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THE 555 TIMER IC


The 555 is commonly called a TIMER IC. It is an 8-pin chip and has a number of different identifications: LM555CN from National and SE555/NE555 from Signetics are just two manufacturers. These numbers all refer to the most common and cheapest version, we will call the 555.

The 555 contains more than 28 transistors and it is basically a chip containing a number of building blocks that end up very similar to an oscillator without the TIMING COMPONENTS.
It needs two or three external components to produce an oscillator capable of operating at
a frequency from 1Hz to 500kHz. When it oscillates at a frequency less than 1Hz, the circuit is called a Timer or Delay. The chip also has a pin (pin 2) that prevents the chip from starting the Timing cycle until it is taken LOW. Another pin (pin 4) stops the chip from oscillating (or continuing with a delay-time) when it is taken LOW and a pin (pin 5) that can adjust the mark-space ratio of the waveform.

The diagrams below show the names of each pin and a simplified block diagram of the internal workings.



THE FUNCTION OF EACH PIN
Pin 1 Ground

The ground (or common) pin is connected to the 0v rail - commonly called the negative rail or EARTH rail.

Pin 2 Trigger
This pin connects to the lower comparator and is used to set the control flip flop. When it is taken LOW, it causes the output to go HIGH. This is the beginning of the timing sequence for a monostable operation. Triggering is accomplished by taking the pin below 1/3 of rail voltage - in digital terms, this is called a LOW. The action of the trigger input is level-sensitive, allowing slow rate-of-change waveforms, (as well as pulses), to be used as trigger sources. The trigger pulse must be of shorter duration than the time interval determined by external R and C. If this pin is held low for a longer period of time, the output will remain high until the trigger input is high again.
If the trigger input remains lower than 1/3 rail voltage for longer than the timing cycle, the timer will re-trigger upon termination of the first output pulse. When the timer is used in monostable mode with trigger pulses longer than the output pulse, the trigger duration must be shortened by external circuitry. The minimum pulse-width for reliable triggering is about 10uS.

Pin 3 Output

The output of the 555 comes from a high-current totem-pole stage. This provides both sinking and sourcing current. The high-state output voltage is about 1.7 volts less than the supply. At 15 volt supply, the chip can sink 200mA with an output-low voltage level of 2 volts. High-state level is 13.3 volts. Both rise and fall times of the output waveform are quite fast, typical switching being 100nS. To make the output HIGH, the TRIGGER PIN (pin 2) is momentarily taken from a HIGH to a LOW. This causes the output to go HIGH. This is the only way the output can be made to go high. The output can be returned to a LOW by making the THRESHOLD PIN (Pin 6) go from a LOW to a HIGH. The output can also be made to go LOW by taking the RESET PIN to a LOW state.

Pin 4 Reset

This pin is used to make the OUTPUT PIN (Pin 3) LOW. The reset pin must go below 0.7 volt and it needs 0.1mA to reset the chip.
The RESET PIN is an overriding function. It will force the OUTPUT PIN to go LOW regardless of the state of the TRIGGER PIN (Pin 2). It can be used to terminate an output pulse prematurely, to gate oscillations from "on" to "off." The pin is active when a voltage level between 0v and 0.4 volt is applied to it. When not used, it is recommended that the RESET PIN be tied to the positive rail to avoid the possibility of false resetting.

Pin 5 Control Voltage

This pin allows direct access to the 2/3 voltage-divider point. This is the reference level for the upper comparator. When the 555 timer is used in a voltage-controlled mode, the voltage-controlled operation ranges from about 1 volt below rail-voltage to 2 volts above ground (0v). Voltages can be safely applied outside these limits, but they should be confined to between 0v and rail voltage. By applying a voltage to this pin, it is possible to vary the timing of the chip independently of the RC network. The control voltage may be varied from 45 to 90% of the Vcc in the monostable mode, making it possible to control the width of the output pulse independently of RC. When used in the astable mode, the control voltage can be varied from 1.7v to the full Vcc. Varying the voltage in the astable mode will produce a frequency modulated (FM) output. If the control-voltage pin is not used, it should be bypassed to ground, with a 10n capacitor to prevent noise entering the chip.

Pin 6 Threshold

Pin 6 is one input to the upper comparator (the other is pin 5). It makes the OUTPUT PIN go LOW.
To make the output go LOW, the Threshold pin is taken from a LOW to a level above 2/3 of rail voltage. This pin is level-sensitive, allowing slow rate-of-change waveforms to be detected. A dc current, termed the threshold current, must also flow into this pin from the external circuit. This current is typically 0.1µA, and will determine the upper limit of total resistance allowable from pin 6 to rail. For 5v operation the resistance is 16M. For 15v operation, the maximum resistance is 20M.

Pin 7 Discharge
This pin is connected to the open collector of an NPN transistor. The emitter goes to ground. When the transistor is turned "on,'" pin 7 is effectively shorted to ground. The timing capacitor is connected between pin 7 and ground and is discharged when the transistor turns "on". The conduction state of this transistor is identical in timing to that of the output stage. It is "on" (low resistance to ground) when the output is LOW and "off" (high resistance to ground) when the output is HIGH.
Maximum collector current is internally limited by design, so that any size capacitor can be used without damage to the chip. In certain applications, this open collector output can be used as an auxiliary output terminal, with current-sinking capability similar to the OUTPUT (pin 3).

Pin 8 Rail
This pin (also referred to as Vcc) is the positive supply voltage pin for the 555. Supply-voltage operating range is +4.5 volts to +16 volts. The chip will operate over this voltage range without change in timing period. The only change is the output drive capability, which increases in current as the supply voltage is increased.