Yet Another Simple Pot-controlled 555 PWM Generator

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Introduction: 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.

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.

Step 2: What Will Be Needed

The schematic that this circuit is based on is so general, that instead of giving hard values for each component, the author gave relations that would allow for any combination of parts.

However, for the sake of getting it right the first time, I'll list the values that did work.

Components:
  * 1 - Generic 555 IC (NE555 was used)
  * 2 - 1K Resistors (R1, R3)
  * 1 - 100K Linear Potentiometer ( R2 = 100 * R1 )
  * 1 - 1n4004 Diode (Pretty much any diode will do)
  * 1 - .1uF Ceramic Capacitor (Unsure about the relation of the value of this to the resistors)
  * Breadboard

Step 3: Build It!

There's nothing much else to this circuit except for just going for it and building it.

Know of the proverb "Measure twice, cut once"? The same thing applies. Build the circuit, check the circuit, then apply the power.

Using the circuit is simple! Your PWM signal will be coming from pin 3. From there, make the standard led circuit, except route the voltage to pin 3. Play with the pot and enjoy!

Step 4: Credits / Sources

Article discussing the schematic: http://electronicdesign.com/article/analog-and-mixed-signal/wide-range-pulse-width-modulator-uses-555-timer110.aspx

Schematic: http://archive.electronicdesign.com/files/29/11028/figure_01.gif

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34 Comments

Got this working, with asspumtion they print north to south, with assumption pin 7 comes in at same point as r1 off +.

Too many wires. But I can't see a breadboard working with a 555 as pin spacing is 1.25m, while bread board is 2.5mm. http://www.intersil.com/content/dam/Intersil/docum...

Either I am wrong with bb spec, or my 555 is wrong brand.

1 reply

Please slow down and try to absorb things step by step. Always look through the datasheet for any part you're using, it will list what marking is used to indicate pin 1. In the case of 555s, they are usually interchangeable as far as pinout but may have small differences in details of spec and performance. Keep in mind that some parts come in different packages, they're not all made to work with your breadboard. Almost all breadboards are 0.1"/2.54mm spacing, and most all *DIP* (dual inline package) chips are 0.1"/2.54mm spacing as well and should plug right into a breadboard. It sounds like you got an SOIC package which has 0.050"/1.27mm spacing and is for surface mounting, not meant for breadboard without a DIP adapter. Using pots can also be searched and studied, they use certain conventions that you just have to learn first. The "CW" with arrow indicates which direction you turn the pot to move the wiper (pin 2) from one voltage to the other, thus sweeping its output. The pot forms a voltage divider from pin 1 to pin 3 voltage, with the divided value being output on pin 2.

My 555 that arrived looks nothing like the wikipedia, where there is a dot or two in the north side. How are we supposed to figure out the north south orientation of the 555 without burning out the chip, if it doesn't look like the wikipedia picture? There is random number on it too small to read. I could assume they would print north to south. Or I could assume they would print south to north.

Of course, I will go with the detailed diagram over rough. Trial and error and google on the diode/cap. However, I don't want to trial and error pot hookup. This pot hookup needs explanation of what is actual hookup, described by the three arrows pointing into r2.

The rough and detailed schematic don't jive. On the detailed pin 5 is not hooked up. On the rough schematic, it is hooked to 2 and 6 pin.

Also, on rough schematic, what does 2 cw refer to?

I haven't yet built, but studied with microscope the laid out schematic.

Firstly, I do not yet understand the pot hookup. Most pots have three pins. I assume only two to be tapped, not three. But assuming as nothing mentioned in diagrams. Now assuming only two pins the Vs through R1 and pin 7 go into center pin of the pot, while third pin of the pot goes to r3. ?????????????????

Also, I find the schematic a mere jumping off point for a physical diagram, which takes another half hour to draw out. On an actual diagram, I see these pins shorted together: 8:4 , 5:2 and 7 to 6 via Diode. If I am right, 7 and 8 are shorted through r1. And 7 and 5 are shorted/connected through pot and r3. And 5 and 1 through the capacitor.

Not having built circuits recently, I am also going cold on diode and cap orientation. Hope it is obvious, when building.

1 reply

Blowing up the pot hookup on the jpg of the breadboard, I am even more confused, since they all look connected together.

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.

2 replies

So when you say "any diode", do you really mean ANY? I really don't have to worry about any of the specs? Could you confirm this one would be fine: http://www.mccsemi.com/up_pdf/1N4148X(SOD-523).PDF...

Thanks

Yes 1N4148X will work.

Good luck!

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 ? )

1 reply

The capacitance controls the PWM frequency by changing the time it
takes to charge to a given voltage through a current determined by the
resistor value.

You're right, increasing the frequency will create more heat. The reason is switching losses in the transistor: it spends most of its time in either the cutoff (~infinite resistance, ~0 current, ~0 power dissipation) or saturation (~0 resistance, ~0 voltage drop, ~0 power) regions, but each transition moves through the linear region where current is flowing through an intermediate resistance for a short amount of time. Each transition dissipates some energy, and the more transitions per second (higher frequency), more power is dissipated overall. (This is why SMPS designers use lower PWM frequencies to increase efficiency... but that also slows response to large load current steps, so for things like CPU power supplies they also have to use other tricks like synchronous multiphase topologies to get the right performance)

I generally bypass it to ground with a small cap. (Internally it's connected to the other side of the threshold comparator so you can override the 2/3Vcc with a different control voltage if necessary.)

Awesome, this circuits works brilliantly.

I also had a similar issue like you - other circuits on Instructables did not work for me (maybe I didn't wire them correctly?), but this one works great! Thanks!

what is the frequency range of this PWM generator.

BTW AMAZING instructable

1 reply

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.