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555 |
In Brief .. This astable circuit uses a 555 Timer Integrated Circuit (I.C.) to produce an output voltage that continuously switches between "off and on". It is not stable in either state. This signal can be used to switch, or pulse other devices. The 555 Timer can sink or source an output with current up to 200mA (see OP1). This makes it a useful device when used as part of a bigger system. The I.C. is built on a small element called Silicon. Its circuit consists of 22 transistors, 2 diodes and 16 resistors and its package is smaller than one square centimetre. The I.C. is commonly used in electronic devices and only costs a few more pence than a single transistor. The 555 Integrated Circuit is also described in P8, where it is used in an monstable circuit. In this application it can make its output turn "on" for a brief period, before returning back to rest in its "off" state.
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Circuit Diagram |
How the circuit works.... The 555 Timer uses R, Ra and C to set the frequency, or rate at which the output turns on and off. (Note: The connection marked "I/P" must be taken to + V before this will start to happen). The capacitor C is charged up by the current flowing through Ra and R (see P3 for further details). Whilst it is charging, the 555 output is equal to the power supply voltage: this is known as the high period. When the voltage stored in the capacitor reaches 2/3 of the power supply voltage, the 555 timer starts to discharge the capacitor via resistor R. Whilst this is happening the 555 output goes to 0 volts: this is known as the low period and this cycle repeats. If R (eg 100K) is much larger than Ra (eg 1K), then the on and off times are nearly equal. This is known as an even mark/space ratio, as shown below.
Calculating the On/Off rate (frequency) Frequency (in Hz) = 1 / cycle = 0.72 / R x C
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