This is the first article in a series where I’ll explain the three modes of the 555 timer. Click here to go to part 2: 555 Timer Basics – Bistable Mode, or here to go to part 3: 555 Timer Basics – Astable Mode.

Introduction to the 555 Timer

The 555 timer could possibly be the most common IC chips used in DIY electronics projects. The name comes from the three 5K Ohm internal resistors used as voltage dividers in the inner circuitry if the chip. The 555 timer is small, inexpensive, and versatile. It is the core of many electronic devices which feature flashing LEDs, beeping noises, or anything that depends on a reliable source of oscillating current. This is why it is referred to as a timer. It can be used to output a pulse of electricity for a specific amount of time, for example to turn on a light for 5 seconds once a button is pressed. It can be used to generate quick pulses of electricity, to make an LED blink on and off for example. It can also be used to generate high frequency pulses (as a square wave) that are able to produce sound when connected to a speaker or buzzer.

Three modes of output are possible with the 555 timer – monostable, bistable, and astable. Each mode has different characteristics, and will determine how the 555 timer outputs current.

Here is the datasheet of the 555 timer from Texas Instruments for more technical information:

Circuit Basics PDF Icon 555 Timer Datasheet

555 Timer555 Timer Monostable Mode Circuit

  • Pin 1 – Ground: Connected to 0 V
  • Pin 2 – Trigger: Turns on the output when the voltage supplied to it drops below 1/3 of Vcc.
  • Pin 3 – Output: Outputs up to 200 mA of current at about 1.5 V.
  • Pin 4 – Reset: Resets the timing operation of the output when it is connected to ground (0 V).
  • Pin 5 – Control: Controls timing output independently of the RC circuit when the voltage supplied to it is above 2/3 Vcc. When not in use, it is usually connected to ground via a 0.01 μF capacitor to prevent fluctuations in timing of the RC circuit.
  • Pin 6 – Threshold: Turns off the output when the voltage supplied to it reaches above 2/3 Vcc.
  • Pin 7 – Discharge: When output voltage is low, it discharges the capacitor in the RC circuit to ground.
  • Pin 8 – Vcc (supply voltage): Can range from 4.5 V to 15 V.

Monostable Mode of the 555 Timer

In monostable mode, the 555 timer outputs a single pulse of current for a certain length of time. This is sometimes referred to as a one-shot pulse. For example, in the schematic below, by pressing the push-button switch once, the LED connected to the output pin will light up, then turn off automatically after the predetermined length of time. The length of time the LED remains on is determined by the resistance and capacitance values of R1 and C1. We can calculate the time from the equation:

t = (1.1)(R)(C)

Where t is the length of electrical output in seconds, R is the resistance of R1 in Ohms, and C is the capacitance of C1 in Farads.

As shown in the equation, larger values of resistance or capacitance will increase the time the LED stays on.

How Monostable Mode Works

Before the button is pressed, the voltage on the trigger pin is high. The trigger pin is connected internally to the discharge pin, and when the trigger pin voltage is high, the discharge pin is open to ground and prevents charge from building up on capacitor C1.

When the button is pressed, the voltage on the trigger pin drops low. When the trigger pin voltage is low, the output pin turns on. Also, the discharge pin opens and stops C1 from draining to ground. Now the capacitor can charge and collect voltage. However, the capacitor takes time to charge, and while the voltage across it is below 2/3 Vcc, the threshold pin remains low, and the output pin stays switched on. When the charge finally builds up enough to make the voltage across it above 2/3 Vcc, the output pin switches off because the voltage at the threshold pin is now high. Now that the voltage at the threshold pin is high, the internal comparator switches the discharge pin back on, and the charge on the capacitor flows to ground until the button is pressed again.

The length of time the LED remains on is a function of the time it takes for the capacitor to become charged enough so that the voltage across it is 2/3 Vcc. The time the LED stays on is also determined by R1 since this resistor prevents the flow of current to the capacitor and thus increases the time it takes to reach the state of charge where there is 2/3 Vcc across it.

Watch the video to see the 555 timer in monostable mode:

Example Monostable Mode Circuits

To observe the monostable mode of the 555 timer, connect up a circuit like this:

555 Timer Monostable Mode

R1: 10K Ohms

R2: 10K Ohms

R3: 470 Ohms

C1: 470 μF

C2: 0.01 μF

When you press the button once, the LED should turn for about 5 seconds, then turn off. The length of time the LED stays on is dependent on the values of R1 and C1. Larger resistance or capacitance values will increase the time the LED stays on.

A good way to observe the dependency of time on resistance in this circuit is to replace R1 with a variable resistor (potentiometer):

555 Timer Monostable Mode w Potentiometer

Now adjust the potentiometer and see how it affects the time the LED stays on. The effect is quite dramatic. For a great resource on the 555 timer, OpAmps, and other IC’s check out the Engineer’s Mini Notebook: Timer, Op Amp, and Optoelectronic Circuits & Projects. There are 24 different 555 timer circuits in this book!

Click here to continue on to part 2, 555 Timer Basics – Bistable Mode.

If you have any questions about this circuit, or are having trouble getting it to work, please leave a comment below!

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