Electronic Engineering Glossary Terms

555 Monostable Multivibrator

MULTIVIBRATOR. A form of relaxation oscillator which comprises two stages that are coupled so that the input of one is derived from the output of the other. A multivibrator is basically two amplifier circuits arranged with regenerative feedback. One of the amplifiers is conducting while the other is cut off. In this case both amplifier circuits are contained within a single 555 IC.

There are three types of multivibrators: ASTABLE, MONOSTABLE, BISTABLE.

555 Monostable Multivibrator

The Monostable multivibrator circuit has no stable state. With no external signal applied, the internal transistors alternately switch from cutoff to saturation at a frequency determined by the RC time constants of the coupling circuits. So an Astable multivibrator is an oscillator which could either be used as a pulse generator or square-wave generator depending on the value of the resistor and capacitors used in the circuit. A related transistor circuit configured as an Transistor Monostable multivibrator.

Circuit Description:
Power: With any integrated circuit always Bypass the Vcc pin to ground via a capacitor, normally a 0.1uF ceramic capacitor [not shown in the circuit above]. Pin 8 is Vcc and Pin 1 is ground; however the capacitor should be connector to Vcc and taken to the closest possible PCB ground, which may not necessarily be pin 1 of the IC. The 555 IC may be operated with any voltage between 4.5 volts and 16 volts [see data sheet info below].
Control Voltage: Pin 5 of the 555 is an input and is not used in this application. However the control voltage should be bypassed using a 0.01uF between pin 5 and ground. The capacitor will filter noise on the internal resistor ladder and fix the voltage level on pin 5 so the input does not float to a different voltage.
Output: Pin 3 is the output of the circuit. The output will toggle between ground and Vcc each time a trigger is applied to the circuit. With Vcc set at 5 volts the output will switch to a minimum voltage of 2.75 volts or a typical voltage of 3.3 volts. Using a 15 volt Vcc as another example the 555 will typically reach 12.5 volts [200mA source], 13.3 volts [100mA source]. The rise and fall times of the output pulse will be about 100nS.
Trigger: Pin 2 is connected to the input trigger voltage for the circuit. Each time a Trigger is applied the output will toggle high for a predetermined amount of time, based on the value of the resistor [R1] and capacitor [C1] used. A high to low transition [low pulse] on the trigger input will cause the output to switch; however, the trigger should be taken high before the output pulse times out. In other words the output pulse width should be longer than the input trigger width, or the trigger pulse must have a shorter duration than the RC time constant of R1 and C1. The minimum trigger pulse width is 1uS.

555 Monostable Waveforms

The top waveform is the input trigger pulse applied to pin 2. The center waveform represents the charging voltage on capacitor C1. While the bottom voltage waveform shows the resultant output on pin 2.
Threshold: Pin 6 is also connected to C1 which develops the voltage to the pin. The Threshold is also connected to the Trigger pin which also sees the same voltage.
Discharge: Pin 7 is connected to capacitor C1. Once charged the capacitor discharges into pin 7.
Reset: Pin 4 is connected to Vcc in the graphic above only as an example when not using the reset pin. However in the monostable mode, the 555 may be reset from Voh to Vol on the negative slope of a reset pulse. Once the rest is returned high, the output will remain low only if the trigger is high. If the trigger is low when reset is returned high, the output will go high. The minimum reset pulse width is 2uS.

Timing Equations:
Charge Time; t1 = 0.693*(R1 + R2)*C1
Discharge Time; t2 = 0.693*R2*C1
Total period; T = t1 + t2 = 0.693*(R1 + 2*R2)*C1
Oscillation frequency; f = 1/T = 1.44/[(R1 + 2*R2)*C]
Duty cycle = D = R2/(R1 + 2*R2)

Generic Part Numbers: LM555, SE555, NE555, CMOS; LMC555
Generic Packages: 8-pin SOIC, 8-pin SOP, 8-pin DIP
Check the data sheet for the complete part numbers.
The 555 Timer has an operating temperature range of 0C to 70C
The 555 Timer has an storage temperature range of -65C to 150C
Also refer to How to Derate an IC.

Dual Version: Dual 555 timer in a single IC; LM556, SE556, NE556
Generic Packages: 14-pin SOIC, 14-pin DIP.

Circuit Enhancement; The schematic above shows the trigger signal running to pin 2 of the timer. A Pull-up Resistor can be added between pin 2 and Vcc to insure that the pin is set high with out any input or when not being pulled low. In addition a small 0.001uF capacitor could also be added between the input trigger and pin 2. The capacitor would smooth out any variations with the rising edge of the incoming trigger signals, which might have caused a false trigger. The actual values of the resistor and capacitor are not that important as neither one of them effect the timing of the device.

Of course the RC components that make up the input voltage filter do effect timing, but only indirectly. The filter changes the point that the input reaches the voltage threshold of the trigger point. The addition of the capacitor on the input causes a delay in the voltage reaching a particular value because the capacitor needs to discharge to that value.
In the monostable mode, the device is triggered on the negative slope of a trigger pulse. The trigger pulse must be of a shorter duration than the RC time interval [R1 x C1]. The minimum pulse width is 1 microsecond, set by R1 and C1 and has nothing to do with the pull-up resistor or input capacitor just added.
The voltage across the input capacitor increases [or decreases] at an exponential rate based on the capacitor value and a resistor value [which in this case is the output resistance of the device generating the trigger voltage]. The Time Constant, or product of the resistor and capacitor determine the charging rate of the capacitor [determined by the equation found by the link], and so change the point at which the 555 sees the trigger pulse fall below the threshold point.