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# 555 Timers

Updated: Feb 24

Course: ME353 Mechatronics with Prof. Cusack

Purpose: Learn how to use 555 Timers

555 Timers can be used in many modes (astable, monostable, PWM, and more). The 555 timer chip is called so because it has five 5 kiloOhm resistors in series. It uses one flip flop and two comparators to charge and discharge an RC circuit which makes it great for systems with periods things like blinking LED lights or a timed delay response.

Vc = Vs (1 - e^(-t/RC))

Vc = voltage across the capacitor

Vs = voltage source

RC = time constant tau

t = time

Changing the resistor and capacitor values allows for changes in the time constant, tau, value. This is the time it takes for the system to charge the capacitor from an initial voltage to ~63% of the initial voltage or the time it takes for the system to discharge the capacitor to ~37% of its initial voltage. For instance, increasing R and C values by choosing different resistors and capacitors will result in an increased time constant meaning the system will take longer to charge/discharge.

The 555 timers in the chip function as voltage divisions for the comparators. 2/3 of the initial voltage in the 555 timer gets compared to the voltage in the external RC circuit whose period is being timed. Once the voltage from the external circuit exceeds 2/3 of the initial voltage from the timer, the comparator in the timer tells the output to switch from a 0 to a 1. Effectively, the comparators compare a nominal value from an RC circuit to a trigger value determined by 2/3 or 1/3 of the 555 timers initial voltage to flip flop an output signal.

For the monostable mode, you get a single pulse. For this mode, once the timer gets a trigger, it turns on for time T = 1.1RC

. The time is derived from calculating how long it takes for a system to get to 0.66 of the initial voltage. In the monostable mode, triggering the timer means going from a high signal to a low signal.

Here's a video of an LED being triggered in monostable mode by a push button. The LED only turns on for a period of time when I trigger the push button.

For astable mode, the timer uses the output signal to compare to 1/3 and 2/3 Vcc. This means that the capacitor charges and discharges between 1/3 and 2/3 of the initial voltage across the resistors in the chip. This automatic trigger of the charge and discharge generates an output square wave. The period of this signal is a function of the resistors connected to pins 6 and 7.

Here's a video of an LED turning on and off in astable mode. The LED switches between on and off periodically.

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