Modes of Operation[edit | edit source]

Astable - Toggling / Blinking[edit | edit source]

555 in A-Stable Configuration
555 in A-Stable Configuration

An astable mode is when the timer is not stable in either state. In the context of the 555, an astable configuration can be accomplished by making use of its discharge pin to drain a capacitor in order to reset itself. The 555 timer can be used as a variable clock by using a potentiometer as RB and its frequency can be sub mHz up to 100kHz.

RA and RB form a voltage divider and charges the capacitor C. The 555 timer has an internal voltage divider that divides the input voltage evenly by 3 (Eg. 1.66v and 3.33v with a 5v input voltage) as well as an internal SR-latch. The trigger pin will set the internal SR-latch when it is below 1.66v. When triggered, the output will be high while the discharge pin will not allow current to flow. As a result, as the output is high, the capacitor will also begin to charge. Once the capacitor exceeds the threshold voltage (Eg. 3.3v), the SR-latch is reset causing the output to be low and the discharge pin to allow current to flow and discharges the capacitor. When the voltage drops below 1.66v, the cycle repeats.

The duty cycle and frequency is determined by the values of RA, RB, and the capacitance of C. This is because:

  • How fast the capacitor drains is dependent on RB.
  • How fast the capacitor charges is dependent on RA+RB
  • How long it takes to charge C is based on C's capacitance

The formulas for the time the output is high or low and the frequency can be found in the datasheet:

The maximum current the discharge pin can sink should not exceed 10mA (https://sound-au.com/articles/555-timer.htm). The combined resistance of RA and RB should also not exceed 3.4M at 5V or 10M at 15V (refer to the datasheet).

Monostable - One Shot[edit | edit source]

555 in monostable configuration
555 in monostable configuration

A monostable mode is when the timer is stable for one state but is unstable in another (hence the mono in the name). This is useful for one-shot triggers of a fixed duration such as to debounce a button or to run something for a fixed amount of time after a trigger.

In this configuration, the state is to have the output be off and the discharge pin to be discharging current. When the trigger pin is pulled low, the output will be driven high for as long as it takes until the capacitor C is charged via RA. During this period, any additional triggers on the input has no effect.

As per the datasheet, pin 5 should have a 0.01uF/10nF capacitor to ground. Again, the maximum current the discharge pin can sink should not exceed 10mA.

Bistable - Flip Flop[edit | edit source]

555 in bistable configuration
555 in bistable configuration

A bistable mode is when the timer is stable for both on and off states. You may want to implement this to debounce a latched on/off switch for instance.

When using the 555 as in a bistable configuration, in essence, you are just exposing the internal SR-latch where the trigger pin is the latch set and reset pin as the latch reset.

The discharge pin can be left disconnected. Pull both reset and trig high by default, and pull low to trigger.

Applications[edit | edit source]

Here are some applications of the 555 timer.

Voltage Multipler / Negative Voltage Inverter[edit | edit source]

You can use the 555 in an astable configuration to drive a charge pump in order to multiply a voltage or invert the voltage. Example circuits are given below.

Sine Wave Generator[edit | edit source]

555 Sine Wave Generator with LC Circuit
555 Sine Wave Generator with LC Circuit

You can use the 555 in an astable configuration to drive a LC circuit in order to generate a sine wave.

The formula to calculate the output frequency is:, where R is the resistance of R1 and C is the capacitance of C1. Eg.

See Also[edit | edit source]