Introduction: Build Your Own 555 Timer
The 555 timer. A chip so versatile that it has been used in everything from toys to spacecraft. A chip that can act as an oscillator, a schmitt trigger, PWM driver, a siren/alarm, a light or dark detector, and much much more. It is the most popular IC of all time having been around since 1971 and now selling over 1 billion annually.
This instructable will show you how to build your own 555 timer using only transistors and resistors, no ICs!
Why build this?
Good question. There are a few different reasons:
1. To Learn:
Learning may be a scary thought to some people, but this project has taught me a lot about comparators and analog circuits as well as a lot of the basics of the 555 timer. The 555 timer combines both digital and analog circuitry and while digital circuitry is taking over, analog is still important.
2. To Understand:
The 555 timer is a very versatile and useful chip. That's why it is the number 1 most produced chip. It is used very often so it is important to understand how the thing works. Now, you can read about it or even see a simulation, but nothing it quite as good as actually making it yourself.
3. It's Fun:
If you like working with electronics, especially breadboarded electronics, this should be a fun little project. You will break the black plastic barrier that stands between you and your integrated circuits and see the circuit in all its glory (well almost, making your own transistor could be difficult)!
Now that I've hopefully convinced you somewhere within that intro, lets get started!
Step 1: 555 Internals
So what the heck is going on inside the 555 timer? Well here are a couple of schematics from the National Semiconductor datasheet to help explain it.
In the first picture we can see that there are two comparators, one on the trigger pin and one on the threshold pin. We can also see that they are connected to a voltage divider. One input of the Threshold comparator is at 2/3 Vcc and one input of the Trigger comparator is at 1/3 Vcc. The outputs of the comparators lead to a flip flop. Although it doesn't say on the image, the flip flop happens to be a SR flip flop. From the flip flop there is the output stage which leads to the output pin and the transistor that controls the discharge.These are the basic parts of the 555 timer.
This is the basic theory of operation:
When the trigger voltage goes below 1/3 Vcc (its reference voltage) the comparator Sets the flip flop, which pulls the output high and turns off the discharge. When the threshold swings higher than 2/3 Vcc (its reference voltage) the comparator Resets the flip flop, which pulls the output low and turns the discharge on. This basic operation allows the 555 timer to operate in various ways with various configurations.
I don't want to get into all of the ins and outs of how the 555 timer works, so if you know, great! If you don't know here is a good tutorial with lots of theory and operation information. It is my personal favorite.
If we look at the schematic diagram in the datasheet (second image), we can see what is actually happening inside the chip. The comparators are differential amplifiers, or long tailed pairs with a few added components to increase gain and sensitivity. The differential amplifier is the basis of the comparator, it greatly amplifies the difference in voltage to the point where millivolt differences result in rail to rail swings (voltage swings between 0v and Vcc). What's interesting here is that the threshold comparator uses NPN transistors whereas the trigger comparator uses PNP transistors. I don't know if that has an effect on the operation, but I just kept them like that in my circuit. The threshold comparator also has several extra transistors not present in the trigger comparator, along with a slightly different configuration. They perform the same function though, so I just replicated the trigger comparator but using NPNs instead.
The flip flop circuit is rather interesting. There is a lot going on for what is just a SR flip flop with a second force reset. That can be made with 3 transistors so I discarded that circuit and made my own.
The output driver is fairly simple. It is composed of two transistors, with the signal to one inverted so that when one is on, the other is off. This allows the output to operate in push-pull mode. This means that the output can source current, the output is shorted to Vcc, when it is high and sink current, the output is shorted to ground, when it is low.
Step 2: My Circuit
When designing my circuit, my goal was functional simplicity. I designed it to be simple, yet still be able to provide all the functions of the actual chip.
The comparators are based on the trigger comparator from the actual timer schematic. I started with much simpler comparators, but they did not give me the precision I had hoped to get. Even after adding more transistors to increase sensitivity, the comparators were just not working out. The comparators in the actual timer have a few extra features that allow them to be much more accurate and precise than what I had designed. I have more information about the comparators in a later step.
The voltage divider is very straight forward, it just takes three 4.7K ohm resistors. Short of having 5K ohm resistors, I used 4.7K instead. You may want to select resistors that have close values for the voltage divider so that the 1/3 Vcc and 2/3 Vcc are as close as possible. For example, a 4465 ohm (-5% tolerance) a 4700 ohm and a 4935 ohm (+5%) would be a bad selection for the divider, but 4600, 4650, 4570 would be a good selection. The values don't have to be close to 4700, just close to each other.
I have created Set-Reset flip flops before but I am using a simpler circuit here that requires only two transistors. It is called a bistable multivibrator which means it has two stable outputs. The circuit is similar to a lot of the little two transistor LED flashers.
My output stage is different from the one in the schematic. It is a lot simpler, just a PNP and NPN transistor set up in complementary mode. It still provides all of the functionality of the original output stage, in fact because you have a choice as to what transistors to use, you could replace the small signal transistors with large power transistors. That way you aren't limited to 200 mA, you could potentially drive a few amps!
Step 3: What You Will Need
- 2 1k ohm resistors
- 2 10k ohm resistors
- 11 4k7 ohm resistors (4k7 means 4.7k)
- 11 2n3904 NPN transistors or similar
- 9 2n3906 PNP transistors or similar
*You will also need a large bypass capacitor (100µf) on the supply rails when testing and running the timer.
If you plan on making this a permanent you will also need a circuit board to solder everything to.
Step 4: Building the Trigger Comparator
We are going to start with the trigger comparator. For this step, you will need 4 PNP transistors, 3 NPN transistors, a 10k ohm resistor and a 4k7 ohm resistor.
You can build the comparator how you want just by following the schematic, but to make it easier if you don't, here are lots of pictures to help.
First, put two PNP transistors on the breadboard so that the left one has the rounded edge facing you and the right has the flat edge facing you. Have their emitters (the two innermost leads) in the same row so that they are connected. Also in this row, add the 10k ohm resistor and connect it to Vcc.
Next, add two more PNP transistors on the outsides of the first two. They should have the same orientation. Do NOT connect them to the first two!
After that, add jumpers between the emitters of the outer transistors to the bases of the inner transistors. Also add wires from the collectors of the outer transistors to Ground (GND).
Then spin the board around and add two NPN transistors exactly how you did it for the first two transistors. The should be in the same rows as the middle two PNPs, but on the other side of the board so they are isolated.
Next, add a jumper that connects the collector and base together on the right transistor. It is important to remember which transistor it is because that determines what input is the reference. Then add a jumper between Ground and the emitters, they are the two connected together.
Next, add two jumpers across the middle divider from the outside leads of the NPN transistors to the outside leads of the middle PNPs. The picture may be helpful here. Then connect the bases (middle lead) of the NPN transistors together.
After that, add the third NPN transistor to the left of the first two. It must be opposite of the transistor with its base and collector connected together. It should have it's flat side facing you and not be connected to the other two in any way.
Then add a wire from the leftmost lead, the emitter, to Ground. Also add a jumper between the base of the third transistor and the leftmost wire of the middle transistor. Finally add the 4k7 ohm resistor between Vcc and the collector of the third transistor, the rightmost lead.
Finally add the inputs. For this instructable, a yellow wire will indicate an input and blue will indicate an output. The inputs should be on the base, middle lead, of the outer PNP transistors. The output on the side of the third NPN transistor will be the reference input and the other will be the trigger lead input. The output should be connected to the collector of the third transistor, the same lead
as the 4k7 ohm resistor. The reference input is on the opposite side of the NPN with its collector and base connected in the schematic. That applies for real too.
You have now completed the trigger comparator! Now to test it...
Step 5: Testing the Trigger Comparator
I always hate when I wire up a big, complex circuit and find that it doesn't work because of something silly like a missing or misplaced wire, which is very hard to track down. Because of that, I like to test all of the smaller parts of a big build.
Testing is simple. The first step is to add power to the board, anywhere from 3V to 12V should be fine, but lower voltages are more forgiving. Add power and cut it after a second or two. Feel the transistors and make sure none are warm or hot. If they are, you have a short or wrong connection somewhere. Next, make a simple voltage divider with 2 resistors and connect that to the reference input. Add some sort of indicator to the output, I used an LED. To make it light up, I had to switch out the 4k7 ohm resistor for a 470 ohm resistor. If you do so, remember to put the 4k7 ohm resistor back in! Now add power and connect the trigger input to ground. The LED should light up. When you connect it to Vcc, the LED should go dark. If nothing happens, there is an issue somewhere. If the LED lights up when the input is high, connected to Vcc, and goes dark when the input is low, switch the inputs around.
If everything works, you can move on to the Threshold comparator.
Step 6: Building the Threshold Comparator
The construction of the threshold comparator is very similar to the construction of the trigger comparator. You will need 4 NPN transistors, 3 PNP transistors, and a 10k ohm resistor.
To start, place two NPN transistors on the breadboard so that the left one has the rounded edge facing you and the right has the flat edge facing you. Have their emitters (the two innermost leads) in the same row so that they are connected. Then add the 10k ohm resistor to this row and connect it to Ground.
Add two more NPN transistors to the outside of the first two. They should be of the same orientation but not be connected. Add jumpers from the outermost leads, the collectors, of these transistors to Vcc. Then add jumpers from the bases of the middle transistors to the emitters of the outer transistors.
Next, spin the board 180 degrees and add two PNP transistors exactly how you added the first two transistors. It helps to have them on the same rows for wiring purposes.
Add a wire from the emitters, which are connected together, to Vcc. Also add a wire between the collector and base of the left one. Once again keep track of which transistor it is because that determines the reference and threshold inputs.
Next, add two wires across the middle divider from the outside leads of the PNPs to the outside leads of the middle NPNs. The picture may again be helpful here. Then connect the bases (middle lead) of the PNP transistors together.
Add the third PNP transistor to the left of the first two but have its flat edge facing. Then add a wire from its emitter, the leftmost lead, to Vcc. Also add a wire from its base to the collector of the middle transistor.
Finally its time to add the inputs and output. The output goes on the collector of the third PNP transistor. This lead shouldn't have anything connected to it yet. The inputs are connected to the bases of the outer NPNs. In the schematic, the side with the PNP with its base and collector wired together is the reference side. This is also true in the actual circuit.
Now you have the threshold comparator too!
Step 7: Testing the Threshold Comparator
Testing the threshold comparator is similar to comparing the trigger comparator. First apply power for a second or two to make sure nothing it getting hot. Then make a voltage divider from two resistors and connect that to the reference input. Add an LED or other indicator to the output. Keep in mind that there is direct connection to Vcc so you will need a resistor if you use an LED. Turn power on. When the threshold input is tied to Vcc, the LED should light up. When it is tied to ground, the LED should be off. If the reverse happens, swap the inputs.
If everything works, then congrats, you now have working trigger and threshold comparators! If not, check your wiring!
Step 8: More on the Comparators
A large amount of my time and research was spent on the comparators. My goal was to research some about them then design my own circuit. Eventually that failed and I ended up using a tweaked version of what is shown in the datasheet schematic. Throughout the process I have learned a lot about the theory of how these circuits work so I want to share some of that with you.
I have already said that the heart of these comparators is the differential amplifier, specifically the long tailed pair. What the heck is that? I also mentioned that a diff. amp. amplifies the difference between two voltages. The long tailed pair is a common type of diff. amp. that gets its name from the resistor or other current limiter that tails off of two transistors (or vacuum tubes). The first image shows a basic long tailed pair made using 2 transistors and 3 resistors. The tail resistor connects to the emitters of the transistors; it should be larger than the two resistors that connect the collectors to Vcc. Those two resistors should be equal. Here is a (very) basic explanation:
When both inputs are at the exact same voltage, let's say 1/2 Vcc, the transistors are "on" the same amount so equal current flows through them and into the tail resistor. There is a voltage drop on both collectors but the difference of voltage between the two is 0. When the voltage of one input rises some, that transistor becomes "more on" so more current can flow through, raising the voltage at the emitters and dropping the voltage of the collector more. This rise in emitter voltage creates "back flow" into the other transistor which turns it off more. When that transistor turns off, less current flows through it so the voltage at its collector rises. So now the difference between the two collector voltages is very large, due to a small change in input voltage.
Like I said, basic. There are a few issues with the circuit: 1. Small value differences in the collector resistors unbalances the output. 2. The output is a difference of two signals, we need a single output signal. 3. When the input voltages ave very close, the output voltage difference becomes very small. How do we solve these issues? Well, there are two easy ways to improve the design. One amplifies the input and the other amplifies the output, while also solving the other issues.
Let's start with amplifying the output. Enter the current mirror. In the second image you can see the basic current mirror consists of 2 PNP transistors, one of which has its collector and base connected, a resistor tying that transistor's collector to ground, and an output from the other transistor's collector. The current mirror limits the current of the left side (in this image) to the current that flow through the right side. The transistor with its base and collector tied together acts as the control side. When the resistor drops the voltage toward ground, the transistor starts to turn on because the voltage of its base is being lowered. Then current starts to flow through the transistor which raises the base voltage until there is a balance between the amount of current flowing through the transistor and resistor. The bases of the two transistors are connected so the second transistor also turns on but only to the amount that the first one is at. Thus, the current of the second transistor is limited to the current flowing through the control transistor. How does this help the diff. amp?
First off, it helps to balance the collector current of the transistors because the current of one side is actively limited by the current of the other side. Secondly, it gives one single ended output instead of a differential output. Because one side is being used to control the current, only the other side can be used for an output. Finally, it amplifies the output. Whenever there is a difference between the inputs, one side of the diff. amp. allows more current to flow and the other side allows less. If the side that allows more current is the side that the mirror control transistor is on, then the current mirror allows more current on both side. The excess of current on the other side raises the voltage. When the opposite happens and the side of the diff. amp. that allows more current is on the mirror's load side, the control side allows very little current so there is a voltage drop on the load side. The amount that the voltage varies is much greater than before and now its only on one output.
Now the diff. amp. works much better; it is balanced much better, it has only one output, and the output has been amplified some. The first two issues have been solved, but the third hasn't been solved fully, yet. The diff. amp. with a current mirror makes a good differential stage for an OP amplifier, but it still isn't optimal for a comparator. When the input voltages become close, the output still doesn't have a sharp transition. The way to improve that is to amplify the input.
If you take a look at the third image, you can see a diagram of the Darlington transistor, or sometimes Darlington pair when two discrete transistors are used. Invented by Sydney Darlington in 1953, the transistor is designed to create very high gain by using two transistors. A signal applied to the input gets amplified by the first transistor. The current flows through the transistor and out of its emitter. The current then goes into the base of the second transistor, which amplifies it further. The result is a gain approximately equal to the gain of the first transistor multiplied by the gain of the second. If we replace the two transistors of the long tailed pair with Darlington pairs, we can increase the amplification of the diff. amp. greatly because each input will be amplified by two transistors instead of just one.
If you look at figure 4, you can see the long tailed pair with a current mirror and Darlington transistors. This circuit is very good as a comparator because it has very high gain from the current mirror and Darlington transistors which allows the inputs to be extremely close together without the output "dropping off." The circuit is also very balanced and has a single output because of the current mirror.
This is a very well designed circuit and has taught me a lot. Hopefully you have learned some too (if you didn't already know or understand it).
Step 9: Building the Flip Flop
The next step is to make the flip flop. The build is tricky in that there is a lot of resistors to keep track of but it isn't to difficult provided you look at the images and I write good instructions! For the flip flop, you will need 2 NPN transistors, 2 1k ohm resistors, and 5 4k7 ohm resistors. For reference, the Emitter is the outer lead, the base is the middle lead, and the collector is the inner lead for both transistors.
Start by putting the transistors on the board. The left should have its flat edge facing you and the right should have its rounded edge facing you. Have a gap of 2-3 rows in between them, this keeps things from getting crowded. Then add jumpers from their emitters to ground.
Add a 4k7 ohm resistor from the base of one transistor to the collector of the second. Then add a 4k7 resistor from the base of the second transistor to the collector of the first.
Now on the left transistor, add a 4k7 ohm resistor to the base and have its other lead go out to the left. This will be the set input. On the right transistor, add 2 4k7 ohm resistors to its base. Their other leads should not be connected together. These will be the reset inputs.
Then add a 1k ohm resistor from the collector of a transistor to Vcc. Then repeat for the second transistor.
Now its time to add the input and output leads. The outputs are connected to the collectors of the transistors. The left output is the inverting output. It goes high with a Reset signal. The right output is the non-inverting output and it goes high with a Set signal. The inputs are connected to the three resistors with only one connection. There should be one on the left side and two on the right.
That is the completed SR flip flop. If you want to test it go to the next step, if not skip to step ten.
Step 10: Testing the Flip Flop
We are going to test the flip flop now. The flip flop is easy to test, first add an LED or other signaling or measurement device to each of the outputs. What to look for when testing the flip flop is that when power is first applied, one of the LEDs should light up. If both or neither light up that isn't good. Once passed that, if you connect the SET input to Vcc the currently lit LED will stay lit or the other will light, depending on which state the flip flop was in when power was applied. If you remove the input from Vcc, the LED should stay lit. Then connect one of the RESET inputs to Vcc. The other LED should light up. If you remove the input from Vcc, it should still stay lit. If all of this works properly for you, then you have successfully made the SR flip flop.
Step 11: The Comparators and the Flip Flop
By now, the board is getting cluttered with a bunch of input and output wires. Let's clean things up a bit by connecting the comparators to the flip flop.
Remember which is the threshold comparator and which is the trigger comparator? Good. (neither do I!) If we refer back to the schematic, we see that the threshold output connects to the reset of the flip flop. So find the output of the threshold comparator, MAKE SURE THAT IT IS THE THRESHOLD COMPARATOR!!! If you mix up the comparators, the timer will not work! Now connect the output to the reset of the flip flop. The reset side had the two inputs, choose one, it doesn't matter because they both do the same thing. To keep clutter down and to make it easier to see, I've replaced the blue and yellow wires for a green wire to identify a connection between two parts of the circuit. (Blue + yellow = green)
Now, find the output of the trigger comparator, the one you didn't use. Also find the SET input of the flip flop, this is the side with only one input and also the opposite side that you just connected the threshold to. Connect the output of the trigger comparator to the SET input.
Now that we've tied up some loose *wire* ends, we can move on to the last of the building: the little stuff!
Step 12: The Other Stuff
The other stuff includes the reset input, the discharge pin, the output driver, and the voltage divider with the control voltage pin.
We will start off with the voltage divider. For it you will need the three 4k7s that you specifically chose for it. If there is open space between the comparators, you can build it there but if there isn't space, just pick a spot with access to both comparators' reference. Start by putting a resistor on and connect one lead to Vcc. Then place another a few rows away with a lead connected to ground. Finally take the third resistor and connect it to the other two resistors. Instead of putting output wires on it, let's just connect it to the comparators now. Find the reference inputs of the comparators. It is very important that you use the reference inputs because using the wrong input will keep the timer from functioning properly. The threshold reference connects to the resistor tied to Vcc and the Trigger reference connects to the resistor tied to ground. Also on the resistor tied to Vcc, add a wire for the Control Voltage input.
Next, let's make the output driver. For it you will need an NPN and a PNP transistor. Start by placing the NPN transistor on the board with its rounded side facing you. Then put the PNP to the right of it with its flat edge facing you. The inner leads (the emitters) should be connected. Add a wire from the outer lead of the NPN transistor to Vcc and add a wire from the outer lead of the PNP transistor to ground. Finally add a wire that connects the bases of the transistors together. The input wire to the driver is connected to the bases of the transistors and the output wire connects to the emitters, the two inner leads.
After that is the reset transistor. The external reset pin of the timer connects to this transistor which controls the flip flop. You will need a PNP transistor and a 1k ohm resistor. Find the resistor of the second RESET input to the flip flop. Replace the wire with a PNP transistor. The collector should connect to the resistor so if the resistor is to the left, the rounded edge of the transistor should face you. If it's on the right, the flat edge should face you. Then add a wire from the emitter to Vcc and a 1k ohm resistor to the base. The other lead of the resistor will be the reset input so add an input wire to it.
The last thing to make is the discharge input. You will need an NPN transistor and a 4k7 ohm resistor. Start by placing the transistor on the board so that its rounded edge faces you. Then add a wire from the emitter, right lead, to ground. Add a 4k7 ohm resistor to the base, the middle lead. Finally add a wire to the collector of the transistor, the lead with nothing connected to it, this is the discharge. Also add a wire to the unconnected lead of the resistor. The flip flop controls the discharge transistor so that input will connect to the flip flop.
Now that the other stuff is done and over with, its time to wire all of it up to the rest of the circuit.
Step 13: Putting Everything Together
The last thing to do before testing the timer is to make the last couple of connections and gather all of the inputs and the output together.
Since the reset transistor and the voltage divider were already connected to the RESET input and comparator references, respectively, we only need to connect the discharge transistor and the output driver to the flip flop. The control input of the discharge connects to the RESET output of the flip flop. The output is on the side of the SET input, NOT the RESET inputs! The input of the output driver connects to the SET output which is on the RESET inputs' side.
After that, its time to gather all of the wires that are the pins of the IC. These include: Trigger, Threshold, Control Voltage, Discharge, Output, Reset, Vcc, and Ground. In my opinion, the easiest way to align all of the pins is by the pinout of the IC. This way, you can make the test circuit exactly how you would if you were using the chip.
Step 14: Try It Out
Now its time to make a circuit to test the timer. My favorite circuit to use it the astable oscillator. It turns the timer into an oscillator that can blink LEDs or make sounds. You will need 2 resistors 1k ohms or larger, a capacitor and some sort of output or measurement device. I used an LED and a little piezoelectric speaker. With the speaker I also used a potentiometer to vary the frequency. REMEMBER TO USE A BYPASS CAPACITOR BETWEEN VCC AND GROUND! Like the actual IC, there is a large current spike when the output changes state due to both transistors being on for a brief period during the transition.
When trying for the first time, assume there is going to be a short circuit, so shut it off after a second or two and make sure nothing is getting hot. If the LED starts blinking, the speaker makes sound, or whatever would appear on your measurement device, that is a very, very good sign! It means that the timer works and you can turn it on without worry.
There is always the good chance that it won't work, in fact the second time I built the circuit (for the Instructable) it didn't work. I tore apart all of the different parts and saw that they all worked fine and I found out that when switching out a yellow wire for a green one, I misplaced the green one! It is OK if it doesn't work the first time, consider yourself lucky if you don't release any blue smoke like I did when building the output driver for the first time; both transistors died! If you do release blue smoke, find the blown transistor and figure out if you have it facing the right way. If you had it facing the wrong way, replace it with a new one this time placing correctly. If you had it correct, check all of the connections to and from the transistor and make sure they are right.
If nothing gets hot and no blue smoke was release but the darned thing just won't work, start checking connections. Start with the last connections and wires you placed and work backwards from there. Also check transistor placement. A wrong facing transistor usually doesn't result in a blown transistor just an incorrectly functioning one. Since you know that the comparators and the flip flop work - that is if you tested them - you can skip checking them, but do check the connections between them. Anything that you have built without testing should be checked.
By going back through all of your connections, you should be able to track down the bug and correct it to get a working timer.
Step 15: Going From Breadboard to Circuit Board
The next task, should you choose to accept it, is to make the timer permanent by putting onto a PC board. The easiest way to do so is to keep the circuit on the breadboard and build a second one on the PC board. This requires you to have double of everything but is by far the easiest method because you have a concrete reference to look at when placing components. If you don't have enough parts to make a second one, take them off the breadboard one or two at a time and place them onto the PC board. This method would destroy your reference circuit, but by only taking a few components off at a time you can preserve it long enough to layout everything on the PC board.
I will not describe the steps I took to lay out the PC board in high detail like I did with the breadboard. I did that so that you could get a feel for how to lay out things like the comparators efficiently and to keep less experiences persons from possible getting overwhelmed. The PC board layout is very similar to the breadboard layout, though it won't look like it at first glance. The key to the PC board is that you have 2 dimensions of freedom instead of just one, as the rows on the breadboard prohibited much of that. You can tighten up all of the circuits by creatively placing components. The image for this step shows that well. Behind on the breadboard are the comparators, and in the foreground is a layout for a comparator on the PC board. The layout will be explained next step.
Step 16: Transferring Components: Part 1
We are going to start with the comparators again. They lend themselves to use board space efficiently on the PC board because many of the connections between the transistors allow them to be placed very close together. The easiest way to go about the comparators is to start with the same transistors we started with on the breadboard, then move on to the outer transistors and finally the three on the other side.
The two middle transistors can be placed the same exact way they were on the breadboard, same orientation and everything. The outer two transistors can be place on the board with the same orientation. Their emitters connect to the bases of the middle two so instead of placing them to the outside of the middle pair, place the in front with their emitters in the same row as the bases of the middle pair. The 10k ohm resistor can then sit nicely between the two. The current mirror pair of transistors can be placed on the same rows as the middle pair, just like on the breadboard. Positioning the third transistor is up to you. Its a good idea to place it so that the reference input will be on the side which the other comparator will be built so that the voltage divider can be placed between the two.
What's really cool about the comparators is that they are identical in layout. The only difference which supply rail gets connected at various places. What this means is that once you have one built and soldered down, you can replicate the layout EXACTLY, remember to use the right transistors though! That includes all of the solder bridges and wires.
Once you have made the comparators, you can go ahead and make the voltage divider. If you planned ahead you would have put the references beside each other. Once again the divider is easy, a resistor from Vcc to the threshold reference, a resistor from ground to the trigger reference, and a resistor connecting the reference inputs together.
Step 17: Transferring Components: Part 2
Now its time to build the flip flop and put the output driver, reset and discharge transistors on the board. The flip flop tends to be more organized the way it was made on the PC board as compared to on the breadboard. It is a good idea to run all of the power connections as you are constructing the timer that way you can be sure to get all connected.
I think the best way to do it is to put the emitters of the two transistors together since they are both tied to ground. Then add the resistors that connect one collector to the other base and vice versa. This arrangement looks very similar to the transistors of the comparators. The next thing to do is to add the inputs from the comparators through 4k7 ohm resistors and add the 1k resistors that tie the collectors to Vcc.
It is up to you to place the discharge transistor and the reset transistor but as for the output driver, I have a suggestion. Since the emitters and bases of the transistors are connected together, I think it is easiest to have the transistors in the same rows so that it only takes a solder bridge to connect the emitters and bases. Refer to images 2 and 3.
If you so desire, you can put a bypass capacitor right on the board so that should you forget to add one on the breadboard, it will still be there.
Step 18: Finishing Touches
Now that everything is on the PC board and wired up, we need a way to use the timer by adding the external "pins". You could do it however you want to, but I prefer making a DIP 8 plug that can be inserted into a breadboard or IC socket. The way to do that is to get some male pin headers and cut off 4x 4 pin sections. On two of the sections, either cut off or pull out all of the pins. Then superglue all four sections together, with the two with pins on the outside. This creates a Dual Inline Package with eight pins. In a DIP the two rows of pins are 0.3" apart. Since all of the headers have 0.1" spacing, putting two in between creates a 0.3" space.
Next start running wires from the board to the plug. I found it easiest to start from the side closest to the board and work out. For me, that meant start with Vcc on pin 8 and ground on pin 1. Then move to Discharge on 7 and Trigger on 2 and so on. Leave some room between the board and the plug So that it can be plugged in easier and to give you some room when soldering the connections. Finally, add a copious amount of hot glue to keep the wires from making contact and to give it some more strength.
And that is it, you can now try it out!
Step 19: Plug It in and Hope for the Best
Since testing the individual parts of the timer on the PC board would be difficult at best, there is really nothing much to do but plug it in and hope you did everything right. If it works, then congratulations! You have just built your own 555 timer!
The timer should perform just as well as the IC. I have done some frequency tests and found that in performed even with the little capacitance of the scope probes. My goal was to design a circuit that would perform all of the functions of the real thing and I think I am successful. Of course there are some other things to note about the circuit. I wouldn't recommend running it below 4.5V like the real IC because of the darlington pairs which need at least 1.2V (and Vcc-1.2V for the PNPs) to operate and 1/3Vcc when Vcc = 4v is cutting it close. I have run it at 3.3V but just barely. I don't know what the maximum it can run at, I've run it at 17v with no problem (the IC datasheet recommends about 16-18v). If somebody would like to push it to the limit, please comment or message me your results! One thing that I've mentioned was using power transistors on the output driver. This would allow you to draw more than the measly 200mA from the output. You would only be limited by your transistors and the power wires!
Of course, there is the possibility that something will go wrong when soldering. It is much harder to figure out where the issue is. The best advice I can offer is to start checking connections and probe different connections with a multimeter or oscilloscope when it is powered on. If you have a bad connection that causes a transistor to heat up or burn out, that could be much easier to deal with because you know where the problem is so you can track it down easier. All I can really say is good luck.
Step 20: Final Product
If you have made it through the instructable and now have a working timer either on the breadboard or on a PC board, congratulations! You have just proved that the little plastic chip is not magic (just the little plastic TO-92 transistor packages!)
Hopefully you have enjoyed this project and possibly learned something. If you have, please vote for it in the hurricane lasers contest!
If you have any suggestions of things to figure out and build, such as a certian IC or something of the nature please leave your suggestion in a comment below!
Good luck and happy making!