Difference between revisions of "555 Timer"
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Inside the 555 timer is a voltage divider the divides +V<sub>CC</sub> into thirds. A voltage comparator compares the '''trigger''' input (pin 2) with 1/3 V<sub>CC</sub>, and another comparator compares the '''threshold''' (pin 6) with 2/3 V<sub>CC</sub>. The point at 2/3 V<sub>CC</sub> on the voltage divider is connected to the '''control voltage''' (pin 5). This pin can be used to modify the values of 1/3 V<sub>CC</sub> and 2/3 V<sub>CC</sub> without having to change V<sub>CC</sub>. However, if this input is not going to be used, it should be connected to a bypass capacitor (0.01uF) to protect it from noise. |
Inside the 555 timer is a voltage divider the divides +V<sub>CC</sub> into thirds. A voltage comparator compares the '''trigger''' input (pin 2) with 1/3 V<sub>CC</sub>, and another comparator compares the '''threshold''' (pin 6) with 2/3 V<sub>CC</sub>. The point at 2/3 V<sub>CC</sub> on the voltage divider is connected to the '''control voltage''' (pin 5). This pin can be used to modify the values of 1/3 V<sub>CC</sub> and 2/3 V<sub>CC</sub> without having to change V<sub>CC</sub>. However, if this input is not going to be used, it should be connected to a bypass capacitor (0.01uF) to protect it from noise. |
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The '''trigger''' and '''reset''' inputs are level-sensitie [[Digital#Active_Low_Input|active low]]. To activate the trigger, the voltage on the pin must be pulled down to under 1/3 V<sub>CC</sub>. To reset the timer, the voltage on the pin must be pulled under 0.4V. The ''reset'' input will override other inputs and set the output to "low". |
The '''trigger''' and '''reset''' inputs are level-sensitie [[Digital#Active_Low_Input|active low]]. To activate the trigger, the voltage on the pin must be pulled down to under 1/3 V<sub>CC</sub>. To reset the timer, the voltage on the pin must be pulled under 0.4V. The ''reset'' input will override other inputs and set the output to "low". If the ''reset'' input is not going to be used, it should be wired to V<sub>CC</sub> to prevent false signals. |
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==Monostable and Astable mode== |
==Monostable and Astable mode== |
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Revision as of 13:14, 3 July 2006
Connection digram for the 555 IC timer (click for larger image).
Inside the 555 timer is a voltage divider the divides +VCC into thirds. A voltage comparator compares the trigger input (pin 2) with 1/3 VCC, and another comparator compares the threshold (pin 6) with 2/3 VCC. The point at 2/3 VCC on the voltage divider is connected to the control voltage (pin 5). This pin can be used to modify the values of 1/3 VCC and 2/3 VCC without having to change VCC. However, if this input is not going to be used, it should be connected to a bypass capacitor (0.01uF) to protect it from noise.
The trigger and reset inputs are level-sensitie active low. To activate the trigger, the voltage on the pin must be pulled down to under 1/3 VCC. To reset the timer, the voltage on the pin must be pulled under 0.4V. The reset input will override other inputs and set the output to "low". If the reset input is not going to be used, it should be wired to VCC to prevent false signals.
Monostable and Astable mode
The 555 family of timer chips can be used in either monostable mode or astable' mode.
