Introduction: 555 Timer Circuits

Picture of 555 Timer Circuits

The 555 Timer uses 3 main functions: Astable, Monostable, and Bistable. The Astable outputs a high pulse on a steady repetitive pace. The Monostable when triggered holds the output high for a predetermined time period. The Bistable can be held high and low by pressing the pushbuttons. If you want to learn more, visit

Step 1: Astable 555

Picture of Astable 555

This build can be used in a variety of circuits: Timers, clocks, drive servo motors, and IR Sensors. The Astable is able to do this because it can change the cycle from milliseconds to minutes using two resistors and a capacitor.

Step 2: Monostable 555

Picture of Monostable 555

Using the monostable, you can output a high pulse for a preset time period. Using a capacitor and resistor, the time period can vary from milliseconds to minutes.

Step 3: Bistable 555

Picture of Bistable 555

Another way to think about this circuit, is simply as a switch. If you press the bottom pushbutton the LED will go high and pressing the top pushbutton it will stay low.


cedric23 (author)2016-01-17

Has the capacitor been well connected?

Phil B (author)2015-01-16

Everyone has high praise for the 555 timers. So far, I have found only one 555 circuit I wanted and needed to build. I could not say in which mode the timer operates. The circuit uses a red LED and connections to various resistances. It is a capacitor tester for capacitors 10 micro-farad and higher (if I remember correctly). It was very handy when I was working with some old tube radios and performed very well.

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