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555 Timer Astable Mode


In this chapter, we will build an Astable 555 Timer circuit. Before we can do that, we need to discuss an experimenter board, sometimes called a breadboard. There are many types of these available, made by many manufacturers. The one I use is shown below.

They are all pretty much the same except for a few things. The biggest difference is the size of the boards, the number of holes, and some are labeled differently.

They all have a couple of things in common; and they are all used to help someone put a circuit together quickly, usually temporarily. They have metal contacts under the plastic to assist in making the circuit you want. There are basically two types of connections.

First, let’s talk about the one at the very top labeled + and shown in red. There is one strip of metal contacts under that entire row. Anything you plug into any one of those holes will be electrically connected to everything else in that row. Normally, we hook the positive terminal of the power supply or battery there. Then anything that is supposed to go to the + side also plugs into one of the holes in that row.

The second row is built exactly the same, but is normally used for the negative or – connection, sometimes called ground. Some breadboards only have + at the top and – at the bottom. This one has two sets. The set at the bottom is just like the set at the top, but they are NOT connected to each other internally. That means if you want to use them for + and -, you need to run a jumper from the top + to the bottom + and the same with the minus row.

The short vertical rows of five holes are connected differently. All five holes are connected vertically to each other, but not to anywhere else.

The wide space between the upper group of 5 holes and the lower group is the perfect spacing for a standard chip.
OK, let’s build our Astable 555 Circuit

Here we have added the 555 chip, and two jumpers that tie the upper + and – rails to the bottom ones. Plus is wired to plus and minus to minus. I like to use a RED wire for the plus and a BLACK wire for the minus.

The chip has to go in the right way. First, it fits between the upper and lower vertical slots. Second, there is a small circle on the chip that identifies pin number 1. It is plugged into the hole in row “E” in the seventh position from the left. That hole would be called “E7”. There is only one “E7” on the entire board.

Honestly, I never pay attention to those letters and numbers, once you gain some experience, you won’t either. Since I am showing this in such a way that you could follow along if you wanted to, by building one at home, it would be better if we both use the exact same holes.

If the board you are using has more holes or is labeled differently, we can still make it work. For example, if your board was labeled A thru E up top, just ignore the letters and plug the chip in with pin 1 in the top of the bottom set, in the seventh hole from the left, just like mine.

Next we add the three resistors, 1K, 1M, and 390 ohms. They have no polarity, so you can install them in either direction.

Here we have added 2 capacitors and an LED. The small .01 mfd capacitor from I10 to the upper negative rail has no polarity, just like the resistors.

The 1uF capacitor is an ELECTROLYTIC capacitor and therefore does have polarity and won’t work if it is installed backwards. In fact, it might even explode if it isn’t installed correctly! It should have a minus sign (-) marked on it. That lead has to go down on the lower minus rail.

The LED also has to be installed in the correction. The longer lead has to go to the left.

We are only two steps away now! Here we have installed all of the jumpers except one. These make all of the connections from the chip to the various components.

Finally, we hook that last jumper up. It is the yellow one that goes right across the chip. Remember, the chip has eight pins numbered 1 thru 8 and pin 1 has the circle and is installed here on the bottom left at E7. Then, still on the bottom row, counting left to right, we have pins 2, 3, and 4. Then we go up to the top row, above pin 4, and counting back to the left, we have pins 5, 6, 7, and 8. So that yellow jumper goes from pin 2 to pin 6 on the chip.

Don’t hook the battery up yet.

This is the final board drawing for this lesson. On the next page, I have shown an actual picture of the one I built. They are hooked up exactly the same. You need to do a very careful inspection all of your connections to make sure everything is connected as shown.


After you are sure that all connections are correct, you can hook the battery up and your LED should be blinking! If not, you either hooked something up wrong, or you have a defective component or battery.

The most likely problem would be that something is hooked up wrong or not making good contact. Ninety-nine out of one hundred times, if I have a problem, it’s because I was one hole off and didn’t catch it during my inspection before installing the battery. As the circuits get more and more complex, it gets harder to see if everything is correct. If you skip the inspection, you might get lucky. You might also damage components!

In the next chapter, we will discuss the effects of changing the timing components.