Circuit Tutorials

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Prof ETK (ELECTRONIC TUTOR KITS)

The Best in Practical Electronics Training
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Professor ETK’s 555 Astable Kit has been designed to both teach and entertain the relative newcomer to Electronics.

·     DANGER TRAIL: While the components are fairly robust, sometimes there is a danger of component failure. The most common fault is the incorrect connection of the power leads to the IC. The following wiring mistakes can, however, result in the sudden death of a component. * The battery / power supply leads being connected to the wrong terminals.

* Pin 7 of a 555 being shorted to +V.

* Pin 3 of a 555 being shorted to either +V or -V (0V)

* LED legs connected the wrong way around Electrolytic Capacitor (C1) legs connected the wrong way around.

* Transistor legs connected the wrong way around.

The Internal Components

The 555 consists of a reference voltage divider producing two reference voltages: 33% of the supply voltage and 66% of the supply voltage. An op-amp comparator connects to each of these references, with the other inputs connecting to the external pins 6 and 2. Both comparator outputs connect to a flip-flop, which can be RESET by a 0V pulse on pin 4. The output of the flip-flop goes to an inverting current-gain amplifier as well as to the base of a crowbar transistor. When the flip-flop output is HI, the transistor shorts pin 7 to 0V and output pin 3 goes LO. Pin 5 connects to the 66% voltage reference point and allows one to externally alter both reference voltages on the voltage divider.

·         The 555 can be connected so as to have no stable state (astable) by connecting pins 6 and 2 to the charging capacitor and by connecting pin 7 between two charging resistors. What happens is this... when power is first applied to the circuit; C1 is discharged so pin 2 is LO. This causes the flip-flop to set, the output goes HI, the transistor gets no base current and so C1 starts charging up through both R1 and R2. When the 66% voltage level is reached, the comparator switches, the flip flop resets, pin 3 goes LO, pin 7 crowbars to 0V and C1 starts discharging through R2 ONLY. When C1 reaches the 33% voltage threshold, pin 2 sets the flip-flop again because it is directly connected to the capacitor and C1 starts charging up again. The process will continue charging and discharging until either the power is cut, or if the flip-flop is manually reset with a 0V potential on pin 4. The graph opposite shows the frequencies obtained for various capacitor and resistance values. It can be seen that the MARK time and the SPACE time will not be the same since R1 and R2 are used to charge C1, but only R2 is used to discharge it. It can also be quickly seen that the SPACE time will always be shorter than the MARK time.  The following formulae apply:    The formula is a little simpler if R1 and R2 are identical, because the MARK will be exactly double the SPACE. If a diode is used to short out R2 in the charging direction, it is possible to create a MARK/SPACE ratio of 1:1.

The 555 can be connected so as to have no stable state (astable) by connecting pins 6 and 2 to the charging capacitor and by connecting pin 7 between two charging resistors. What happens is this: when power is first applied to the circuit; C1 is discharged so pin 2 is LO. This causes the flip-flop to set, the output goes HI, the transistor gets no base current and so C1 starts charging up through both R1 and R2. When the 66% voltage level is reached, the comparator switches, the flip flop resets, pin 3 goes LO, pin 7 crowbars to 0V and C1 starts discharging through R2 ONLY. When C1 reaches the 33% voltage threshold, pin 2 sets the flip-flop again because it is directly connected to the capacitor and C1 starts charging up again. The process will continue charging and discharging until either the power is cut, or if the flip-flop is manually reset with a 0V potential on pin 4. The graph opposite shows the frequencies obtained for various capacitor and resistance values. It can be seen that the MARK time and the SPACE time will not be the same since R1 and R2 are used to charge C1, but only R2 is used to discharge it. It can also be quickly seen that the SPACE time will always be shorter than the MARK time.  The following formulae apply:    The formula is a little simpler if R1 and R2 are identical, because the MARK will be exactly double the SPACE. If a diode is used to short out R2 in the charging direction, it is possible to create a MARK/SPACE ratio of 1:1.

Circuit Schematic of Astable Multivibrator

This circuit is a variation of the Mono-stable. Whereas the Mono-stable has one stable and one tempory state, the Astable has no stable state and is free runing from the one state to the other continuously.

Component overlay on PCB