555 Timer Chip - One Shot Lesson



The 555 chip is a precision timer chip that can time intervals from microseconds to hours. It has two modes:

  1. monostable
  2. astable multivibrator

Monostable means that after being triggered, there will be a time delay before the chip changes states. An example around the house would be the light in my garage - it is normally off but when I drive in, the light goes on and stays on for a minute after I close the garage door, and then goes out again. The time delay can be modified by a single resistor and capacitor connected to the chip.

Astable means that the chip output switches states, changing from on to off, at a given frequency and with a certain duty cycle. In computers, the CPU and motherboard operations are controlled by a timer chip (eg. 3 GHz processor). One complete switch from on to off to on again, is called a cycle. The length of time for one cycle is called the Period, and measured in seconds. Frequency is the amount of number of times the chip's output changes states in one second, which is the reciprocal of Period and measured in Hertz (Hz), which is cycles/second. The duty cycle is the ratio of the length of time the output is low to the length of time in one cycle (the period). The frequency and duty cycle can be controlled with two resistors and one capacitor connected to the chip.


Draw a diagram of the chip and label the pins with pin numbers and their meanings. Use the Texas Instrument datasheet.

Monostable Operation


Monostable operation can be created with the circuit shown above.

The threshold level (pin 6) is normally 2/3 of Vcc (approximately 3.33 V). The trigger level (pin 2) is normally 1/3 of VCC (approximately 1.67 V).

The trigger (pin 2) should initially be high, above the trigger level. When the trigger if high, the 555 output is low and there is a path from discharge (pin 7) to ground. Because of the path between discharge and ground, the current flows to ground, rather than charging capacitor C

The input on trigger (pin 2) is then set to low. As the voltage on the trigger drops below the trigger level (1.67V), the 555 sets the the output high, and breaks the path from discharge to ground. Capacitor C then is gradually charged through RA until the voltage across the capacitor reaches the threshold voltage of the threshold (pin 6). Once the threshold voltage is reached on pin 6 and if TRIG has returned to a high level, the output of the 555 output goes low, the path from discharge (pin 7) to ground is re-established and capacitor C discharges through pin 7.

The time delay can be calculated with the equation:

t = 1.1 x RA x C



Calculate the time delay for a 2.2 microFarad capacitor and 1.5M Ohm resistor.

t = 1.1 x 1.5M Ohms x 2.2 uF
  = 1.1 x 1,500,000 Ohms x 0.0000022 Farads
  = 3.6 seconds


Using a combination of at least 2 different capacitors and 3 different resistors. Calculate the expected time delay for each combination. Complete the table below.

Show the teacher your work.

Resistor Value (Ohms) Capacitor Value (uF) Calculated Time Delay (seconds) Measured Time Delay (seconds)
560 K 2.2    
1.5M 2.2    
3.3 M 2.2    
560 K 10    
1.5 M 10    
3.3 M 10    

Create the circuit for pictured above, with one capacitor and resistor. Show the teacher your work.

Run a test of each circuit and measure the time delay. Complete the table above.

Use a multi-meter connected from ground to pin 6 to watch the voltage on the capacitor slowly rise as it charges through the resistor when the output is high, and then quickly discharge through the discharge pin (7), as the 555 output is set low.

last updated 05-Mar-2006