Introduction: Pulse Generator Using a 555 Timer

Hello! For my very first instructable I wanted to show how to build a pulse generator circuit using the ever so popular 555 timer chip. This is a very simple, yet very important, circuit to build and understand. It provides a great introduction into integrated circuits and the 555 chip finds its way into many more sophisticated circuits. So understanding the basics of the 555 is essential.

Step 1: Gather the Stuff You Need!

For this circuit you will need:

  • 100Ω resistor (x1)
  • 1K Ω resistor (x1)
  • 100K Ω resistor (x1)
  • 1µF capacitor (x1)
  • 0.1µF capacitor (x1)
  • LED (x1)
  • 555 Timer (x1)
  • Wires (to hook up a battery to the circuit)
  • PCB (circuit board)
  • 9v Battery
  • 8-pin IC socket (optional. recommended if you want to reuse your 555 chip elsewhere)

You will also need to have solder and a soldering iron as well as a wire cutter/stripper. You can set up this circuit on a breadboard and forgo the PCB if you like.

Step 2: Putting It All Together!

Note! Before you starting adding the 555 chip, be sure to ground yourself since static electricity can give your chip a lethal jolt! Or you can do it on purpose to live out your Emperor Palpatine fantasies.

On the top of the chip there will be either a small notch or dimple. The notch or dimple is located between pins 1 (negative pin) and 8 (positive pin) on the chip (see figure 1-2). The pins are numbered counterclockwise starting with pin 1 on the top-left of the chip when the dimple/notch edge is facing away from you.

Figure 1-1 shows the circuit diagram. There are multiple ways to arrange the components on the PCB so where to start is up to you really. I usually begin with soldering the chip's socket on the PCB first then adding the other components. You can use an 8-pin IC socket so you don't have to solder the 555 chip directly to the PCB. I like to make a drawing and highlight sections completed with a highlighter as I go to keep track of my progress and to ensure I didn't forget anything.

Be careful when soldering the 1µF capacitor (C1) and the LED as they are polarized. The negative side of the capacitor will be marked with negative symbols on the capacitors's body. The flat edge on the LED indicates the negative side or you can look at the LED's legs. The shorter leg is the negative lead.

Tip: Connecting pin 2 to pin 6 can be a bit tricky. One trick is to connect pins 2 and 6 from the bottom using a lead that you trimmed off earlier (see photo).

When you're done soldering all the components, connect the circuit to the 9v battery. Is the LED blinking? It is!? Then great! You have wired everything correctly!

If its not blinking don't be sad! Check to make sure everything is grounded correctly and that you solder joints are solid. Also check to make sure you have things oriented properly. If it still isn't working maybe you went too Sith Lord on the 555 chip......or you have a dead battery.

Step 3: What's Going on Here?!

I could write a long and, most likely, a very boring essay on what's going on in this circuit so i'll cut to the chase.

The way this circuit is set up puts the 555 timer in astable mode (basically a fancy way to say pins 2 and 6 are connected). Astable mode causes the 555 timer to trigger itself, producing a stream of pulses as long as its hooked up to a power supply. A lot of other 555 timer projects have the chip in astable mode, most of which, are used for creating sounds and light patterns when used with other IC chips.

Pins 2 and 6 are connected so that the circuit acts as an oscillator. Capacitor C1 is being charged through two resistors in series (R2 and R3). However, C1 discharges only through resistor R3. Which means C1 is charging more slowly than it's discharging; causing the LED to be on longer than it is off. Which may be hard to see without the aid of an oscilloscope.

You can change the frequency (that is how fast or slow the LED will blink) by experimenting with different resistors and capacitors. Changing the values of R2, R3, and C1 will change the frequency of output pulses that come out of pin 3. Changing the values of R2 and R3 will effect the length of the on cycle. Changing the value of R3 only will changed the duration of the off cycle. You can use the formula shown in figure 1-3 to calculate a desired frequency.

I hope this was helpful and clear! Please let me know if anything needs clarification and click follow if you'd like to see my future instructables!

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