Better 555 Duty Cycle Control (PWM).

Рет қаралды 8,314

Күн бұрын

Пікірлер: 38

@paulpkae 26 күн 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!

@GIRLYMECHANIC 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 Ай бұрын

Thanks for sharing.

@CXensation 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 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.

@michaelrtreat Ай бұрын

Nice trick using pot to adjust duty cycle without affecting frequency

@paulpkae 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.

@Pyroteknikid 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 Ай бұрын

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

@paulj-ms9vz Ай бұрын

Fantastic

@JaenEngineering 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 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 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 Ай бұрын

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

@KkkKkk-re9il Ай бұрын

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

@95rav 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 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.

@grahamnichols1416 Ай бұрын

@95rav Great Scott's video does not diminish this video in any way. Your negative, and frankly dismissive comment helps nobody. Rather appreciate the effort and generosity of this content creator for the useful spreadsheet that he has provided. Liked and subscribed for the effort.

@VIDEOAC3D Ай бұрын

@grahamnichols1416 Agreed.

@dalenassar9152 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 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 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!!

@lamp007 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 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 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 29 күн 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?

@foogod4237 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 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 27 күн 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 26 күн 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.

@y2ksw1 Ай бұрын

Plus the resistance of the diodes.

@paulpkae 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).

@JusticeAlways 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 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.