Status:
Ready for Proof Reading.
Errors found in illustrations, corrected, illustrations finished.
Schematics verified, experiments complete.
Minor update, SubML packed 091127

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555 MONOSTABLE MULTIVIBRATOR



PARTS AND MATERIALS

• One 9V Battery
• Battery Clip (Radio Shack catalog # 270-325)
• Mini Hook Clips (soldered to Battery Clip, Radio Shack catalog # 270-372)
• A Watch with a second hand/display or a Stop Watch
• A wire, 1½" to 2" (3.8 mm to 5 mm) long, folded in half (shown as red wire in illustration)
• U1 – 555 timer IC (Radio Shack catalog # 276-1723)
• D1 – Red light-emitting diode (Radio Shack catalog # 276-041 or equivalent)
• D2 – Green light-emitting diode (Radio Shack catalog # 276-022 or equivalent)
• R1,R2 – 1 KΩ ¼W Resistors
• Rt – 27 KΩ ¼W Resistor
• Rt – 270 KΩ ¼W Resistor
• C1,C2 – 0.1 µF Capacitor (Radio Shack catalog 272-1069 or equivalent)
• Ct – 10 µF Capacitor (Radio Shack catalog 272-1025 or equivalent)
• Ct – 100 µF Capacitor (Radio Shack catalog 272-1028 or equivalent)

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 1, chapter 13: Electric fields and capacitance
Lessons In Electric Circuits, Volume 1, chapter 13: Capacitors and calculus
Lessons In Electric Circuits, Volume 1, chapter 16: Voltage and current calculations
Lessons In Electric Circuits, Volume 1, chapter 16: Solving for unknown time
Lessons In Electric Circuits, Volume 4, chapter 10: Monostable multivibrators



LEARNING OBJECTIVES

• Learn how a Monostable Multivibrator works
• Learn a practical application for a RC time constant
• How to use the 555 timer as a Monostable Multivibrator

SCHEMATIC DIAGRAM

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ILLUSTRATION

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INSTRUCTIONS

This is one of the most basic 555 circuits. This circuit is part of this chips datasheet, complete with the math needed to design to specification, and is one of the reasons a 555 is referred to as a timer. The green LED shown on the illustration lights when the 555 output is high (i.e., switched to Vcc), and the red LED lights when the 555 output is low (switched to ground).

This particular monostable multivibrator (also known as a monostable or timer) is not a retriggerable type. This means once triggered it will ignore further inputs during a timing cycle, with one exception, which will be discussed in the next paragraph. The timer starts when the input goes low, or switched to the ground level, and the output goes high. You can prove this by connecting the red wire shown on the illustration between ground and point B, disconnecting it, and reconnecting it.

It is an illegal condition for the input to stay low for this design past timeout. For this reason R3 and C1 were added to create a signal conditioner, which will allow edge only triggering and prevent the illegal input. You can prove this by connecting the red wire between ground and point A. The timer will start when the wire is inserted into the protoboard between these two points, and ignore further contacts. If you force the timer input to stay low past timeout the output will stay high, even though the timer has finished. As soon as this ground is removed the timer will go low.

Rt and Ct were selected for 3 seconds timing duration. You can verify this with a watch, 3 seconds is long enough that we slow humans can actually measure it. Try swapping Rt and Ct with the 27 KΩ resistor and the 100 µF capacitor. Since the answer to the formula is the same there should be no difference in how it operates. Next try swapping Rt with the 270 KΩ resistor, since the RC time constant is now 10 times greater you should get close to 30 seconds. The resistor and capacitor are probably 5% and 20% tolerance respectively, so the calculated times you measure can vary as much as 25%, though it will usually be much closer.

Another nice feature of the 555 is its immunity from the power supply voltage. If you were to swap the 9V battery with a 6V or 12 battery you should get identical results, though the LED light intensity will change.

C2 isn’t actually necessary. The 555 IC has this option in case the timer is being used in an environment where the power supply line is noisy. You can remove it and not notice a difference. The 555 itself is a source of noise, since there is a very brief period of time that the transistors on both sides of the output are both conducting, creating a power surge (measured in nanoseconds) from the power supply.


THEORY OF OPERATION

Looking at the functional schematic shown below, you can see that pin 7 is a transistor going to ground.

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This transistor is simply a switch that normally conducts until pin 2 (which is connected through the comparator C1, which feeds the internal flip flop) is brought low, allowing the capacitor Ct to start charging. Pin 7 stays off until the voltage on Ct charges to 2/3 of the power supply voltage, where the timer times out and pin 7 transistor turns on again, its normal state in this circuit.


<Continued on next post>

 

The following will show the sequence of switching, with red being the higher voltages and green being ground (0 volts), with the spectrum in between since this is fundamentally an analog circuit.

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This graph shows the charge curve across the Ct.

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Figure 1 is the starting and ending point for this circuit, where it is waiting for a trigger to start a timing cycle. At this point the pin 7 transistor is on, keeping the capacitor Ct discharged.

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Figure 2 shows what happens when the 555 receives a trigger, starting the sequence. Ct hasn’t had time to accumulate voltage, but the charging has started.

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Figure 3 shows the capacitor charging, during this time the circuit is in a stable configuration and the output is high.

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Figure 4 shows the circuit in the middle of switching off when it hits timeout. The capacitor has charged to 67%, the upper limit of the 555 circuit, causing its internal flip flop to switch states. As shown, the transistor hasn’t switched yet, which will discharge Ct when it does.

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Figure 5 shows the circuit after it has settled down, which is basically the same as shown in Figure 1.

 

I have a thought, it doesn't really have anything to do with the operation of the experiment per say. More of a format for the setup. If green is usually ground, and red is the plus lead, should the LEDs reflect this?

My thought was green is go, red is stop, so green is 1, and red is 0, but in drawing some diagrams I use red for the plus terminal, and green for ground, which isn't that unusual a convention. What does every one else think.

I was completely wrong in how it worked, but the nice thing about electronics is it is impartial, and will let you experiment. I did the experiment, and learned some new things. I will be doing some major rewrites, but it's all good in the end.

 

Excellent article so far, Bill!

 

OK, I have some stuff left to check out on the layout, but I'm done with the bulk of it again. Please proof it and ask if you have any questions.

I tried adding a bit more color for the illustration. What do you think people?

Orange (as in one of the resistor codes) just doesn't look right on my personal monitor, but OK on others. Not sure if there is anything I can do about that.

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looks good so far.

 

OK, in the colored version - I had a real hard time seeing where the leads were on those dark green caps! That's actually about the right color for certain caps, but... the connection needs to be visible.

Also, the upper power rails aren't connected internally to the lower power rails on ANY of my breadboards. Are they on yours?
It would help "newbies" a great deal to briefly show how the breadboard internal connects go.

Also, on some breadboards, the power rails are not contiguous across the entire breadboard! I have a few that are like that, one is an Archer Universal Breadboard (used to be #276-169a). There are two power rails along the top, and two along the bottom. 5 groups of 5 sockets have continuity, then there is a slight gap, then another 5 groups of 5 sockets have continuity. If you want them to have continuity all the way across, you have to jumper them in the middle.

Have a look at the attached protoboard drawings - I grabbed one of your protoboard graphics, and modified it to show you what I mean.
Note that in the 1st drawing, the spacing of the power rails is slightly different than what you have - there is an extra space between the 5th and 6th groups of power rail pins.
The 2nd drawing shows the internal connections of the power rails. The colors don't mean anything other than the two ajacent rails aren't connected to each other. So with these boards, I have 8 distinctly separate power rails. This is not convenient; I've been bitten by it more than once.

 

OK, in the colored version - I had a real hard time seeing where the leads were on those dark green caps! That's actually about the right color for certain caps, but... the connection needs to be visible.

Good point, I'll look into it more. I actually measured and drew the real components Radio Shack sells under that number, and the color is match too.

Also, the upper power rails aren't connected internally to the lower power rails on ANY of my breadboards. Are they on yours?
It would help "newbies" a great deal to briefly show how the breadboard internal connects go.

Check rows 13 and 19, there is the interconnect. Sad side story, I about ruined my 9V battery, I had stashed a single jumper off to the side... across the + and - strips. Took longer than I like to admit to find, I was tired. I measured 1 amp out of that little battery.

Also, on some breadboards, the power rails are not contiguous across the entire breadboard! I have a few that are like that, one is an Archer Universal Breadboard (used to be #276-169a). There are two power rails along the top, and two along the bottom. 5 groups of 5 sockets have continuity, then there is a slight gap, then another 5 groups of 5 sockets have continuity. If you want them to have continuity all the way across, you have to jumper them in the middle.

Have a look at the attached protoboard drawings - I grabbed one of your protoboard graphics, and modified it to show you what I mean.
Note that in the 1st drawing, the spacing of the power rails is slightly different than what you have - there is an extra space between the 5th and 6th groups of power rail pins.
The 2nd drawing shows the internal connections of the power rails. The colors don't mean anything other than the two ajacent rails aren't connected to each other. So with these boards, I have 8 distinctly separate power rails. This is not convenient; I've been bitten by it more than once.

I thought about that, but I had to settle on some standard. This one I have seen at Tanner's (an electronic parts store), Fry's (electronics/computers/appliances), and interestingly, Radio Shack. I have older versions too, but I had to settle on something. BTW, the column/row designations are really on the breadboard, I have found it pretty convenient already, the image you have is one of my old ones.

The eBook mentions the protoboards, http://www.allaboutcircuits.com/vol_6/chpt_1/2.html , and what is funny is I see the jumpers for the power stripes your talking about. I may take this up with Dennis in the near future, but short of adding or modifying a chapter (which is a real possibility) I don't see an easy answer. I have 4 different makes and models, all based on the same patents, but all slightly different. I don't have the hair left to pull out on this one.

Thanks for the inputs.

One article researching this claims the monostable will oscillate if put into the illegal configuration I mentioned. I couldn't see anything on my DVM under AC or Freq, but I can believe it. If I verify this on my Oscope I'll add the blurb. The reason I think it might be true is Ct never reached Vcc, but stayed slightly above the 2/3 point. I suspect a very narrow pulse is involved.

BTW, I just bought a used 19" LCD monitor to improve my personal graphics quality. My 19" KDS model CRT monitor is slightly blurry at the 1024 X 768 I like to use, and I have seen MUCH better pictures on LCDs (including the 17" in my back room). Orange just doesn't come through well either, I have to wonder how common this problem is.

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How about this one?

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Or I could loose the coloring off the caps altogether.

 

Yes, the white shows up much better on the caps.

As far as all the jumpers between the busses - that's a lot of jumpers! I'd prefer to use a couple of long jumpers than a bunch of short ones; less to troubleshoot (and later remove!)

As an aside, have you ever tried using pin 5 (Ctrl) with your PWM circuit? Try connecting a 1k pot with the ends on Vcc and GND, and the wiper on pin 5. You might be surprised...

 

As far as all the jumpers between the busses - that's a lot of jumpers! I'd prefer to use a couple of long jumpers than a bunch of short ones; less to troubleshoot (and later remove!)

I plan on reusing the layouts as much as I can for the experiments. That and I tapped off the slots (could have gone for the source I guess, it's a matter of taste). I also like short jumpers for another reason, most of them are leads from the diodes, caps, resistors....

I've started a small hobby tote tray for this kind of junk, with conductive foam inserts for ESD sensitive stuff. Not the best, but it's mostly local and somewhat organized.

As an aside, have you ever tried using pin 5 (Ctrl) with your PWM circuit? Try connecting a 1k pot with the ends on Vcc and GND, and the wiper on pin 5. You might be surprised...

I was planning on using that as a later project, showing how a 555 can be used as a VCO. I'm not sure the editors have bought into it, but my thought is a chapter devoted to 555 circuits, it would give us something to point to on a lot of noob questions. Besides, I like 555's, and I find I'm learning new things just doing this, which is a bonus.

 

OK, I rebuilt this circuit to finish some experiments. All that's left is to pack it for the book.

Thanks for the help everyone.