555 Timer IC

 

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The 555 timer is the most well known IC (integrated circuit) in electronics. You can easily make timing and other circuits once you get familiar with 4 or 5 pins.

Links to pages with demonstration circuits are listed near the bottom of the page. The component will probably seem confusing until you build some of those basic circuits.

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Basic properties:

555 timer pin layout illustrative diagram by electronzap electronzapdotcom
555 timer pin layout illustrative diagram by electronzap electronzapdotcom

Not guaranteed to be accurate. Always verify specifications with the datasheet.

  • Supply voltage (Vcc): Min 4.5V – Max 16V
  • Max power dissipation (Pd): 600mW
  • High output: The voltage at the output is as close to the positive supply as it can get. Probably a volt or so below it.
  • Low output: The voltage at the output is connected to fairly well to ground (negative supply rail).

Pin layout below, starting top left and working counter clockwise. 

  • 1. Ground 0V reference point supply power pin. Negative supply voltage.
  • 2. Trigger pin: 1/3 or less supply voltage sets the output pin high as long as the reset pin doesn’t have a low input.
  • 3. Output: Either high (close to positive supply voltage) or low (almost direct connection to ground).
  • 4. Reset: Low input voltage to reset pin sets the output pin low no matter what. 
  • 5. Control pin: You can make some adjustments to the internal voltage divider (not done much). Common to add a small capacitor headed to ground to help stabilize the voltage. 
  • 6. Threshold: 2/3 supply voltage applied to this pin sets the output and discharge pins to low.
  • 7. Discharge: Connects to ground when output is low. Disconnects electrically altogether (like an off switch) when the output is high.  
  • 8. Supply. Positive side of the power supply. 
Three equal value resistor voltage divider for one and two thirds supply voltage divider schematic diagram by electronzap electronzapdotcom
Three equal value resistor voltage divider for one and two thirds supply voltage divider schematic diagram by electronzap electronzapdotcom

Internally, the 555 timer has 3 equal value resistive areas (some claim 5K each, which they say is where the number 555 comes from) that form a voltage divider.

The internal circuitry of the 555 looks at those 1/3 and 2/3 supply voltages to  determine when an input signal has more or less than 1/3 or 2/3 supply voltage applied to it.

Pin 5 can adjust those voltages. Often an approx. 10nF capacitor is connected to pin 5 and ground to helps stabilize those voltages. 

A capacitor should be attached across the supply pins (1 and 8) to help stabilize the supply voltage as far as the 555 time is concerned. 

Video

Quick look at 555 NE555 timer flip flop circuit from integrated circuit kit by electronzap review

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Quicker lessons:
 

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