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.

what is the frequency range of this PWM generator.

BTW AMAZING instructable

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?
Xavierxf5 months ago
What is pin 5 connected to? Is it +5 or GND?
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%.
rimar20003 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?
ZeroTruths (author)  rimar20003 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
ZeroTruths (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.
omnibot3 years ago
Nice one.
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