Instructables

Yet Another Simple Pot-controlled 555 PWM generator

Something that a project that I'm working on has me doing is using a serial to parallel IC (think 74HC595) to control leds. However, rather than drive the leds directly from the pins, I opted for the use of transistors. After testing this out, it became apparent to me that perhaps the leds might be too bright, so I went in search of a simple PWM generator.

Of course, there are a couple of instructables that already feature such a circuit, but I was unable to get them to work correctly for whatever reason. This being the case, I will now present the circuit that I came across and works very well.
 
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Step 1: Yet another overview

PWM stands for Pulse Width Modulation, which is a simple way of efficiently supplying varying amounts of power.

For example. Say that you wanted to control the brightness of an led (note: there are many ways to do this, but for the sakes of an example, I'll only note two). The first way would be to put a variable resistor in series with the led. This would alter the amount of current that went through the led, while holding the voltage constant. If you put the variable resistor at 40%, the led would be 40% as bright as it could be.

The second way would be to connect a led in series with a resistor and a power supply that could be turned on and off really quickly. Let's say that you were able to turn on and off the power supply quick enough to the point where 40% of the time it was on, and 60% of the time it was off. This would be reflected by the led being on full brightness, but only for 40% of the time, giving the illusion of being 40% as bright as it could be.

Two different methods, for the same result. What's the difference? About 60% of the energy gets burned off as heat in the resistor in the first circuit, while in the second circuit, almost all of the energy supplied is used.

This is why PWM is useful. It allows a signal to range from completely off to completely on. If a signal is turned on and off quick enough, given a certain ratio, a signal can appear to be that ratio, without suffering from much power loss at all.
stefco1 month ago

i believe the cap value is regulating the pwm frequency , i have 0 experience with pwm this is my first project , but i believe lower value caps = higher frequency and vice versa. You don't want too low because it will either be audible ( i've heard motors produce a sound from the controller frequency if it's within hearing range) and you don't want too high because well... i have no idea (maybe it will heat up ? )

what is the frequency range of this PWM generator.

BTW AMAZING instructable

You can play with various values of R and C in this calculator:

http://www.ohmslawcalculator.com/555_astable.php

The calculator does not incorporate the diodes that allow this circuit to go below 50% duty cycle, so I don't know how that might affect the frequency. To simulate the effect of turning the pot all the way one way and then all the way the other way, I set R1 to 1 kOhm and R2 to 101 kOhm, and vice versa. I got a frequency of 70 and 140 Hz, so it may be the case that the frequency changes depending on which way the pot is turned.

interesting. I also played around with the capacitor as well, and it also affects the frequency directly. So i was thinking of trying to put multiple capacitors in series and parallel with switches to modify the capacitance. By having them in series allows you to increase the frequency and having it in parallel gives a lower frequency.

I built a version of this circuit in the Java Circuit Simulator App. You should be able to view it using this link. You can use the slider on the right to move the pot around and see the resulting frequency. The graph at the bottom should show frequency in the upper left, as long as the frequency has been stable for long enough.

http://www.falstad.com/circuit/#%24+1+5.0E-6+19.056626845863+50+5.0+50%0A165+464+224+480+192+2+12.0%0AR+528+128+528+80+0+0+40.0+12.0+0.0+0.0+0.5%0Aw+592+256+592+128+0%0Aw+592+128+528+128+0%0Ar+592+288+688+288+0+100.0%0Ag+688+288+736+288+0%0Aw+464+352+384+352+0%0Aw+464+320+384+320+0%0Aw+384+320+384+352+0%0Aw+464+256+384+256+0%0Az+384+256+384+320+1+0.805904783+5.6%0A174+336+224+384+288+0+100000.0+0.9950000000000001+Duty+Cycle+Pot%0Aw+528+128+528+192+0%0Ar+336+224+336+128+0+1000.0%0Aw+336+128+528+128+0%0Ar+336+288+336+352+0+1000.0%0Aw+384+352+336+352+0%0Ac+336+352+336+416+0+1.0E-7+6.872346013941149%0Ag+336+416+336+448+0%0Ao+4+64+0+46+20.0+0.1+0+-1%0A

Sorry. It appears that the link isn't working right. Try opening the applet at http://falstad.com/circuit and then clicking file/import and pasting in this text:

$ 1 5.0E-6 19.056626845863 50 5.0 50

165 464 224 480 192 2 12.0

R 528 128 528 80 0 0 40.0 12.0 0.0 0.0 0.5

w 592 256 592 128 0

w 592 128 528 128 0

r 592 288 688 288 0 100.0

g 688 288 736 288 0

w 464 352 384 352 0

w 464 320 384 320 0

w 384 320 384 352 0

w 464 256 384 256 0

z 384 256 384 320 1 0.805904783 5.6

174 336 224 384 288 0 100000.0 0.9950000000000001 Duty Cycle Pot

w 528 128 528 192 0

r 336 224 336 128 0 1000.0

w 336 128 528 128 0

r 336 288 336 352 0 1000.0

w 384 352 336 352 0

c 336 352 336 416 0 1.0E-7 6.801312165289001

g 336 416 336 448 0

o 4 64 0 46 20.0 0.1 0 -1

Xavierxf10 months ago
What is pin 5 connected to? Is it +5 or GND?

just leave it unconnected

I understand all electrical parts/components have a mathematical equation to them.
What I am Currently in need of, is the math relating to pulse frequency and high voltage.
IE; as High frequency as possible and as high voltage as possible.
Resistors, Capacitors, Transistors, and whatever else is required to Increase DC voltage and frequency of number of pulses per second.
What I desire it to have a circuit, on the Cheap, that I can Easily adjust the frequency and voltage output.
Type of signal is currently not important, although, I'll eventually be using Scalar waves frequency which is more Potent than Radio waves.
Cold Current Generation.
Can you Help me out?
gwood61 year ago
Hi - Thanks for this! It looks nice and simple and very like a simple 555 pwm that I constructed recently and actually worked so thanks and well done : )
May I ask what you would do with pin 5 on the diagram..????
Would it go to ground or connect with a .01uf cap on its way to ground ??
Thats what my present simple 555 does..!
Sir Dean1 year ago
Any diode will work, schottky just has a lower voltage drop but that doesn't affect things in a meaningful way here.
When R2 is at the top, the Capacitor is charged via R1 and the PWM is High. So
roughly speaking, the smallest PWM High time is R1 (1K) * C1 (0.1u) = 0.1ms, and the Max Low PWM time is then (R2 (100K) + R3 (1K) ) * C1 (0.1u) = 10ms. Therefore the max On duty cycle is 99%, and the min On duty cycle is 1%. One can play with these by varying the values of Rs and C1.
lesizz1 year ago
Thanks for the 'ible. I'm gonna use it tomorrow. Looks like a good simple circuit.

The diode symbol in your schematic (not the hand-drawn one) is the symbol for a schottky diode. In your parts list the diode specified is a 1N4004. The 4004 is a general junk diode and not a Schottky. A regular diode symbol has just a simple straight bar without the switchbacks. Although a Schottky would probably work fine.

Another PWM generator on this site did only 10% to 90%. From the looks of the scope pictures it looks like yours does more like 1% to 99%.
rimar20004 years ago
Good work! Now, a question: can I use this PWM to vary the velocity of a desk fan? And a drill? In other words: how many potency can this circuit manage?
MrCruz (author)  rimar20004 years ago
Directly? No. If you were to directly connect the output pin to the device, the most current you'd get would be something around 30mA, I believe. However, that's still not to say that it couldn't be done with this circuit. What you could instead do is use the output of this circuit and use it to control the gate pin on an NPN transistor, similar to what I did on the very last image. You'd have to make sure that the transistor could take the kind of currents that you need (drills and fans tend to use a lot of current, around the range of Amps)
Can you show that part of the circuit?
Thanks very much for your response. Electronic, as you can see, isn't one of my skills...
coolstuff143 years ago
Did you know you can play tetris on that oscilloscope?
rcisneros3 years ago
Thanks for the post.
I know it's my own ignorance, but there is not info here for me. The levels of the readers electronics knowledge varies greatly.
Like for me, I don't see how this even works since I can't find where to attach the power, where it comes out. I have to figure out ground, might be the (-). I know it's me, but help a guy out and label those things.
Keep in mind that the people that can read this cold, probably know how to do it already.

Thanks
MrCruz (author)  rcisneros3 years ago
Thanks for the reminder. It's true that I do often overlook this.

The 555 IC can take a voltage of between 5-15v, which means that you could power it with a simple 9v battery. In schematics, Vs stands for Voltage Source, which is where you would plug the positive (+) terminal of the voltage source, while the triangle (in the first picture in this step) is where you would connect the negative (-) terminal.

Schematics that have multiple ground symbols in actuality only have one. When creating the circuit, all of those ground symbols will connect together and meet at the battery's negative terminal.

The output voltage, where the signal is generated, comes from pin 3 on the IC.

Remember, all grounds meet together, unless otherwise specified.
omnibot4 years ago
Nice one.