FM TELEPHONE BUG

1. Asimple telephone transmitter circuit that is ideal for transmitting the telephone conversation through small distances.The circuit is very simple and uses only few components.
2. The entire circuit can be easily included in the telephone itself or in the junction box.The circuit is powered from the telephone line itself.
3.  The transistor Q2 is wired as a a Colpitts oscillator to produce oscillations in the FM transmission band.
4.  The audio signal from the telephone line is coupled to the the base of Q2 to obtain the frequency modulation.The LED D1 acts as a power on indicator.
   
   
   
   
   
   
   NOTES:
   

   • Any radio frequency  PNP transistor like A933   can be used for Q2.
   • For L1 make  8 turns of  0 .6mm dia enameled copper wire  on a 3.6mm dia steel bolt.
   • For antenna use a 15cm long 0.6 mm dia copper wire.

   
   

SIMPLE IR AUDIO LINK

1. Here is a simple IR audio link that can be used to transmit audio signals up to 4 meters.
2. The signal to be transmitted is applied to the base of Q1 via resistor R4.
3.  The transistor Q1 drives the IR transmitting diodes D1 and D2.The audio input will be modulated to the IR signals transmitted.
4. The transmitted IR signals will be picked by the photo transistor Q2.
5.  The emitter voltage of the transistor Q2 will change according to the sound modulated to the IR signal.
6. The transistors Q3 and Q4 amplifies this signal to drive the speaker or headphone.C1 and R3 forms a filter to avoid interference from stray IR signals.

   
   

   
   NOTES:
   

   • The circuit can be assembled on a general purpose PCB.

   • Use 9V PP3 batteries for powering the transmitter and receiver.

   • The phototransistor Q2 can be any NPN phototransistor like PNZ154, PNA1605F, BPW77NA or BPW85.

   • An 8 Ohm speaker or a headphone can be used to hear the sound.

TELEPHONE TRANSMITTER

1. This is a simple, but very useful circuit that can be used to transmit telephone conversations.
2. When the telephone receiver is on hook the voltage across the lines will be about 48 volts. 
3. The preset R7 is so adjusted to obtain a 24.7 V across between the cathode of D2 and ground. 
4.  At this voltage the Zener diode D2 will be in breakdown and the transistor T1 will conduct. This makes the transistor T2 OFF.
5.  When the receiver is off hook, the line voltage drops to about 11 volts. This makes the transistor T1 OFF and subsequently the T2 ON.
6. The T2 in switched ON condition will provide a DC path for the transistor T3 used in the FM transmitter section.
7.  The transistor T3 is wired as a common emitter radio frequency oscillator. In simple words the transistor T2 serves as an ON/OFF switch for this oscillator.
8. The modulated signal will be available at the collector of transistor T3 and the signal id fed to the antenna via capacitor C5.

   
   

   
   NOTES:
   

   • Assemble the circuit on a good quality PCB.
   • For L1 make 45 turns of 36 SWG enameled copper wire on the resistor R6 itself.
   • The resistor R6 must be a 1M, 1 watt resistor.
   • For L2 make 3 turns of 21 SWG enameled copper wire on a 12 mm plastic former.
   • For antenna, use a 1 meter insulated copper wire.
   • The capacitor C3 can be a 50pF trimmer.

   
   

TELEPHONE USE IN INDICATOR

1. Here is a simple circuit that can be used as a telephone status indicator.
2. When the telephone is in use (off hook) the transistor Q1 switches ON making the red LED D2 glow.
3.  When the telephone is not in use (on hook) the Q1 turns OFF and Q2 turns ON.
4.  This makes the red LED D2 off and green LED D3 ON.
5. The circuit is powered from the phone line itself and no external power supply is required.

   
   
   
   
   
   
   NOTES:
   

   • The circuit can be assembled on a general purpose PCB.
   • IF 2A ampere bridge is not available, make one using diodes like 1N4007.
   • Note that some countries prohibit people from connecting other devices to the phone line.

   
   

FM TRANSMITTER USING UPC 1651

1. Here is the circuit diagram of an FM transmitter using the IC UPC1651. UPC1651 is a wide band UHF Silicon MMIC amplifier. 
2. The IC has a broad frequency response to 1200MHz and power gain up to 19dB.The IC can be operated from 5V DC.
3.  The audio signals picked by the microphone are fed to the input pin (pin2) of the IC via capacitor C1.
4.   C1 acts as a noise filter.
5.  The modulated FM signal will be available at the output pin (pin4) of the IC. Inductor L1 and capacitor C3 forms the necessary LC circuit for creating the oscillations.
6. Frequency of the transmitter can be varied by adjusting the capacitor C3.

   
   

   
   NOTES:
   

   • The circuit can be assembled on a Vero board.
   • Inductor L1 can be made by making 5 turns of 26SWG enameled copper wire on a 4mm diameter plastic former.
   • A ¾ meter insulated copper wire can be used as the antenna.
   • Do not give more than 6V to the IC.
   • Mic M1 can be a condenser microphone.

   
   

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.