostable mode) and two stable outputs (bistable mode). The last option for this IC is to have neither state be stable, or astable mode. The output continually switches back and forth between the two states at constant rate, which is just an oscillator or frequency generator. This rate is fully adjustable, and is very reliable. Let's see what that looks like.

It's hard to tell exactly which state the 555 will start in, either high or low output, but let's assume the capacitor starts discharged and output pin 3 is high. Trigger pin 2 is tied directly to threshold pin 6, so we can already tell that the internal flip-flop will switch back and forth as the voltage on the capacitor rises and falls. The rate of that flipping and flopping is determined by both R1 and R2.

We started with threshold pin 6 low, so output pin 3 is high while C2 charges through both R1 and R2 until it reaches 2/3 source voltage. That triggers the internal flip-slop, driving output pin 3 low. Pin 3 is then low while C2 then discharges through R2 and discharge pin 7. Once C2 reaches 1/3 source voltage, the internal flip-flop drives output pin 3 high, and C2 charges through R1 and R2 again, starting all over. This constantly changing high/low state on output pin 3 can be heard as a tone on a small 8Ω speaker. You can also use the output as a pulse width modulated signal, driving small motors with variable speed control. Remember to use a transistor between the output pin 3 and the load if the load requires a larger current. The 555 is pretty robust, but the LM555 from TI can only output 200mA, so be sure to protect it.

Changing the value of R1 will adjust the recharge time of C2, but the discharge time will be the same, so the width as well as the frequency of the pulse will be affected. Changing R2 will affect both charge and discharge, so only the frequency changes. A couple of adjustable pots work very nicely here. Changing C2 will change the frequency as well.

Step 5: So Now What?

There is a plethora of 555 circuits out there. A simple unfiltered Google search can provide hours of fun. Hans Camenzind stated in an interview that he was still surprised, 40 years later, with the varied applications people had come up with to use the 555.

If you just want a reference to a great site, check out Colin Mitchell's website talkingelectronics.com. Start with this page if you want another (and probably better and more thorough) explanation of how the 555 works. Otherwise go here for a list of circuits. Truthfully, the entire site is a treasure trove of knowledge and just plain awesomeness. Be sure to come up for food, water, and/or air every now and then.

Charles Platt in Make: Electronics has a great reaction timer that uses all three 555 modes (pg 170). One 555 is set up with a start switch as a one-shot to provide a delay to start the circuit. (Looking back at the schematic for the one-shot, R4 charges C2, which is what gives us our delay. Replacing R4 with a potentiometer will allow for a variable delay.) That triggers the next 555, set up as a flip-flop, to output. The flip-flop flops high, enabling the 7-segment numeric display to start showing the count. Once the user sees the count start, a stop button is provided to flip the second 555 output back to low, disabling the count and allowing the user to see the number they landed on. A third 555 is set up in astable mode to continuously run a counter. That counter drives the numeric display and has a reset button to zero said display. If it sounds complicated, it is but to be fair, it's also a bit difficult to explain in words. But it's also a great circuit to see the 555 hard at work. It is also not the only circuit in which he utilizes the 555 in some way or another.

Forrest M. Mims III in his Basic Electronics I workbook has some really good basic 555 circuits, like a simple keyboard tone generator, a voltage controlled oscillator, a siren synthesizer, and a frequency meter.

Ultimately, the limits of the 555 are only matched by your imagination. There aren't many circuits where you can't use a 555 in some type of application.

Thanks for reading. Please don't hesitate to ask questions in the comments below. You never know when someone else has the same question and that way we can all learn and help each other get better. Have fun building!

Also, please check out the Digilent blog where I contribute from time to time.



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    16 Discussions


    1 year ago

    In all three circuits, what is the role of C1. What would happen if it take it out or vary in size? Thanks a bunch

    1 reply

    Reply 1 year ago

    C1 is a filtering cap on the power supply. It's a universal component in most electronic circuits to filter out small but rapid fluctuations in the voltage source. Depending on the stability of your source, you can remove it, but it's good practice to always use one. The value is dependent on the power requirement of your circuit.

    The idea is that if the input voltage from the source drops, the capacitor acts like a short term backup battery and tries to maintain the input voltage until either the source recovers or the cap runs out. Larger caps will take longer to drain, but also take longer to fill. If your circuit requires a fair bit of power, a small cap may not be able to supply enough electrons to properly filter the source fluctuations since it would drain too quickly.


    2 years ago

    In oscillator mode, could you please explain to me where your resistor and cap values come from? i only see you mention C3 (unless i missed the others), so does this mean that the other values are standard and you will only manipulate C3?

    1 reply

    Reply 2 years ago

    I apologize for the confusion. The text should have referred to C2 and not C3. I have now fixed that. The values for C1, C3, and C4 as shown in the schematic are default values that I almost always use when I connect a 555, but none of them have any effect on the 555's performance as an oscillator.

    C2, R1, and R2 are the components that directly effect the oscillation frequency and duty cycle. Those values are chosen based on your requirements for your circuit. The nominal values as shown were chosen at random for this particular project, but will produce a 3.1 kHz signal at 66.67% duty cycle. You can do the math the long way or use a calculator like this one.

    I hope that explains it better.


    3 years ago

    Thanks for explaining it so simply.


    Thanks for posting this. I have never taken a formal electronics class and I have several 555's in a box of components that I picked up. Now I got an idea what to do with them.

    4 replies

    Reply 4 years ago

    Any pointers on what things to look for that I may harvest 55 from? I like recycling as much as I can.


    Reply 4 years ago on Introduction

    I got mine from craigslist a guy dropped out of tech schol and was selling his electronics tool bag inside was a box of of chips and resistors and other components.


    Reply 4 years ago on Introduction

    Truth be told, as ubiquitous as they are and as many parts as I've salvaged stuff from, I can't remember ever salvaging a 555. It's possible that they are embedded as part of the core IC since ICs can be made so much smaller than in years past and the circuit is pretty small. Small, simple toys and such don't need much computing power so there is probably room on the die. But that's total speculation. Sorry I couldn't help.


    Reply 4 years ago

    Stupid spell checker. 555s


    Reply 4 years ago on Introduction

    That was my goal. Just keeping it basic, nothing fancy. I usually find projects for the 555 but not a lot of the "how" explanation. This was more to help me than anything else!


    4 years ago

    I love how all of your instructables have the same "flavor". And as always, so detailed!

    1 reply