How to simulate 555 Timer as Monostable Multivibrator in Proteus

 In this electronics tutorial we show how to simulate 555 monostable timer circuit in proteus electronics design software. Before making the actual circuit on breadboard or on PCB you want to know whether the circuit will work as intended or not. This is why circuit simulation is important.

NE555 Timer & Proteus Professional

NE555 Timer is a a versatile linear IC introduced by Signetics in around 1970 which can be used in variety application. It has three operating modes- monostable multivibrator, bistable multivibrator and astable multivibrator. Application examples are linear ramp generator, frequency divider, missing pulse detector, pulse stretcher, pulse width modulation etc. As an application example of monostable multivibrator to use PIR sensor output to the input of 555 timer to generate an output pulse with certain pulse duration during which a buzzer connected at the output of 555 monostable timer can be turned on to sound alarm.

Proteus professional is a circuit simulation software with PCB design capability. It has also instruments like oscilloscope to check signals in the circuit and many other real behavior components like buzzer, PIR sensors, arduino etc.

555 monostable timer circuit

Shown below is a schematic diagram of 555 monostable timer circuit drawn in Proteus Electronics Design Software. Note that voltage input and load needs to be connected at the trigger input(TR pin 2) and output(pin3).

In the monotstable mode, the input to the timer is the Trigger(TR) pin 2 and the Output(Q) is from pin 3. In monostable multivibrator mode, when we apply a negative going pulse to the trigger input and while this pulse voltage amplitude is falling and becomes less the 1/3 of Vcc(5V) then the timer will produce a pulse at the output pin 3 and the timer returns to stable state and does not produce any further pulses at the output. This is why monostable mode of 555 timer is also called One Shot Circuit. The output pulse width is set by the resistor(R1) and (C1) and how to calculate it is shown later below. The 555 timer needs power supply which can be between 4.5V to 18V. Here we have supplied 5V to the timer, connected to supply pin 8 and ground pin 1. The Reset(R) pin 4 is active low and will reset the internal flip flop and hence the timer when a low signal is applied to this pin. That's why here we have connected to positive supply. The Control Voltage(CV) pin 5 is used to change the reference voltage input to one of the comparator inside the timer which is not required here so it is grounded via 10nF capacitor(C2) to reduce noise pickup. The Discharge(DC) pin 7 and the Threshold(TH) pin 6 are tied together and connected to the RC network as shown in the circuit diagram above.

Determination of Resistor and Capacitor values for Monostable Multivibrator 555 Timer Circuit
Let's say we want output signal with 5ms pulse duration. The equation for calculating the resistor R1 and Capacitor C1 values are derived as follows. The voltage across capacitor(C1) increases exponentially and is given by,\(V_c = V_{cc}(1-e^{t/R1C1})\)\(2/3V_{cc}=V_{cc}(1-e^{t/R1C1})\)Solving we get,

\(t=1.1R1C1\)

Let's choose R1 =10KOhm, and since t = 5ms, we get,C1 = t/1.1R1 = 5ms/1.1*10KOhm

or, C1 = 0.45uF

Choosing closest standard value, C1 = 0.47uF

This is how we have chosen the R1 and C1 values in the above circuit diagram.

You can also use the 555 monostable timer online calculator.Simulation of 555 monostable timer circuit in Proteus

For simulation we provide negative going pulse signal of 500us to the Trigger input of the 555 monostable timer. Also we hook up an oscilloscope to see the input and output signal. The wiring diagram in proteus for this will look like the following.

The input signal pulse setting is shown below.

After running the simulation, the oscilloscope will display the following input and output signal waveform.

The upper yellow signal waveform on channel A is the input 500us signal. The lower blue waveform on channel B is the output signal of pulse width 5ms. As you can see from the oscilloscope, for the output pulse we have 5.15 division and each division is 0.97ms/div(see the knob value on the oscilloscope panel). Hence we have total pulse width of,

Pulse Width = 5.15 divx0.97ms/div = 5ms

Hence the simulated monostable multivibrator 555 circuit is working as expected.

Video Demonstration

The following is demonstration of 555 timer as monostable multivibrator simulation in proteus.