LM555

SNAS548D – FEBRUARY 2000 – REVISED JANUARY 2015

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8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

The LM555 timer can be used a various configurations, but the most commonly used configuration is in

monostable mode. A typical application for the LM555 timer in monostable mode is to turn on an LED for a

specific time duration. A pushbutton is used as the trigger to output a high pulse when trigger pin is pulsed low.

This simple application can be modified to fit any application requirement.

8.2 Typical Application

Figure 17 shows the schematic of the LM555 that flashes an LED in monostable mode.

Figure 17. Schematic of Monostable Mode to Flash an LED

8.2.1 Design Requirements

The main design requirement for this application requires calculating the duration of time for which the output

stays high. The duration of time is dependent on the R and C values (as shown in Figure 17) and can be

calculated by:

t = 1.1 × R × C seconds

(6)

8.2.2 Detailed Design Procedure

To allow the LED to flash on for a noticeable amount of time, a 5 second time delay was chosen for this

application. By using Equation 6, RC equals 4.545. If R is selected as 100 k

Ω, C = 45.4 µF. The values of R =

100 k

Ω and C = 47 µF was selected based on standard values of resistors and capacitors. A momentary push

button switch connected to ground is connected to the trigger input with a 10-K current limiting resistor pullup to

the supply voltage. When the push button is pressed, the trigger pin goes to GND. An LED is connected to the

output pin with a current limiting resistor in series from the output of the LM555 to GND. The reset pin is not used

and was connected to the supply voltage.

8.2.2.1 Frequency Divider

The monostable circuit of Figure 11 can be used as a frequency divider by adjusting the length of the timing

cycle. Figure 18 shows the waveforms generated in a divide by three circuit.

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