The simplest 555 oscillator takes output pin 3 to capacitor C1 via resistor R1.
When the circuit is turned on, C1 is uncharged and output pin 3 is HIGH. C1 charges via R1
and when Pin 6 detects 2/3 rail voltage, output pin 3 goes LOW. R1 now discharges
capacitor C1 and when pin 2 detects 1/3 rail voltage, output pin 3 goes HIGH to repeat the
cycle.
The amount of time when the output is HIGH is called the MARK and the time when the
output is LOW is called the SPACE.
In the diagram, the mark is the same length as the space and this is called 1:1 or 50%:50%.
If a resistor and capacitor (or electrolytic) is placed on the output, the result is very similar to
a sinewave.
C1 to POSITIVE RAIL
C1 can be connected to the positive rail. This is not normal practice, however it does work.
The output frequency changes when the capacitor is changed from the negative rail to the
positive rail. Theoretically the frequency should not change, but it does, and that's why you
have to check everything. The frequency of operation in this arrangement is different to
connecting the components via pin7 because pin3 does not go to full rail voltage or 0v. This
means all the output frequencies are lower than those in the "555 Frequency Calculator."
The table shows the frequency for the
capacitor connected to the 0v rail and 12v
rail:
C1 to 0v rail C1 to 12v rail
1k 1n 505kHz 1k 1n 255kHz
1k 10n 115kHz 1k 10n 130kHz
1k 100n 23kHz 1k 100n 16kHz
10k 1n 112kHz 10k 1n 128kHz
10k 10n 27kHz 10k 10n 16kHz
10k 100n 3700Hz 10k 100n 1600Hz
CHANGING THE MARK-SPACE RATIO
This ratio can be altered by adding a diode and resistor as shown in the following diagrams.
In the first diagram, the 555 comes ON ("fires-up") with pin 3 low and pin 2 immediately
detects this low and makes pin 3 HIGH. The 10n is quickly charged via the diode and 4k7
and this is why the MARK is "short." When the capacitor is 2/3Vcc, pin 6 detects a HIGH
and the output of the 555 goes LOW. The 10n is discharged via the 33k and this creates the
long-duration SPACE (LOW). The second diagram creates a long-duration HIGH:
HOW TO REMEMBER THE PINS:
THE FASTEST 555 OSCILLATOR
The highest frequency for a 555 can be obtained by connecting the output to pins 2 and 6. This arrangement takes about 5mA and produces an output as shown. The max frequency will depend on the supply voltage, the manufacturer, and the actual type of 555 chip.
View the output on a CRO. Our 555 "Test Chip" produced a frequency of 300kHz at 5v and also at 12v. (CMOS versions will operate at a higher frequency.) Note the very short LOW TIME.
When the circuit is turned on, C1 is uncharged and output pin 3 is HIGH. C1 charges via R1
and when Pin 6 detects 2/3 rail voltage, output pin 3 goes LOW. R1 now discharges
capacitor C1 and when pin 2 detects 1/3 rail voltage, output pin 3 goes HIGH to repeat the
cycle.
The amount of time when the output is HIGH is called the MARK and the time when the
output is LOW is called the SPACE.
In the diagram, the mark is the same length as the space and this is called 1:1 or 50%:50%.
If a resistor and capacitor (or electrolytic) is placed on the output, the result is very similar to
a sinewave.
C1 to POSITIVE RAIL
C1 can be connected to the positive rail. This is not normal practice, however it does work.
The output frequency changes when the capacitor is changed from the negative rail to the
positive rail. Theoretically the frequency should not change, but it does, and that's why you
have to check everything. The frequency of operation in this arrangement is different to
connecting the components via pin7 because pin3 does not go to full rail voltage or 0v. This
means all the output frequencies are lower than those in the "555 Frequency Calculator."
The table shows the frequency for thecapacitor connected to the 0v rail and 12v
rail:
C1 to 0v rail C1 to 12v rail
1k 1n 505kHz 1k 1n 255kHz
1k 10n 115kHz 1k 10n 130kHz
1k 100n 23kHz 1k 100n 16kHz
10k 1n 112kHz 10k 1n 128kHz
10k 10n 27kHz 10k 10n 16kHz
10k 100n 3700Hz 10k 100n 1600Hz
CHANGING THE MARK-SPACE RATIO
This ratio can be altered by adding a diode and resistor as shown in the following diagrams.
In the first diagram, the 555 comes ON ("fires-up") with pin 3 low and pin 2 immediately
detects this low and makes pin 3 HIGH. The 10n is quickly charged via the diode and 4k7
and this is why the MARK is "short." When the capacitor is 2/3Vcc, pin 6 detects a HIGH
and the output of the 555 goes LOW. The 10n is discharged via the 33k and this creates the
long-duration SPACE (LOW). The second diagram creates a long-duration HIGH:
HOW TO REMEMBER THE PINS:
THE FASTEST 555 OSCILLATOR
The highest frequency for a 555 can be obtained by connecting the output to pins 2 and 6. This arrangement takes about 5mA and produces an output as shown. The max frequency will depend on the supply voltage, the manufacturer, and the actual type of 555 chip.
View the output on a CRO. Our 555 "Test Chip" produced a frequency of 300kHz at 5v and also at 12v. (CMOS versions will operate at a higher frequency.) Note the very short LOW TIME.
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