MOBILE INCOMEING CALL INDICATOR

1. This circuit can be used to escape from the nuissance of obile phone rings when you are at home.This circuit will give a visual indication if placed near a mobile phone even if the ringer is deactivated.
2. When a call is coming to the mobile phone, the transmitter inside it becomes activated.The  frequency of the transmitter is around 900MHz.
3.  The  coil L1 picks up these oscillations by induction and feds it to the base of Q1.This makes the transistor Q1 activated.
4.  Since the Collector of Q1 is connected to the pin 2 of IC1 (NE555) , the IC1 is triggered to make the LED connected at  its output pin (pin 3) to blink.The blinking of the LED is the indication of incoming call.
   
   
   
   
   
   
   NOTES:
   

   • The coil L1 can be made by making 150 turns of 36 SWG enameled copper wire on a 5mm dia plastic former.Or you can purchase a 10 uH coil from shop if available.
   • The circuit can be powered from a 6V battery.
   • Assemble the circuit on a good quality PCB.
   • C1 & C3 are to be polyester  capacitors.
   • The electrolytic capacitor C2 must be rated 10V.

   
   

1KHz IR TRANSMITTER CIRCUIT

1. This circuit was designed in response to a request from my reader. What he asked for was a 1KHz IR transmitter circuit for some remote control application.
2. I think this circuit may satisfy him. Any way this circuit can be used where ever a low power IR transmitter of 1 KHz operating frequency is needed. This transmitter can transmit up to a distance of about 10 meters.
3. The circuit is based on a NE555 timer IC (IC1) which is wires as an astable multivibrator to produce 1KHz pulses. 
4.  The output pulses of the IC1 will be amplified by the Q1(SK100) to drive the two IR transmitter LEDs wired serially.
5. The resistors R1, R2 and capacitor C2 determines the operating frequency of the IC.The circuit starts emitting IR pulses when ever the push button switch S1 is pressed.

   
   
   NOTES:
   

   • The circuit can be powered from a 9V PP3 battery.
   • The switch S1 is a push button type switch.
   • The IC 1 must be mounted on a holder.

   
   

   
   

QRP ANTENNA TUNNER CIRCUIT

1. Low power ( 3 to 30 MHz)  transmitters constructed by hams are generally called QRP’s. For such transmitters a well tuned antenna is a must.
2.   If the impedance is not properly matched there will be a little or no output.But if properly matched there will be great results.A circuit for matching the antenna properly with the transmitter id given below.
3.  The output of the transmitter is given to the input of the tuner( connector BNC1). The output of the tuner(connector BNC2) must be connected to antenna.
4.  Then adjust the L1 and C1 to obtain the maximum transmission power.The transmission power can be checked using a SWR meter.  

         

           NOTES:

• Assemble the circuit on a goos quality PCB or common board.
• If the matching is not satisfactory then change the values of L1,C1,C2&C3  to the next close value and tune again.
• Proper tuning requires some trial and error.
• The circuit can be enclosed in an aluminum casing for better performance.

AM RECEIVER USING ZN 414

1. This is the circuit diagram of the simplest single chip AM radio. The circuit is designed around the IC ZN414Z which is a ten transistor tuned radio frequency receiver.
2. The IC has only three leads and is available in the TO92 package. All necessary circuits required for an AM receiver (RF amplifier, detector and AGC are incorporated inside the IC).In the circuit given below, capacitor C1 and resistor R1 forms the tank circuit which is essential for tuning.
3.  Capacitor C4 decouples DC from the output of the IC and C1 bye-passes the noise. Transistor Q1 and associated components forms a classic driver stage for the headphone.
4. Head phone is connected across the resistor R5 and R4 gives necessary biasing for the transistor Q1

    
    

    

   NOTES:
   

   • Assemble the circuit on a good quality PCB.
   • The circuit can be powered from a 1.5V cell.
   • Do not give more than 1.8V to the circuit as it will destroy the IC.
   • For L1 make 80 turns of 0.3mm enameled copper wire on a 5cm long ferrite rod.
   • K1 can be a high impedance crystal earpiece.

   
   

SPEAKER TO MICROPHONE CONVERTER CIRCUIT

1. This circuit is a simple approach for converting a loud speaker into a microphone.
2.  When the sound waves fall on the diaphragm of a speaker, there will be fluctuations in the coil and there will be a small proportional induced voltage.
3. Usually this induced voltage is very low in magnitude and useless. Here in the circuit the low voltage is amplified using transistors to produce a reasonable output.
4. The transistor Q1 is wired in common base mode and produces the required voltage gain. The transistor Q2 is wired as an emitter follower to produce enough current gain.
5. The voice quality of this circuit will not be as much as a conventional microphone but quite reasonable quality can be obtained. 
6. To set up the circuit, keep the preset R2 at around 10 Ohms and connect the battery. Now adjust R2 to obtain the optimum sound quality.
   
   
   
    NOTES: 

• Assemble the circuit on a general purpose PCB.
• Power the circuit from a 9 V PP3 battery.
• A 3 inch speaker can be used as K1.
• All capacitors must be rated at least 15V.
• An 8 Ohm speaker or head phone can be connected at the output to hear the picked sound.

FREQUNCY TO VOLTAGE CONVERTER USING LM 331

1. LM331 is basically a precision voltage to frequency converter from National Semiconductors. The IC has a hand full of applications like analog to digital conversion, long term integration, voltage to frequency conversion, frequency to voltage conversion. Wide dynamic range and excellent linearity makes the IC well suitable for the applications mentioned above.
2. Here the LM331 is wired as a frequency to voltage converter which converts the input frequency into a proportional voltage which is extremely linear to the input frequency.
3. The frequency to voltage conversion is attained by differentiating the input frequency using capacitor C3 and resistor R7 and feeding the resultant pulse train to the pin6 (threshold) of the IC.
4. 
   
   
   The negative going edge of the resultant pulse train at pin6 makes the built-in comparator circuit to trigger the timer circuit. At any instant, the current flowing out of the current output pin (pin 6) will be proportional to the input frequency and value of the timing components (R1 and C1). As a result a voltage (Vout) proportional to the input frequency (Fin) will be available across the load resistor R4.
   

   
   

   
   

   NOTES:

   • The circuit can be assembled on a vero board.
   • I used 15V DC as the supply voltage (+Vs) while testing the circuit.
   • The LM331 can be operated from anything between 5 to 30V DC.
   • The value of R3 depends on the supply voltage and the equation is R3= (Vs – 2V)/ (2mA).
   • According to the equation, for Vs = 15V, R3=68K.
   • The output voltage depends on the equation, Vout = ((R4)/(R5+R6))*R1C1*2.09V*Fin.
   • POT R6 can be used for calibrating the circuit.

   
   

   
   

   
   

LONG DURATION TIMER CIRCUIT

1. This timer circuit can be used to switch OFF a particular device after around 35 minutes.
2. The circuit can be used to switch OFF devices like radio, TV, fan, pump etc after a preset time of 35 minutes. Such a circuit can surely save a lot of power.
3. The circuit is based on quad 2 input CMOS IC 4011 (U1).The resistor R1 and capacitor C1 produces the required long time delay. When pushbutton switch S2 is pressed, capacitor C1 discharges and input of the four NAND gates are pulled to zero. 
4. The four shorted outputs of U1 go high and activate the transistor Q1 to drive the relay. The appliance connected via the relay is switched ON. When S2 is released the C1 starts charging and when the voltage at its positive pin becomes equal to ½ the supply voltage the outputs of U1 becomes zero and the transistor is switched OFF.
5. This makes the relay deactivated and the appliance connected via the relay is turned OFF. The timer can be made to stop when required by pressing switch S1.

   
   

   NOTES:
   

   • Assemble the circuit on a good quality PCB or common board.
     
   • The circuit can be powered from a 9V PP3 battery or 12V DC power supply.
     
   • The time delay can be varied by varying the values of C1&R1.
     
   • The push button switch S2 is for starting the timer and S1 for stopping the time.
     
   • The appliance can be connected via contacts N1 & N2 of the relay RL1.
   • The IC U1 is 2 input quad NAND gate 4011.

   
   

   
   
   
   

NI-Cd BATTERY CHARGER CIRCUIT

1. This circuit can be primarily used for charging 12V Ni-Cd battery packs. Any way 6V and 9V battery packs can be also charged by using this circuit a little compromise on power efficiency.
2. he built in automatic current regulator regulates the charging current to somewhat 4 amperes. When the charging current reaches 4A, the voltage across resistor R1 becomes 0.7V and switches the transistor Q1 ON.
3. The transistor Q2 which is now in ON state will shorts the base of Q3 to ground and inhibits the biasing of Q4 through which the charging is done. That is how the current regulation is achieved.
4. 
   
    When charging low voltage battery packs, the excess voltage will be dropped across Q4.It is advised to use a heat sink with transistor Q4.
   

   
   
   NOTES:
   

   • The circuit can be assembled on a Vero board.
   • T1 can be a 230V primary, 12V secondary,4A step down transformer.
   • Bridge D1 can be made by using 1N5400 diodes.
   • Fuse F1 can be a 500mA type.

   
   
   
   

   
   
    

CABLE T.V. AMPLIFIER

1. This is a very simple cable TV amplifier using two transistors. This amplifier circuit is most suitable for cable TV systems using 75 Ohm coaxial cables and works fine up to 150MHz.



2. Transistor T1 performs the job of amplification. Up to 20dB gain can be expected from the circuit.T2 is wired as an emitter follower to increase current gain.
   

NOTES:

• Use 12V DC for powering the circuit.



• Type no of the transistors are not very critical.



• Any medium power NPN RF transistors can be used in place of T1 and T2.



• This is just an elementary circuit. Do not compare it with high quality Cable TV amplifiers available in the market.
  

DING-DONG SOUND GENERATOR

1. This is the circuit diagram of a ding dong sound generator based on two NE555 timer ICs.
2. The circuit is designed to toggle between two adjustable frequencies to produce the ding dong sound. The first NE555 (IC1) is wires as an astable multivibrator operating at 1Hz.
3. The frequency of the second NE555 (IC2) is modulated by the output from the first IC. This is attained by connecting the output of first IC to the control pin (pin5) of the second IC.
4. he tone of the sound depends on the frequency of the second IC and the changeover time depends on the frequency of the first IC.

   
   
   

   
   NOTES:
   

   • The circuit can be assembled on a Vero board.
   • Use 9V PP3 battery for powering the circuit.
   • POT R4 can be used to adjust the tone of the sound.
   • POT R2 can be used to adjust the change over time.
   • IC1 and IC2 must be mounted on holders.
   • K1 can be a 8 ohms, 1/2 watt tweeter

   
   
   

   
   
   
   

   
   
   

SIMPLE ELECTRONIC COMBINATION LOCK USING IC LS 7220

1. This is the circuit diagram of a simple electronic combination lock using IC LS 7220.This circuit can be used to activate a relay for controlling (on & off) any device when a preset combination of 4 digits are pressed.
2. The circuit can be operated from 5V to 12V.
3. To set the combination connect the appropriate switches to pin 3,4,5 and 6 of the IC through the header.As an example if S1 is connected to pin 3, S2 to pin 4 , S3 to pin 5, S4 to pin 6 of the IC ,the combination will be 1234.
4. This way we can create any 4 digit combination.Then connect the rest of the switches to pin 2 of IC.This will cause the IC to reset if any invalid key is pressed , and entire key code has to be re entered.
5. When the correct key combination is pressed the out put ( relay) will be activated for a preset time determined by the capacitor C1.Here it is set to be 6S.Increase C1 to increase on time.
6. For the key pad, arrange switches in a 3X4 matrix on a PCB.Write the digits on the keys using a marker.Instead of using numbers I wrote some symbols!.The bad guys will be more confused by this.

   
   
   

   
   
   

   PARTS LIST:
7. C1 1 1uF 25V Electrolytic Capacitor
   C2 1 220uF 25V Electrolytic Capacitor
   R1 1 2.2K 1/4W Resistor
   Q1 1 2N3904 NPN Transistor 2N2222
   D1 1 1N4148 Rectifier Diode 1N4001-1N4007
   K1 1 12V SPDT Relay Any appropriate relay with 12V coil
   U1 1 LS7220 Digital Lock IC
   S1-S12 12 SPST Momentary Pushbutton Keypad (see notes)
   HD1 1 12 Position Header

   
   
   

   
   
   
   

DIGITAL STOP WATCH

1. A digital stop watch built around timer IC LM555 and 4-digit counter IC MM74C926 with multiplexed 7-segment LED display.
2. MM74C926 consists of a 4-digit counter, an internal output latch, npn output sourcing drivers for common cathode,
   7-segment display and an internal multiplexing circuitry with four multiplexing outputs.The counter advances on negative edge of the clock.
3. The clock is generated by timer IC LM555.The circuit works off a 5V power supply. It can be easily assembled on a general-purpose PCB. Enclose the circuit in a metal box.
4. with provisions for four 7-segment displays, rotary switch S1, start/stop switch S2 and reset switch S3.

   
   

   
   TESTING:
5. First, reset the circuit by pressing S3 so that the display shows ‘0000.’
6. Now open switch S2 for the stop watch to start counting the time. If you want to stop the clock, close S2.
7. Rotary switch S1 is used to select the different time periods at the output of the astable multivibrator (IC1).

   
   
   

   
   
   

   
   
   

AIR FOLLOW DETECTOR CIRCUIT

• This circuit can give a visual indication of the rate of air flow.It can be also used to check whether there is air flow in a given space.
• The filament of a incandescent bulb is the sensing part of the circuit.When there is no air flow the resistance of the filament will be low.
• When there is air flow the resistance drops , because the moving air will remove some of the heat generated in the filament.This variations in the resistance will produce variation of voltage across the filament.
• These variations will be picked up by the opamp (LM339) and the brightness of the LED at its output will be varied proportionally to the airflow.

  
  
  

  
  NOTES:
  

  • The filament L1 can be made by removing the glass of a 40W incandescent bulb.
  • The circuit can be powered from a 12 V DC power supply.

  
  
  

  
  
  

  
  
  

LIGHT ACTIVATED SWITCH CIRCUIT

• This is the circuit diagram of a light activated switch based on National Semiconductors comparator IC LM 311 and a LDR.
• The circuit is based on a voltage comparator circuit wired around IC 1.The non inverting in put of IC1 is given with a reference voltage of 6V using resistors R3 and R4.
• The input to the inverting input will be the voltage across the LDR that is light dependent.At darkness the resistance of the LDR will be high and so do the voltage across it.
• At this condition the voltage at the inverting input will be higher than the reference at non inverting pin and the out put of the comparator will be low(~o V).
• When the LDR is illuminated ,its resistance drops and so do the voltage across it.Now the voltage at inverting input will be lower than that at non inverting input and the out put of the comparator goes high (~12 V).
• This makes transistor Q1 on and it drives the relay.As a result we get a relay switching according to the intensity of the light falling on the LDR.

  
  

  
  
  

  NOTE:
  
• Adjust POT R1 to set the desired light intensity for switching the relay.
• For this illuminate the LDR with the desire intensity light.
• The relay will be either on or off.Adjust POT R1 slowly so that the state of the relay changes.
• That’s it.Now the circuit is set for the given intensity of light.
  
  

Assemble the circuit on a good quality PCB or common board.

You can use either a 12 V battery or a well regulated & filtered 12V DC mains operated power supply.

The pin 5&6 (Balance & Balance/Strobe ) of IC LM311 are shorted to minimize the chance of oscillations.

The pin out of LM311 is also given together with the circuit diagram

Read more: http://www.circuitstoday.com/light-activated-switch-circuit#ixzz0fNngaXtl
Under Creative Commons License: Attribution

LEAD ACID BATTERY CHARGER

• Here is a lead acid battery charger circuit using IC LM 317.The IC here provides the correct charging voltage for the battery.
• A battery must be charged with 1/10 its Ah value.This charging circuit is designed based on this fact.
• The charging current for the battery is controlled by Q1 ,R1,R4 and R5.
• Potentiometer R5 can be used to set the charging current.As the battery gets charged the the current through R1 increases .
• This changes the conduction of Q1.Since collector of Q1 is connected to adjust pin of IC LM 317 the voltage at the output of of LM 317 increases.
• When battery is fully charged charger circuit reduces the charging current and this mode is called trickle charging mode.

  
  
  

  NOTES:
  

  • Connect a battery to the circuit in series with a ammeter.Now adjust R5 to get the required charging current. Charging current = (1/10)*Ah value of battery.
  • Input to the IC must be minimum 15V to get 12 V for charging the battery .Take a look at the data sheet of LM 317 for better understanding.
  • Fix LM317 with a heat sink.

  
  
  

  
  

  
  
  

  
  
  

  
  
  

SINGLE CHIP FM RADIO CIRCUIT

• This circuit is designed as per the data sheet and the result is excellent.Ideal for all category of electronic enthusiasts.
• The TDA7000 is a monolithic integrated circuit for mono FM portable radios, where a minimum on peripheral components is crucial.
• The only function which needs alignment is the resonant circuit for the oscillator, thus selecting the reception frequency. Spurious reception is avoided by means of a mute circuit, which also eliminates too noisy input signals. Special steps are taken to meet the radiation requirements.

  
  NOTES:
  

  • For L1 and L2 wind 5 turns of 0.6 mm enameled Copper wire on a 4 mm dia plastic former.
  • For antenna use a 50mm long insulated copper wire.
  • IC TDA 7000 can withstand up to 10 V supply voltage.But I recommend 6V.
  • Use an 8 Ohm speaker or Headphone at the audio output.

  
  
  

  
  
  

  
  
  

AUTOMATIC LED EMERGENCY LIGHT

• This is the circuit diagram of a low cost emergency light based on white LED.The white LED provide very bright light which turns on when the mains supply is not there.
• The circuit has an automatic charger which stops charging when the battery is fully charged.
• The IC LM 317 produces a regulated 7 V for the charging of Battery.
• t is always better to connect a heat sink with BD 140.Before using the circuit out put of LM317 must be set to 7V by adjusting the potentiometer.
  
  

INFRARED MOTION DETECTOR

• Here is the circuit diagram of an infrared motion detector that can be used to sense intrusions.Infra red rays reflected from a static object will be in one phase, and the rays reflected from a moving object will be in another phase.The circuit uses this principle to sense the motion.
• The IC1 (NE 555) is wired as an astable multivibrator .The IR diode connected at the output of this IC produces infrared beams of frequency 5Khz.These beams are picked by the photo transistor Q1
• At normal condition ie; when there is no intrusion the output pin (7) of IC2 will be low.When there is an intrusion the phase of the reflected waveforms has a difference in phase and this phase difference will be picked by the IC2.
• Now the pin 7 of the IC 2 goes high to indicate the intrusion.An LED or a buzzer can be connected at the output of the IC to indicate the intrusion.
  
  NOTES:
• Comparators IC2a and IC2b are belonging to the same IC2 (LM1458).So the power supply is shown connected only once.No problem.
• When there is disturbance in the air or vehicles passing nearby,the circuit may get false triggered.
• POT R5 can be used for sensitivity adjustment.

T.V.TRANSMMITER CIRCUIT

• Here, uses FM modulation for sound and PAL for video modulation.
• The audio signal to be modulated is pre-amplified using the transistor Q1 and associated components.
• The transistor Q2 has two jobs: production of carrier frequency and modulation.
• The pre-amplified audio signal is fed to the base of transistor Q2 for modulation. Capacitor C5 and inductor L1 forms the tank circuit which is responsible for producing the carrier frequency.
• The video signal is fed to the emitter of transistor Q2 via POT R7 for modulation. The modulated composite signal (audio+video) is transmitted by the antenna A1.
  
  
• NOTES:
• Assemble the circuit on a good quality PCB.
• Inductor L1 can be made by making 4 turns of 24SWG enameled copper wire on a 6mm dia: plastic former.
• T1 can be a radio frequency transformer with built in capacitor. (Can be found on old transistor radio boards).
• Antenna A1 can be a 1M long copper wire. (Experiment with the length to get optimum performance).
• This transmitter is working in VHF band somewhat between 50 – 210MHz.
• This transmitter is compatible only with PAL B and PAL G systems.

REMOTE CONTROLL TESTER

1. This is a simple remote controller tester circuit based on infrared sensor IC TSOP 1738.
2. When the IR waves fall on the sensor it output changes to low state.This makes the transistor Q1 ON and LED will blink according to the code contained in the sign.
3. So for press of each button the LED blinks in different ways.This is a good indication of the working of remote.The diode D1drops 0.7 V to give the IC ~ 5V supply from the available 6V .R2 is a current limiting resistance.
   NOTES:
4. Use a 6V battery to power up the circuit.
5. This circuit can be used to test remotes operating in the 38Khz carrier frequency.Almost all remotes fall into this category so no problem.