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The basics of a 555 Timer IC and Buidling a One Shot Timer

The 555 Timer Integrated Circuit is a family of ICs that can be used in different modes for circuit timing and pulse generation. The two modes I will be using are the monostable and astable. Monostable mode is sometimes refered to as a 'one shot.' The astable mode will be used as our clock signal generator later on.The specific chip I am going to use here is the NE555P which comes in a dual inline package (DIP) with 8 contacts.

If you wish to skip these basics follow the link to the '555 Asychronous' section. If you are unfamiliar with integrated circuits I recommend you do this first part. If you are interested in more theory I suggest the WWW.

You will need:

  1. Breadboard
  2. NE555P IC
  3. One 10 microfarad capacitor
  4. One 0.01 microfarad capacitor
  5. One each resistors: 220 ohm, 10 kilo ohm, 470 kilo ohm
  6. One momentary contact pushbutton switch
  7. One LED 3.3V 20mA
  8. Hookup wires

The 555 Timer IC

Below you will see a breadboard example of setting up your 555 as a one shot timer. Below that you will find a circuit schematic and pinout diagram. By studying both of these you should be able to build a functional one shot timer.

I have simplified the breadboard layout as much as I can. Notice that it only uses the negative bus on the left and positive bus on the right. The component values are noted in the circuit schematic. Make note of the polarity of the 10 microfarad capacitor, the cathode (long wire) is toward the positive bus. The ceramic capacitor has a 0.01 microfarad value and no orientation. Also notice the LED where the cathode is toward the resistor. And lastly, but very important is to note where the dimple or notch is in the DIP, you can see it on the 555 pointing towards the top of the breadboard (up in the picture). The notch is how you orient an IC so that the pins are in the correct order.

Breadboard Layout of a One Shot Timer

What does it do? Push the button and you should see the LED light up for about 5 seconds, when it goes off you can push the button again.

Bored? What you have build is a pulse generator, a very slow one but never the less. Look at the waveform depiction on the bottom of the drawing below. That is what we have accomplished.

555 One Shot Circuit

An output pulse can be taken from pin 3 where the top of the blue wire is plugged in, just plug into the bus; don't unplug the blue wire. If you save this breadboard you can use it later on to step your decade counter IC/light chaser circuit through its steps one at a time. Be sure to hook the negatives together from the two boards.