Better 555 Duty Cycle Control (PWM).

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Күн бұрын

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@paulpkae 2 ай бұрын

Thanks to @bruceaitken1918 for spotting an error at 10:04. The t1 and t2 results should have both been 0.01525 not 0.0165 Seconds!

@JaenEngineering 3 ай бұрын

2:50 if you place a resistor between the junction of R1,R2 and pin 7 you can normalise the mark/space ratio. Also, as the 555 has a push pull output, you can drive the charge/discharge cycle of the cap from the output via a single resistor

@therealjammit 3 ай бұрын

I've got a story. I needed a signal source, duty cycle and exact frequency not needed. Just something that makes noise. The only 555 timer I found in my junk box just happened to have a blown discharge transistor in it, otherwise it worked. I did what you suggested just to make some noise come out from it.

@paulpkae 3 ай бұрын

Thanks for the comment. Yes the output can indeed sink or source current. But, bear in mind that in the arrangement you suggest, your intended output load will affect the frequency.

@JaenEngineering 3 ай бұрын

@@paulpkae true, although you could also use the discharge pin as an open collector output.

@KkkKkk-re9il 3 ай бұрын

@@paulpkaeYes, but you can buffer your output using an opamp or an emitter follower.

@GIRLYMECHANIC 3 ай бұрын

If you need less than 50% duty cycle from a 555 you just invert the output. Eg if you need 10% just set the 555 to 90% then invert the output.

@paulpkae 3 ай бұрын

Thanks for sharing.

@CXensation 3 ай бұрын

Great description ! If you match the 2 diodes, I believe the frequency would be more stable, when you adjust the duty cycle. Also using a dual potientiometer for Ra, Rb would give a smart frequency adjust, still holding the set duty cucle, but it will suffer due to any mismatch in the dual pot. A selector switch for C, could give you decade ranges for the frequency, switching more or less capacitance in, simply in parallel. A diode in series with the Vcc would probably make the circuit back to its predictable frequncy calculations again ... Some food for a second edition of this video. The third edition would be the usual dual opamp diode sine wave converter circuit ... The fouth edition would be a buffer circuit with output level adjust and selectable output impedance ... Thanks

@paulpkae 3 ай бұрын

Thanks for the comment. There's probably 1000's of different implementations, each with their pros and cons. This is just one, easy to follow example.

@Pyroteknikid 3 ай бұрын

I like to use the 555 "output" to drive the timing part of the PWM, since it is inherently sink/source. Can very easily implement frequency and duty control. Most gate drivers have very high input resistance so they dont affect the timing, or the "discharge" can become the output.

@Mark1024MAK 3 ай бұрын

That arrangement is also handy if you want or need an open collector output.

@michaelrtreat 3 ай бұрын

Nice trick using pot to adjust duty cycle without affecting frequency

@paulpkae 3 ай бұрын

Thanks for the comment. There are countless other arrangements that can be applied to achieve the same outcome, this is just one example.

@95rav 3 ай бұрын

GreatScott! covered this in his Basic Electronics #26 about 7years ago... The design was slightly different to enable 0 to 100 duty cycle. It also covered the difference between bipolar transistor based 555's and CMOS.

@paulpkae 3 ай бұрын

Thanks. There are indeed countless videos on the 555 out there. I have not seen Great Scott's one though, I will take a look.

@VIDEOAC3D 3 ай бұрын

@grahamnichols1416 Agreed.

@dalenassar9152 3 ай бұрын

THANKS FOR THE VIDEO!!! I really like the opening diagram. Most videos don't show the entire circuit in much detail. It would really be nice to see even a more 'intuitive' diagram so that the functions of the 555 can be seen more clearly...not as the exact pin layout...with labels also. I would lay it out with the three 5k resistors in the left side. ...one more thing, it is incorrect to say that pin 7 is "floating". A pin is floating if is effectively not connected to anything and is an input.

@paulpkae 3 ай бұрын

Thanks for the comment. I do agree regarding the "floating" bit, because the pin is actually connected to a voltage source via the resistors. But, I am kind of explaining it in a way that most people are able to follow. If you think of it being isolated from the rest of the external components then, when the Discharge transistor is in an OFF state (i.e. the Base is low), then the pin is effectively not connected (open collector).

@dalenassar9152 3 ай бұрын

@@paulpkae YES, I see your point. However, especially in teaching, it is important to use correct terminology...the pin is INACTIVE, but definitely NOT FLOATING...something that may stick in minds of those pursuing a career in EE, especially when the difference between INACTIVE, and FLOATING is often confusing. A CLASSY VIDEO INDEED!!

@paulj-ms9vz 3 ай бұрын

Fantastic

@y2ksw1 3 ай бұрын

Plus the resistance of the diodes.

@paulpkae 3 ай бұрын

Diodes don't really offer much resistance in forward bias mode. The main parasitic property is really the volt drop, which typically around 0.6-0.7V. The Diode voltage drop is indeed mentioned in the video (jump to 10:16).

@foogod4237 3 ай бұрын

11:14 You really don't need all of those components in the "adjustable duty cycle" configuration. First, you never really needed the diode on Rb to begin with. You can just connect Rb directly to the "discharge" pin (across the other diode). In the charging situation, it means a bit of current will take a path through Rb instead of the diode, but since the diode will still be clamping it to basically the same forward voltage the whole time, the end result will be the same. And now that we've gotten rid of that diode, notice that "discharge", Ra, and Rb are both connected to the same point. You can actually replace *both* of Ra and Rb with a single potentiometer, resulting in a single capacitor, diode, and a pot which can adjust from 0% to 100% duty cycle. (Though, as you said, actually turning it all the way down to 0% (or 100%) is probably not a great idea..)

@paulpkae 3 ай бұрын

Thanks for the comment. I think I would prefer the additional current protection and avoid over powering the internal NPN discharge transistor.

@bruceaitken1918 2 ай бұрын

Confused - I'm making a spreadsheet so I can play with the circuit of 10:25 but my numbers don't match yours - I get t1 = t2 = 0.01525 seconds. Not a huge error, but I can't see how it happened. Clues?

@paulpkae 2 ай бұрын

Well spotted, you are the only person so far to mention that. I can see now that my calc is incorrect and your result is indeed right! My apologies for the confusion. I have been trying to work out how I inadvertently arrived at 0.0165 but can see no obvious reason. It must have been a mad moment or a cut and paste boo boo from something else. I will add a correction note in the comments. Thanks for this.

@lamp007 3 ай бұрын

How would one be able to modify the frequency and duty cycle without the use of potentiometer (and maybe also without the variable resistor)? Would we be able to use something like a counter and then after a certain amount of pulses or something change the output?

@paulpkae 3 ай бұрын

Thanks for the comment and great question. It's difficult to fully explain an example answer in a brief comments text and without knowing the exact details of what's required. There are many way of automatically adjusting the frequency and or duty cycle of the 555 Astable circuit. One example would be to utilise a second 555 configured in a monostable arrangement. It's output would go high after a given period and this could be wired into one or more transistors to either:- introduce further resistance, bypass an existing resistor or introduce/bypass the capacitance. If you wanted the circuit to alternate between one frequency/duty cycle and another, then make the 2nd 555 an astable circuit as well.

@lamp007 2 ай бұрын

@@paulpkae Would you also happen to know if there is such an IC or something that can oscillate from 5v to 0v instead of 2/3rds to 1/3rd the input voltage?

@paulpkae 2 ай бұрын

@@lamp007 The output from the 555 will oscillate between ground and your supply voltage (or very close to it). The 1/3rd and 2/3rd voltages are only internal references. Or have I mis-understood your question?

@andrewspir 6 күн бұрын

Great video! For the Variable Duty Cycle Using Potentiometer, does the value of the potentiometer have an effect on the circuit? I.e., should it be a value close the resistors calculated?

@paulpkae 6 күн бұрын

The pot value will affect the frequency of the output but does not necessarily need to be similar to Ra or Rb. It's essential that Ra is of a reasonable size to prevent too much current flowing from the positive rail through the Discharge transistor to ground.

@andrewspir 14 сағат бұрын

@@paulpkae Thank you!

@emreparlak2729 2 ай бұрын

i didnt know you could do these kind of things with excel. you should also teach excel, man

@paulpkae 2 ай бұрын

Excel is incredibly powerful and useful tool.

@JusticeAlways 3 ай бұрын

I'd like to make a adjustable time delay control for the windshield wipers on my car (has just a plain on/off/fast/slow speed control). The "on" time pulse be long enough to get wipers started from "park position"....the "off" time value be roughly 5 ~ 30 seconds set by potentiometer. Because wiper motor draws high current maybe use a relay to handle load. Energizing / de-energizing relay coil would send voltage spikes to timer's output...possible damage? How to address that issue. Being a novice at electronics...I thought this might a great project...and I'd get time delay control on my car's wipers.

@Mark1024MAK 3 ай бұрын

If driving a relay coil, you need a suitable diode wired in parallel with the coil. The diode anode connects to the coil connection that is wired to the positive part of the circuit and the diode cathode connects to the coil connection that is wired to the negative part of the circuit.

@YodaWhat 10 күн бұрын

This way of getting _constant frequency AND variable duty cycle_ is somewhat roundabout, and is only good when you want *manual control* of the duty cycle. There is another method using an external transistor to discharge the timing capacitor "instantly" (very, very rapidly) that lets you get a default 50/50 duty cycle more simply, while also having electronic control of duty cycle, and being able to _precisely_ set the frequency manually. I found that circuit many years ago in the 555 Timer Cookbook (now missing), but I am still trying to find the circuit again. If anyone knows where to find that book or the circuit, please let me know! It is a very useful old circuit. I used it to make a Class D (digital) Audio Amplifier. By using the PWM pin, pin 5, to inject an analog audio signal through a small DC-blocking capacitor, you get a very nice analog modulation of the 555's square wave output. I also used the CMOS 555 in that instance because it can run at 2 MHz, putting the main digital noise above the AM radio band, and making it very easy to do the low-pass filtering with just a single inductor.

@paulpkae 9 күн бұрын

Many thanks. There are countless iterations of the 555 circuits out there and some have been stretched well beyond what people thought possible. Like all electronic components and ICs, they are just the building blocks for us to play around with and create all sorts of configurations.