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LMC555

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SNAS558M – FEBRUARY 2000 – REVISED JULY 2016

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Typical Application (continued)

9.2.2 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 the equation:

t = 1.1 × R × C seconds

where

•

RC equals 4.545

(7)

If R is chosen as 100 kΩ, C = 45.4 µF. The values of R = 100 kΩ and C = 47 µF was chosen 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-kΩ current

limiting resistor pull up 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 LMC555 to

GND. The reset pin is not used and was connected to the supply voltage.

9.2.3 Application Curve

The data shown in Figure 10 was collected with the circuit used in the typical applications section. The LM555

was configured in the monostable mode with a time delay of 5.17 s. The waveforms correspond to:

•

Top Waveform (Blue) – Capacitor voltage

•

Middle Waveform (Purple) – Trigger

•

Bottom Waveform (Green) – Output

As the trigger pin pulses low, the capacitor voltage starts charging and the output goes high. The output goes low

as soon as the capacitor voltage reaches 2/3 of the supply voltage, which is the time delay set by the R and C

value. For this example, the time delay is 5.17 seconds.

Figure 10. Trigger, Capacitor Voltage, and Output Waveforms in Monostable Mode