Yet Another Simple Pot-controlled 555 PWM Generator





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.

  * 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:




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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.

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

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.

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.