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FUNCTION BLOCK
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DESCRIPTION
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THIS USES LOGIC DEVICES
Logic 1 = High = +V
Logic 0 = Low = 0V
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- This astable is based upon 2 LOGIC gates wired as inverters.
- It is similar in function to the NOT gate astable but can be gated, ie can be switched on or off with a logic signal.
- The speed of this switching can be controlled by 2 components.
- Useful for producing sound and high speed clocks.
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CIRCUIT DETAILS
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- The circuit is basically a LOGIC gate astable with an enable input.
- The output can only sink/source about 2-3mA. If you want to drive a speaker to produce sound, you will need to add a driver
There are two types that are easy to use, the NAND works woith a high logic signal and the NOR with a low:
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Input
- ENABLE
A high input will enable the astable while a low input will stop it functioning.
The 100K resistor is only needed if the enable is connected to a switch. If it is connected to another logic gate, then it can be removed.
Output
The output will generate a square wave at the set frequency when the ENABLE input is high.
When the ENABLE is low, the output will stay low.
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GATED NAND ASTABLE
Circuit Diagram
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Input
- ENABLE
A low input will enable the astable while a high input will stop it functioning.
The 100K resistor is only needed if the enable is connected to a switch. If it is connected to another logic gate, then it can be removed.
Output
The output will generate a square wave at the set frequency when the ENABLE input is high.
When the ENABLE is low, the output will stay low.
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GATED NOR ASTABLE
Circuit Diagram
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In both examples:
Frequency (Hz) = 1 / (2.2 x R1 x C1)
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CALCULATING THE FREQUENCY...
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The frequency of the output can be calculated from the following formula:
Frequency (Hz) = 1 / (2.2 x R1 x C1)
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 frequency you require:
1Hz to 100Hz = 470nF,
100Hz to 10KHz = 10nF,
10KHz to 1MHz= 100pF
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2
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Now rearrange the formula to work out the required resistor value R1
R1 = 1 / (2.2 x Frequency x C1)
To ensure that the caluclations are correct, use either Mohms & uF or Ohms & Farads as units
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3 |
Now make:
or as close as you can get. No higher than 4.7M
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PACKAGE DETAILS
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Both the NOR and NAND gates come in a 14pin DIL package. Any gate can be used for any purpose. CMOS types have been chosen since their supply voltage can range from 3 to 15v.
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4001B
2ip NOR GATE
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4011B
2ip NAND GATE
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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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Unused gates should have their inputs connected to 0v
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| Written by Phil Townshend - 2010 |
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www.edutek.ltd.uk - Working Electronics For Students & Teachers
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