555 Circuits

Basic circuits and information about this timer/oscillator. (11/6/2006)


The 555 is a timer/oscillator and it can be used in a lot of applications like precision timing, pulse generation, time delay generation and sequential timing. Here are a few basic circuits using this integrated circuit.


Before we start, let's learn more about it:


  • It works with 4.5V to 16 Volts DC. Some version works with 4.5 to 15 Volts. A CMOS version can use less current.
  • At 5V, it uses about 5mA. (Read the datasheet)
  • Maximum output current: 200mA.
  • Maximum power dissipation: 600mA.
  • Operating temperature: 0 to +70 oC for the LM555/NE555 and -40 to +85 oC for the SA555.


    source: Philips Datasheet.


    The function of each pin connection:


  • 1 Ground: Connected to VSS, "-" or Negative.
  • 2 Trigger: A negative pulse on this pin triggers a positive pulse at the output.
  • 3 Output: Output pin drived by an internal transistor.
  • 4 Reset: Resets the IC when it is connected to GROUND. For normal operation, connect it to positive, VCC, "+".
  • 5 Control Voltage: Used to stabilize the voltage. Usually it is not connected, but it should have a capacitor connected to ground to avoid voltage variations. Also, it can be used to modulate the frecuency. (According to the datasheet)
  • 6 Threshold: Set the threshold voltage to trigger the output. (Or something like that. That is what I did understand from the datasheet). It is connected to an internal comparator.
  • 7 Discharge: A grounded output drived by a transistor that helps to discharge the capacitor at the "trigger" pin. In other words, it is a "open collector output".
  • 8 Vcc: Connected to Positive for power or "+".


    Here is the "standard" circuit to use the 555 as a monostable. "Monostable" means: "One-shoot timer when the IC is triggered.", so, basically it is a timmer.


    555 monostable timer


    The duration of the pulse is determined by R and C. The formula is: T = 1.1*R*C (That means: 1.1 multiplied by the value of the resistor, multiplied by the value of the capacitor).


    For the astable (oscilator) operation of the 555, here is the circuit needed:


    555 oscillator astable timer


    The formula to calculate the frecuency is: F= 1.49 / (R1+2R2) * C
    It means: The frecuency is equal to the 1.49 divided between the result of the value of R1 plus twice the value of R2 multiplied by the value of C.


    The minimum value of R2 is 3k, that means, do not use any value like 2.2k or 1k for R2, also, do not use high values to reach pulses greater than one minute long, it can burn the 555. Some 555 can be used for long pulses, but you need to read the datasheet.


    On the astable operation, when the power is applied to the circuit, the capacitor have no charge and holds the trigger pin low and it triggers the timer, wich establishes the charge thru R1 and R2. When the capacitor charges and reaches the value of the threshold, the ouput does low and the dischage pin discharges capacitor.


    Soon, I will post many circuits using the 555.




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