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FUNCTION BLOCK
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DESCRIPTION
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- This monostable is based upon a 555 timer IC.
- A monostable produces an output pulse (switches on for a length of time, then off).
- The time of the pulse can be set according to 2 components.
- It is usually triggered by an input (switch or logic gate).
- Useful for switching on devices for certain lengths of time.
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CIRCUIT DETAILS
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Inputs
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Circuit Diagram
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- Trigger
This is activated by connecting it to 0v (active low). If the trigger stays low, the output will stay on. If connected to a switch then it must have a pull-up resistor connected to it.
- Reset
When connected to 0v, the timer will not operate.
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Output
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The output will go high then low for the pre-determined time. The output can only sink/source about 25mA, enough for an LED, buzzer or low wattage bulb.
You will need to add a driver to drive bigger devices such as lamps and motors.
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CALCULATING THE TIME...
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The duration of the pulse is calculated as follows:
Time (secs) = 1.1 x C1 x R1
It can be tricky to choose values to start with, so try the following:
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1
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Choose one of the following values for the capacitor, depending on the time you require:
Less than 1 sec. = 1uF,
Up to 120 secs. = 100uF,
Over 120 secs. = 1000uF
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2
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Now rearrange the formula to work out the required resistor value R1
R1 = Time / (1.1 x C1)
To ensure that the caluclations are correct, use either Mohms & uF or Ohms & Farads as units
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Try using a preset  or a potentiometer  for R1. Then you can adjust the time manually. |
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PACKAGE DETAILS
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The 555 timer comes in an 8pin DIL package. |
Pin
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When using ICs, it is always best to use an IC socket so the IC can be removed easily if needed.
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www.edutek.ltd.uk - Working Electronics For Students & Teachers
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