SNAS558M February 2000 – July 2016 LMC555
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.
The LMC555 timer can be used a various configurations, but the most commonly used configuration is in monostable mode. A typical application for the LMC555 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.
Figure 9 shows the schematic of the LM555 that flashes an LED in monostable mode.
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 monostable figure) and can be calculated by: t= 1.1*R*C seconds.
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:
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.
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:
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.
Design a frequency divider by adjusting the length of the timing cycle.
When the timer is connected in the monostable mode and triggered with a continuous pulse train, the output pulse width can be modulated by a signal applied to the control voltage terminal. Figure 13 shows the circuit, and in Figure 14 are some waveform examples.
Modulator the output pulse width by the signal applied to the control voltage terminal.
This application uses the timer connected for astable operation, as in Figure 15, with a modulating signal again applied to the control voltage terminal. The pulse position varies with the modulating signal, since the threshold voltage and hence the time delay is varied. Figure 16 shows the waveforms generated for a triangle wave modulation signal.
Using astable operation vary the pulse position with a modulating signal applied to the control voltage terminal.
The frequency of oscillation is:
An oscillator with a 50% duty cycle output.