Difference between revisions of "555 Timer"
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The '''trigger''' and '''reset''' inputs are level-sensitive [[Digital#Active_Low_Input|active low]] inputs. 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''' (pin 3) 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. |
The '''trigger''' and '''reset''' inputs are level-sensitive [[Digital#Active_Low_Input|active low]] inputs. 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''' (pin 3) 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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In its initial state (assuming ''threshold'' is ''low'' and ''trigger'' is ''high''), the 555 timer's internal flip-flop connects the '''discharge''' (pin 7) to the ground and sets the output to ''low''. Each time the trigger is pulled down under 1/3 V<sub>CC</sub>, the flip-flop will break the ''discharge'' pin's connection to ground, and set the output to ''high''. It will hold this state until something forces the ''threshold'' pin's voltage equal to 2/3 V<sub>CC</sub>, which will reset the flip-flop. |
In its initial state (assuming ''threshold'' is ''low'' and ''trigger'' is ''high''), the 555 timer's internal flip-flop connects the '''discharge''' (pin 7) to the ground and sets the output to ''low''. Each time the trigger is pulled down under 1/3 V<sub>CC</sub>, the flip-flop will break the ''discharge'' pin's connection to ground, and set the output to ''high''. It will hold this state until something (usually a capacitor) forces the ''threshold'' pin's voltage equal to 2/3 V<sub>CC</sub>, which will reset the flip-flop. |
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==Monostable and Astable mode== |
==Monostable and Astable mode== |
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The 555 family of timer chips can be used in either '''monostable'' mode or '''astable''' mode. |
The 555 family of timer chips can be used in either '''monostable'' mode or '''astable''' mode. In the monostable or "one shot" mode, each time the 555 timer is triggered, the output will go ''high'' for a specified amount of time, then return to ''low'' and await another trigger signal. In the astable mode, the timer triggers itself periodically and becomes an oscillator, sending out a train of pulses. |
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Revision as of 13:53, 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 555 timer also has a flip-flop, which is controlled by the two comparators and the reset input.
The trigger and reset inputs are level-sensitive active low inputs. 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 (pin 3) to low. If the reset input is not going to be used, it should be wired to VCC to prevent false signals.
In its initial state (assuming threshold is low and trigger is high), the 555 timer's internal flip-flop connects the discharge (pin 7) to the ground and sets the output to low. Each time the trigger is pulled down under 1/3 VCC, the flip-flop will break the discharge pin's connection to ground, and set the output to high. It will hold this state until something (usually a capacitor) forces the threshold pin's voltage equal to 2/3 VCC, which will reset the flip-flop.
Monostable and Astable mode
The 555 family of timer chips can be used in either monostable mode or astable' mode. In the monostable or "one shot" mode, each time the 555 timer is triggered, the output will go high for a specified amount of time, then return to low and await another trigger signal. In the astable mode, the timer triggers itself periodically and becomes an oscillator, sending out a train of pulses.
