Types of monostable multivibrator using IC555

A monostable mutivibrator (MMV) is a circuit which has only one stable state. After the input trigger pulse is given, the output goes high. It remains high as long as the voltage across capacitor is below 2/3 * VCC , as the capacitor charges and voltage equals 3.3V ( for 5V VCC ), the output goes low and remains low, unless a voltage of lower than 1/3 * VCC is given to the trigger (pin 2) of timer IC 555. Note that the trigger input is “level triggered” and not “edge triggered”. The time for which the output remains high is given by T = 1.11RC. Below given is the circuit diagram along with the output (green) and input (blue) waveform.

Fig 1 - Monostable multivibrator circuit

In the above circuit diagram R = 1Kohm and C = 1uf, Therefore – T = 1.11RC = 1.11ms

On simulator it’s 1.09ms. See the green output waveform.

Two types of Monostable multivibrators -

1. Non retriggerable

2. Retriggerable

Non retriggerable monostable multivibrator

In Non re-triggerable, After the initial trigger pulse is given, the output goes high. It will remain high for time “T” (T=1.11RC). If another trigger pulse comes when output is high then that trigger is neglected. This means that the output is going to remain high only for “T” seconds irrespective of the number of trigger pulses that come after initial trigger. Refer the below given image, look at the waveforms.

Fig 2 - Non retriggerable example

The initial trigger pulse (blue) set the output to high (green). After that four trigger pulses were generated but that didn’t affected the output pulse time duration “T”. It went low after 1.08ms (simulation) close to 1.11ms (theoritical).

Lets see waveform when trigger pulses are given continuously.

Fig 3 - Non retriggerable input/output waveforms

This shows us that the output will go low after 1.11ms and will remain low for the time the trigger input is high. As soon as the trigger input goes low the output turns high again.

Conclusion-

1. When output is low, only the initial trigger sets the output high.

2. The output will go low after time “T” (T=1.11RC).

Circuit simulation link - https://everycircuit.com/circuit/5784033229012992

Retriggerable monostable multivibrator

In retriggerable mode the output will stay high as long as the circuit is triggered frequently within the time duration “T” (T=1.11RC). This means that each time we trigger the circuit when the output is high, then the time “T” is added to the output. Refer the below given circuit and waveform.

Fig 4 - Retriggerable monostable multivibrator circuit diagram

The retriggerable circuit is different from the Non retriggerable, as you can see here there is a transistor (NPN) used to discharge the capacitor. The trigger is provided by giving high pulse to the base of the transistor. If the trigger pulse are fed frequently then the capacitor never reaches 2/3VCC (which is threshold vltg level) and the output remains high. The output will remain high for time “T” (T=1.11RC) if circuit is not triggered after the initial trigger.

Lets see waveform when trigger pulses are given continuously.

Fig 5 - Retriggerable monostable multivibrator input/output waveforms

The above image shows us that the output will stay high if trigger pulses are given continuously.

Conclusion -

1. When output is low, giving a trigger pulse will set output to high.

2. Giving trigger pulses frequently within T=1.11RC, will keep the output high.

Circuit simulation link - https://everycircuit.com/circuit/5115530159325184

Both the 555IC circuits, retriggerable and non-retriggerable are very useful when designing electronic circuits. The applications are vast. I will use these blocks in my future electronic projects. Watch the below given Youtube video (practical working):- 

https://youtube.com/shorts/KWHORnaqYSM?feature=share

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Make a relay module

1. What is a relay?

A relay is a electromechanical device. In simple words it's a combination of electromagnet and a mechanical switch. The electromagnets magnetic field operates the switch. When the internal electromagnet gets supply it turns on the switch and when the supply is cut, it turns off the switch. The switch in relay is a toggle switch also known as single pole double through (SPDT). The advantage of SPDT is that, we get COM(common), NO (normally open) and NC (normally close) contacts which are very usefull in circuit design. The below given image shows the bottom view of a relay along with the pin function.

5V relay pin diagram

The below given circuit images show how to connect the load to relay and how to turn it on and off.

Coil is not provided 5V , relay is off,  load is not getting supply.

Coil is provided 5V, relay is on, load is getting supply.

Once we know how to operate a relay the next thing is to turn on a relay by microcontroller but that's not as easy as directly connecting the 5V ouput pin to relay because the pin can supply only limited current.

2. How to operate relay on digital signal from a microcontroller or IC?

Relay requires current greater than 40mA to operate. The maximum current that can be drawn from  microcontroller's pin is limited to 40mA. That means if we connect a output device that draws current above 40mA then it's for sure that the microcontroller is going to get damaged. To increase the current we need to amplify the current and that can be done using NPN type transistor. The role of a transistor in this case is not only to amplify the current but also switch ground to relay, as the transistor is switching only ground the VCC (DC power supply voltage) for relay can be set depending on DC operating voltage of a relay. I am using a 5V relay in my circuit. The transistor also provides isolation between the relay circuit and microcontroller circuit. If you need more isolation then you can use opto-isolators, but a single transistor just works fine and gets the job done. Refer the below given circuit diagram.

5V Relay module using BJT BC547 (NPN type)

In the circuit diagram apart from relay and microcontroller there is transistor B547, resistor (1K to 10K) and a diode 1N4007. Bulp is load. The 5V external power supply is for relay as the microcontroller operates on 3V ( just an example). 

The diode is connected in reverse bias accross the relay coil , that is to provide path for the reverse high voltage spike. To know more about reverse high voltage spike click here

You can even connect a led in series with a resistor to the output of transistor to indicate that relay is on. 

3. Drawback of using BJT transistor BC547 as switching device. Why to replace it with MOSFET?

In BJT transistor there is current flow from base of transistor to emitter. So the pin on which the transistor is connected will read LOW (0V) this can cause false triggering of some functions if the pin has multiple functions in some microcontrollers (for eg- esp8266). To avoid that you can replace the transistor BC547 with 2N7000 N-channel mosfet. There will be only few changes in circuit like:-

1. emitter replaced by source.

2. base replaced by gate.

3. collector replaced by drain.

4. Pull down resistor of 100K from gate to source.

Refer the circuit given below:-

Circuit diagram of relay module with MOSFET

4. YouTube video

• Now you know how to make a relay module and get a relay wired quickly with a microcontroller or any IC. 
• If you find this blog usefull. Subscribe to my YouTube channel , Twitter. 
• If you have any doubts regarding this blog you can contact me through my email Id - prachethire@gmail.com

 

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Adjustable frequency divider

1. Introduction

This circuit divides the frequency of a square wave by 10,20,30....,90. Nine switches are provided to set the frequency division factor. S1 is for 10, S2 is for 20, and so on up to S9.

2. Circuit diagram

Adjustable frequency divider

3.Circuit description

1. Square wave signal is generated by a frequency generator and is given to clk pin of  IC4017 which is a decade ring counter. The way a ring counter works is by turning on and off each of the nine pins on the beat of the clk pulse. The output is in sync with clk pulses. 
2. Switches S1 to S9 determine the division factor. If Sx is on then the signal is divided by 1x. S1=1, S2=2,...S9=9. The end node of all the switches is connected to the reset pin through a positive edge detector. The IC will reset i.e start counting from Q0 as it receives a positive pulse.
3. The divided output from Q0 of IC4017 is further used as a clk pulse for another IC4017 which divides the signal by 10. 
4. Thus the final output from Q0 is divide by 1*10=10, 2*10=20,.....9*10=90. 
5. If you are using it as a looped timmer then a microswitch is used to reset the IC to set the starting point for the timmer. 
6. The output is given to a monostable circuit made by 555 timmer IC. Transistor Q2 along with capacitor and resistor form the positive edge detector which is used to trigger the monostable multivibrator. This is how we get a pulse of fix 'on time' given by formula T=1.11RC. 

4. Circuit working demo

Thank you! hope you find this circuit useful.

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Presettable electronic timmer (crystal oscillator)

 This circuit turns on the output (LED and buzzer) for a short duration after a preset time interval of 10 to 90 seconds in steps of 10. Without the clock signal, this circuit can also be called a square wave frequency divider of divide by 10 to 90.

Circuit diagram:-

 Main components required:-

• Crystal oscillator circuit removed from wrist watch's machine or wall clock's machine.
• 1.5V battery for crystal oscillator circuit.
• Transistor BC547 - 2pcs.
• Dual timmer IC 556.
• Decade counter IC4017 - 2pcs.
• Capacitors, resistors, led, 5V piezo buzzer.
• Microswitch.

Why make this circuit?

• Precise clock pulses of 1sec because of crystal oscillator which oscillates at a frequency of 32,768 times/second. If you want to know, how to use a crystal oscillator from wrist watch then watch this video on my YT channel-  https://youtu.be/kxRkMNUYzdg
•  Set time interval from 10 to 90 seconds by input switches. No need of using RC network for time setting. Frequency is divided by cascading two 4017IC (decade counter)
• No need to buy relatively expensive microcontrollers to provide precise timing pulses.

Applications:-

• In Factories and industries where timing signals are required to drive an electrical valve, motor, relays, etc.
• Timing signals for an indication for eg- blinking tail light, workout timmer,
• Measuring pulse rate. beats per minute (bpm)
• Measuring RPM of the motor.

Working demo video:-  https://youtu.be/UKywmi5A__U

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Circuit description

Introduction:-

This project is a simple frequency indicator. It displays number of pulses passed for the set time interval "T". If this time interval is of 1 second then we will get the frequency, whose value will be  indicated by the number of lit led's. These led's are connected to the output of 8 bit shift register ( IC 74595) , this limits the maximum frequency that can be indicated by the led bar graph to 8 Hz. To measure higher frequencies we can decrease the time interval or cascade two or more shift registers to display more number of counts. The circuit diagram of the project is given below:-

Understanding this circuit requires basic knowledge about shift registers and edge detector circuits. I would recommend the reader to watch the below given videos for a clear understanding. It benefited me in making many other circuits.

For shift register:- https://youtu.be/6fVbJbNPrEU  

For edge detectors:-https://youtu.be/cqZLJXYXurg

Working (from left to right):-

1. The light dependent resistor (LDR) is used as a light sensor. When light falls on it it's resistance          drops which indirectly drops the voltage accross it.

2. This drop in voltage is detected by the negative edge detector circuit which outputs a positive              trigger pulse. This positive trigger pulse also acts as a serial clock pulse for shift register. For              triggering the 555 ,The positive trigger pulse is converted into negative trigger pulse by transistor        BC547.

3. The 555 monostable multivibrator (MMV) circuit detects this negative trigger pulse and pulls the        output high for time ( T ) given by formula 1.11RC. In this circuit  T=1 sec. This output is given          to serial input data pin and serial clear pin of shift register. 

4. For the time, the output of 555 is high (which is 1 sec). The data '1' is shifted each time a flash of        light is detected by the LDR. After 1 sec when the output of 555 turns 'low'. A negative edge              detector gives a positive trigger pulse to the register clock pin of shift register. This latches the 8          bit data of storage register to the output pins ( Q0 to Q7 ) of shift register.

5. If the flash rate of light is 2 Hz ,then data in storage resistor will be "11000000". Which will be            latched to the output after 1 sec.

6. Only non-retriggerable monostable multivibrator (MMV) will work for this pulse counting circuit      as the MMV cannot be retriggered for set time interval. The 555 MMV is non-retriggerable type.        watch this video for more info:- https://youtu.be/tyD5Ym1xKn0

    

To see working demonstration of the project visit https://youtu.be/HVh95YBNv2M

If you found this blog useful and want to support me, then you can donate me. The money you give will be used to make more advance electronics projects and setting up better lab.