Voltage Controlled Oscillator

What is an oscillator circuit?

Assume an electrical circuit produces the following waveform output (voltage or current output).

Voltage Controlled Oscillator Waveform
Voltage Controlled Oscillator Waveform

This output is a square wave. It can be considered to be a sequence of repeating the following wave at an interval of time period 4.

Voltage Controlled Oscillator
Voltage Controlled Oscillator

This circuit which is producing a waveform by repeating a wave after a specific time interval is an oscillator circuit. Another example can be of a circuit producing continuous sine wave by repeating one cycle of a sine wave.

What is voltage controlled oscillator (VCO)?

The produced continuous waveform produced by the oscillator circuit has a frequency. A circuit in which the frequency of the produced output can be varied by the magnitude of a separately applied external voltage (other than the main supply voltage VCC) is known as voltage controlled oscillator.

Types of VCO:

  1. Linear or harmonic oscillator: This type of oscillator produces a sine wave. It consists of an LC tank circuit or crystal oscillator. The frequency of a tank circuit can be varied by changing the value of the capacitor. Now, a varactor diode’s capacitance can be varied by varying the applied voltage across it. So a varactor diode if used in an LC circuit converts it to a VCO.
  2. Relaxation oscillator: The output signal is a saw tooth or triangularwaveform. This circuit employs the charging and discharging of a capacitor through a resistance. The output frequency depends on the time of charging and discharging of the capacitor.If it is desired to produce a square wave, a triangular wave can be differentiated to produce so. Also a periodic waveform can be passed through a Schmitt trigger to produce a square wave.

    IC 566

    The IC 566 (or LM566) is an integrated circuit that produces a triangular wave and a square wave output from two different output pins. It is an 8 pin IC shown below:

Pin configuration:

IC 566
IC 566

frequency fo = (2/(R1C1))*((Vcc-Vc)/Vcc)

  1. Ground
  2. No connection
  3. Square wave output
  4. Triangular wave output
  5. Modulating/Control voltage VC
  6. Timing resistor R1 (connected between pin 6 to supply voltage VCC)
  7. Timing capacitor C1 (connected between pin 7 to ground)
  8. Supply voltage VCC

A rough internal circuit is shown below:

Voltage Controlled Oscillator
Voltage Controlled Oscillator

Basically, the principle of operation is as such:

The Schmitt trigger switches the current source from charging and discharging the capacitor.

The IC charges and discharges the external capacitor C1 through the resistor R1. A triangular waveform is obtained by passing the voltage waveform across the capacitor C1 through Buffer Amplifier 2 and obtained as output through pin 4.

The voltage waveform across the capacitor when passed through a Schmitt trigger, produces a square wave which is passed through the Buffer Amplifier 1 and obtained as output through pin 4.

Modulating voltage VC should be in the range of  (3/4)Vcc < Vc < Vcc where VCC is the supply voltage.

VCC should be within 10 to 24 Volts.

The frequency modulation (by applying a varying modulating voltage VC) can be done in 10:1 ratio.

The frequency of the output waveform is f0 = (2/R1C1)*((Vcc – Vc)/Vcc) .

An example circuit is shown below:

Voltage Controlled Oscillator
Voltage Controlled Oscillator

Applications of VCO

  1. Tone generators
  2. Frequency modulation
  3. Function generator
  4. Phase Locked Loop

This article is written by Sayantan Roychowdhury.

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Series Voltage Regulator – Working Principle

We assume that the voltage across a zener diode remains constant.

i.e. DVZ = 0.

In all cases, we indicate load resistance by RL

Series voltage regulator circuit diagram
Series voltage regulator circuit diagram
  • Assume supply voltage increases by DVS

Current through resistance R is IR = (VS-V)/R

or, DIR = DVS / R                                (Equation 1.1)

Also IR = IB + IZ ;  or, DIR = DIB + DIZ ;            (Equation 1.2)

From Equation 1.1 and Equation 1.2, an increase in VS increases base current IB and zener current IZ. Since collector current IC = β*IB so IC also increases.

We know IE = IB + IC;

or,          (since IB is very small compared to IC)


As IC increases, IE will also increase through load RL, thus voltage output VO = IE*RL tends to increase.      (Effect 1: VO tends to increase)

Using KVL in output circuit, VO + VBE – VZ = 0; where VBE = base emitter voltage, VZ = zener voltage

DVO = -DVBE        (Equation 1.3)

Equation 1.3 suggests, an increase in output voltage VO decrease base emitter voltage VBE. As decrease in VBE causes decrease in IC , and conversely IEthrough RL , hence output voltage VO tends to decrease.             (Effect 2: VO tends to decrease)

Effect 1 and Effect 2 neutralise each other and VO is constant.

Opposite happens when VS decreases.

  • Assume supply current IS increases by DIS (keeping VS constant)

IS = IC + IR; or, DIS = DIC + DIR         (Equation 2.1)

IR = IB + IZ; or, DIR = DIB + DIZ         (Equation 2.2)

From Equation 2.1, IS increases IC (also IR). Also this is evident as from Equation 2.2, that increase in IR increases IB and hence increases IC.

As IE≈IC so IE increases through RL. Hence output voltage VO = IE*RL tends to increase.

(Effect 1: VO tends to increase)

Remaining analysis is similar to previous case. VO tends to increase decreasing VBE (like Equation 1.3). This decreases IC, consequently decreasing IE and this tends to decrease VO.

(Effect 2: VO tends to decrease)

Effect 1 and Effect 2 neutralise each other and VO is constant.

Short circuit protection

To prevent short circuit i.e. to prevent an excessive high flow of current, the following arrangement is made.

Short circuit protection
Short circuit protection

A very small resistance RSC is connected in series with the load. The base emitter terminals of BJT Q2 are connected across this RSC resistance. When a high current flows across the load, an appreciable amount of voltage is developed across RSC. Hence base emitter voltage of Q2 increases, collector current of Q2 increases, so IB is shunted away from Q1. As IE≈IC= β*IB , hence IE decreases and a large current flow is prevented.

This article is written by Sayantan Roychowdhury.

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Sending SMS from No Signal Area – Mini Project

Hi friends, in previous article we have seen Zigbee and GPS project which tracks a vehicle. Today we will build another innovative electronic project which will send a SMS from No Signal Area. There are many locations where we get poor range or completely no range. So using this embedded system we can send a SMS from such locations. The only condition we need here is, we should have a mobile network at the receiving end of Zigbee module.

This is low cost project and highly innovative. You can build such projects for your final year engineering submissions also.

Sending SMS from No Signal Area


The main objective of this micocontroller project is to send a SMS from No Signal area which is also known as Black Spot area using Zigbee and GSM module.



  1. 8051 family development board
  2. power Supply
  3. Zigbee modules
  4. GSM modem.
  5. Max232


  1. Embedded ‘C’
  2. RIDE to write code
  3. ISP to burn the chip

Block Diagrams:

Zigbee Transmitter Block Diagram

Sending SMS from No Signal Area Transmitter Block Diagram

Zigbee Receiver Block Diagram

Sending SMS from No Signal Area Receiver Block Diagram

Power Supply:

Power Supply


As already stated, this project is useful for creating signal, using GSM module we can send SMS through that signal to destination. In this project we are using two different frequencies. Zigbee has frequency 2.4GHz and GSM has frequency 1800 MHz.

Main circuitry of this project contains two embedded development boards. One contain Zigbee and Keypad and other contain Zigbee and GSM. We need to place first board in No signal (Black spot) area. Other development board which contains Zigbee receiver and GSM module is kept in area where there is mobile network.

When you type a message using keyboard and hit enter from No signal area, Zigbee transmitter will send a signal with message to the receiver end. Receiver end of Zigbee also has GSM module which will send that SMS to destination mobile.

Watch this Video:

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Vehicle Tracking Using Microcontroller & GPS Module

Hi friends, In this project we are going to make a Vehicle tracking system using 8051 microcontroller and GPS module. You can use this project for you final year submissions which cost you around 2500Rs. This is quite interesting and useful project in our day to day life.

We can easily tack lost vehicle using this Vehicle tracking GPS module. In this project we are using 8051 microcontroller, GPS module to track the vehicle location. This system will send a location of vehicle in the form of longitude and latitude values. If you enhance this project you can also easily track the position of vehicle in the graphical presentation on your PC.

Vehicle Tracking Using Microcontroller and GPS Module


The main objective behind developing this project is to track the vehicle position using microntroller GPS module & Zigbee module along with software to see the track of vehicle on PC.

Block Diagrams:

1) Connecting GPS with ZigBee:

Connecting GPS and Zigbee

2) Vehicle Tracking Block Diagram:

Vehicle Tracking Block Diagram

3) Power supply for Vehicle Tracking Circuit:

Power Supply


In this project we using AVL technology. AVL stands for Automatic Vehicle Location. AVL is an advanced method to track and monitor any vehicle which is connected with Vehicle tracking circuit.AVL is a combination of GPS (Global Positioning System) and GIS (Geographic Information SYstem). All data transmission in this project depends on GPS satellite and receiver on the board and Zigbee.

There are planty of tutorials availbel in Internet on how to connect Zigbee with your GPS module. Once you integrate Zigbee and GPS connect your circuit with Microcontroller. You can refer this article on how to connect GPS with 8051 microcontroller.


For building this Vehicle tracking system you will need Embedded C knowledge, RIDE to write code and ISP to burn the chip.


  • 8051 microcontroller kit
  • power supply
  • Zigbee module
  • GPS module

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Flash ADC or Parallel ADC and its Working Principle

Another type of ADC is parallel ADC. Parallel ADC is called as Flash ADC. Its response is very fast. it converts analog signal into digital signal using parallel set of comparators. As its conversion time is very fast it is called as flash ADC.

Following figure shows circuit diagram of parallel ADC or flash ADC.

Flash ADC or Parallel ADC
Flash ADC or Parallel ADC

n-bit Flash ADC consist of parallel combination of 2^n-1 comparators. Outputs of all comparators are connected to an encoder.

Working Principle of flash ADC

Analog voltage is applied to non inverting terminals of all comparators using a single line. Reference voltage is applied to inverting terminals of comparators using divider circuit.

Each comparator produces digital output in the form of 1 or 0. If unknown analog voltage is greater than reference voltage comparator produces high logic. If analog voltage is less than reference voltage then comparator produces low logic i.e. 0.

Thus all parallel comparator produces digital representation of analog voltage in the form of zero and one. These outputs of comparator are then applied to the fast encoder. Encoder converts those zeros and once into binary number and produces digital binary output.

For example, see below table. When unknown voltage is 5 i.e. lies between 4.375 &5.625 is applied to the flash ADC, first four encoders produces output ‘1’ and last three encoders produces output ‘0’. Encoder converts this ‘1111000’ comparator output into ‘100’ binary number as digital output.

Table shows the outputs of comparators and encoder for a 3 bit flash ADC. The range of operation is given as 0-10V.

Analog input


Comparator Output Encoder Output
C1 C2 C3 C4 C5 C6 C7 D2 D1 D0
0.000-0.625 0 0 0 0 0 0 0 0 0 0
0.625-1.875 1 0 0 0 0 0 0 0 0 1
1.875-3.125 1 1 0 0 0 0 0 0 1 0
3.125-4.375 1 1 1 0 0 0 0 0 1 1
4.375-5.625 1 1 1 1 0 0 0 1 0 0
5.625-6.875 1 1 1 1 1 0 0 1 0 1
6.875-8.125 1 1 1 1 1 1 0 1 1 0
8.125-10.000 1 1 1 1 1 1 1 1 1 1


As the number of bits of ADC increases its resolution increases. But such high bit converter is bulky and expensive.

Also see: Sigma Delta ADC or noise eliminating ADC.

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Sigma Delta ADC & its Working

Hi friends, today we will see Sigma Delta ADC and its working. Sigma Delta ADC is widely used in communication system, professional audio system and high precision measurement system. Sigma Delta ADC has characteristics like, high resolution, low cost and low conversion speed.

Following figure shows block diagram of Sigma Delta ADC.

Sigma Delta ADC
Sigma Delta ADC

Sigma Delta ADC consists of two main blocks, Sigma Delta modulator and digital decimeter. Sigma Delta modulator consist of difference amplifier, integrator, comparator and 1-bit DAC. Digital decimeter is used for digital filtering and down sampling.

Working Principle:

Analog signal which is to be digitized is applied to the non inverting terminal of difference amplifier. Inverting terminal of difference amplifier is connected with either +Vref or –Vref depends on output of 1-bit DAC. Output of difference amplifier is integrated using integrator as shown in diagram.

Output of integrator is applied to the non inverting terminal of comparator. In this case comparator works as 1-bit ADC and produces output as 1 or 0.

Output of comparator is connected to the 1-bit DAC. DAC is used to connect either +Vref or –Vref to the inverting terminal of difference amplifier.

DACs output is them again subtracted from analog input. This process is continuous in closed loop. After each loop 1-bit ADC produces 1 or 0. Density of ‘1’ depends on analog voltage supplied. If analog voltage is high then density will be high and if analog signal is low density of ‘1’ will be low.

Output of 1-bit ADC is also connected to the digital decimeter. It is used for digital filtering and down sampling. It produces n-bit digital output in binary format.

Advantages of Sigma Delta ADC:
  • Sigma Delta ADC is inexpensive since all circuitry within the converter is digital.
  • The output of sigma delta ADC is inherently linear but it has little differential non linearity.
  • It do not require sample and hold circuit. It is because due to high sampling rate and low precision.
  • It is limited to high resolution and very low frequency applications.
  • It takes quite long time for producing first digital output because of digital filtering and down sampling.
  • It is not possible to use Sigma Delta ADC for multiplexed ac input signals.

Note: Sigma Delta ADC is also known as Delta Sigma ADC, oversampling ADC or noise shaping ADC.

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DC to AC Converter using IC 555 – Mini project

It is common problem in most of the mini projects that how to convert DC voltage into AC voltage. When we develop any circuit that gives dc output voltage and if we want to convert it into AC voltage, this DC to AC converter is used. In this project we will see how to control 120V AC supply using 12V DC output.

DC to AC Converter using IC555

Components required
  • NE 555 (Timer IC)
  • NPN Transistor (TIP41)
  • PNP Transistor (TIP42)
  • Resistors and capacitors
  • Inductor of 1µH
  • Variable resistor (potentiometer) of 50KΩ
Circuit diagram

Following diagram shows circuit for DC to AC voltage converter using NE 555 timer IC.

DC to AC Converter
12V DC to 120V AC Converter
Description and working of DC to AC converter

Above circuit can be used for a range of +5V to +15V DC voltage. This DC voltage is applied to the NE555 timer IC. NE555 produces frequency proportional to the input voltage. At the output terminal we have used two transistors. One is NPN and other is PNP. These two transistors are used for driving transformer coil. An inductor of 1µH is used in series with 2200µF capacitor.


  • You can also get output voltage between 120V to 230V AC at 50-60Hz.
  • Output voltage depends on step up transformer used in the circuit.
  • You can also use similar transistor instead of TIP41 and TIP42.

You can use this as your second or third year engineering project. This is simple mini project using IC555. It is easy to uild and all components used in the circuit are easily available.

See also: Rain detector alarm circuit

Police siren alarm circuit

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Weighted Resistor DAC and its Operation

HI friends, today we are going to learn weighted resistor DAC. Many times we require to convert a digital output into its equivalent analog voltage. To perform this operation we are going to use weighted resistor digital to analog converter.

Following figure shows the circuit diagram of weighted resistor DAC. This DAC circuit uses weighted values of resistor like 2R, 4R, 6R, 8R and so on depending on the digital inputs available therefore such type of network is known as weighted resistor DAC.

Weighted Resistor DAC Circuit
Fig: Weighted Resistor DAC Circuit

This circuit consist of a transistor switch (shown by the upward arrow) which turns on the switch when digital input is ‘1’ and if digital input becomes ‘0’ it will opens the switch. When transistor switch gets closed a current flows through the weighted resistor due to reference voltage as shown in circuit diagram.

When all such currents from different weighted resistors get added at summing point (which is also known as virtual ground) of operational amplifier it will produce proportional voltage as its output.

For a 4 bit DAC the output V0 is given as follows


Where S3, S2, S1 and S0 represents the status of the switches i.e. on or off (1 or 0).

If resistors are in binary weights i.e. R3=2Rf, R2=4Rf, R1=8Rf and R0=16Rf, the above equation can be written as,


From the above discussion we can say that for a 4 bit DAC 4 switches produces 16 different combinations of output and hence producess 16 different output voltage.in general n-bit DAC producess 2^n different discrete analog voltages.

Current to Voltage Converter using Op Amp

In the last article we have seen how to convert voltage into current using voltage to current converter. Today we will see how to convert current into voltage form using current to voltage converter.

As we know we can not send output voltage of sensor to a long distance because of addition of noise. So we first convert it into current and then send to the destination. But at the destination we have to convert that current into its original form i.e. in voltage form. Using current to voltage converter we can easily convert the current into voltage.

Following figure shows the circuit diagram of the current to voltage converter. It uses simple operational amplifier and a feedback resistance.

Current to Voltage Converter
Current to Voltage Converter

The output voltage of operational amplifier is directly proportional to the current given to the inverting terminal of the op amp.

The value of the output voltage is given by the following equation

V_{o}= I_{in}\times R

See also: Voltage to Current Converter

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Voltage to Current Converter using Op Amp

Hi friends, in this post we will see how to convert voltage into current using simple circuitry. In most of the cases we get the output of measuring devices in the form of voltage. It is not good to transmit this output voltage to the destination directly. Due to addition of noise and wire impedance the output voltage may get corrupted. So in such cases we have convert that voltage into current form. So let us see voltage to current converter.

Voltage to Current Converter using Op Amp

Following circuit shows the voltage to current converter using operational amplifier. It consist of simple resistance connected to the inverting and non inverting terminals of op amp.

Voltage to Current Converter
Voltage to Current Converter

In this circuit the load is grounded and the current through the load can be calculated as follows.

I_{1}=\left (\frac{V_{in}-V_{2}}{R} \right )

I_{2}=\left (\frac{V_{o}-V_{2}}{R} \right )

The current through the load is given by,

I_{L}=I_{1}+I_{2}=\left (\frac{V_{in}-V_{2}}{R} \right )+\left (\frac{V_{o}-V_{2}}{R} \right ) =\frac{V_{in}+V_{0}-2V_{2}}{R}

The gain of the amplifier is



V_{0}= 2V_{2}

Substituting this value in above equation we get,

I_{L}=\left (\frac{V_{in}}{R} \right )

Thus the current is directly proportional to the applied voltage and the resistance used in the circuit. it should be noted that all the resistances used in the circuit are equal to R.

See alos: Current to Voltage Converter

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