Using 555 Timer/Oscillator

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The NE555 timer is one of the most popular Integrated Circuit (IC) out there.  It is sold since 1971, so it’s not exactly a new IC but it still does his job very well!  Even today, over a billion units are produced every year!

555-picture

The 555 timer is an integrated circuit that can be used to produce different kinds of signal:

  • Monostable: Produce a one-shot signal, of a precise duration, when triggered.
  • Astable: Autonomously (without trigger) produces a flip-flop signal (On/Off/On/Off…) or a square wave if you prefer.
  • Bistable: Another flip-flop mode, but that waits for an external trigger to change its state from high to low and low to high.

Pin Layout

The 555 has 8 pins.

555-pin-layout(Picture taken from here)

How to connect the 555 timer pins:

Pin #1: Is the chip GND and should be connected to GND.
Pin #3: Is the chip output and will always be either high (VCC) or low (GND).
Pin #5: Gives you access to what the chip considers as being 2/3 of the input voltage.  You usually just put a 10nF capacitor between that pin and GND.
Pin #8: Is the chip power pin and should be connected to VCC (usually 4.5 to 15 Volts).

This leaves us with a total of 4 pins to tell the 555 timer what kind of signal we want him to produce: Pins #2 (Trigger), #4 (Reset), #6 (Threshold) & #7 (Discharge).
We will look at how to use these pins to use the 555 timer in its different modes.

Monostable signal

555-monostable Picture taken from here.

Output signal

monostable
In monostable mode, the 555 timer will produce a one-shot signal, of a precise duration, when triggered. That signal will be repeated every time we “manually” trigger the chip.
That’s the 555 timer mode you would use to turn on the lights inside your car, for 1 minute, when someone opens one of its doors.

The duration (in seconds) of the pulse/signal can be estimated to 1.1 X RC where R is in Ohms and C is in Farads.
So, by changing the value of the resistor (R) and the capacitor (C), you can specify the duration of the signal.

Example: 
   R = 1M = 1,000,000Ohms
   C = 10uF = 0.00001F

   Signal duration will be 1,000,000 Ohms X 0.00001 Farad = 10 seconds.

How to use the 4 remaining pins:
Pin #2 (Trigger): Pull that pin to ground to tell the 555 timer to generate the signal. The trigger pin must be pulled HIGH (VCC) for the rest of the time.
Pin #4 (Reset): Connect to VCC.
Pin #6 (Treshold): Connect between resistor (R) and capacitor (C) as in the diagram.
Pin #7 (Discharge): Connect between resistor (R) and capacitor (C) as in the diagram.

Astable mode

220px-555_Astable_Diagram.svg Picture taken from here.

Output signal

astable
In astable mode, the 555 timer will produce a flip-flop (square wave) signal without an external trigger.
You would use that mode for things like flashers on a car.

The duration (in seconds) of the pulse/signal can be estimated to (where R is in Ohms and C is in Farads):

t1 (in seconds) = 0.693 X (R1 + R2) X C
t2 (in seconds) = 0.693 X R2 X C
Signal frequency (in Hertz) = 1 / (0.693 X (R1 + 2 X R2) X C)

By changing the value of the resistors (R1 & R2) and the capacitor (C), you can specify the shape of the signal.

How to use the 4 remaining pins:
Pin #2 (Trigger): Connect between resistor 2 (R2) and capacitor (C) as in the diagram.
Pin #4 (Reset): Connect to VCC.
Pin #6 (Treshold): Connect between resistor 2 (R2) and capacitor (C) as in the diagram.
Pin #7 (Discharge): Connect between resistor 1 (R1) and resistor 2 (R2) as in the diagram.

Bistable signal

555-bistablePicture taken from here.

Output signal

bistable
In bistable (basic flip-flop) mode, the 555 timer’s output (Pin #3) will remain high or low for as long as we do not trigger a state change by using the RESET (pin #4) and Trigger (pin #2) pins.

How to use the 4 remaining pins:

Pin #2 (Trigger): Pull that pin to ground to tell the 555 timer to continuously output a HIGH (VCC) signal. The trigger pin must be pulled HIGH (VCC) for the rest of the time.
Pin #4 (Reset): Pull that pin to ground to tell the 555 timer to continuously output a LOW (0V) signal. The reset pin must be pulled HIGH (VCC) for the rest of the time.
Pin #6 (Treshold): Attach to GND.
Pin #7 (Discharge): Unused

 

Another good reference, about using the 555 chip, can be found here.

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