SINGLE TRANSISTOR AUDIO MIXER

1. This very simple audio mixer circuit uses only one transistor.
2. The base emitter junction of the transistor is biased by the diodes D1 and D2.
3.  The signals to be mixed are directly coupled to the base of Q1.
4. Each input lines are current limited by using a 33K resistor. With the used component values the collector current is around 1mA.
    
    
    
    
   NOTES:
   

   • The circuit can be assembled on a Vero board.
   • The circuit can be powered from 15V DC.
   • Gain of the individual channels can be adjusted by adding a 10K POT in series to the lines.

ELECTRONIC MOSQUITO REPELLER

1. Here is the circuit diagram of an ultrasonic mosquito repeller.
2. The circuit is based on the theory that insects like mosquito can be repelled by using sound frequencies in the ultrasonic (above 20KHz) range.
3. The circuit is nothing but a PLL IC CMOS 4047 wired as an oscillator working at 22KHz.
4. A complementary symmetry amplifier consisting of four transistor is used to amplify the sound.
5. The piezo buzzer converts the output of amplifier to ultrasonic sound that can be heard by the insects.

   
   
   
   
   
   
   
   
   NOTES:
   

   • Assemble the circuit on a general purpose PCB.
   • The circuit can be powered from 12V DC.
   • The buzzer can be any general purpose piezo buzzer.
   • The IC1 must be mounted on a holder.

SIMPLE SOUND GENERATOR CIRCUIT

1. This is a real scream generator circuit suitable for any purpose like alarm or car horn.
2.  The circuit is based on two transistors Q1 and Q2.
3. When you press the switch S2 the siren starts up moving to a high frequency.
4. When the switch is released the tone slips down until you shift it up again by pushing the switch S2.

   NOTES:

• Adjustment of tone quality can be obtained by different values for C2.
• If the alarm oscillates before S2 is pressed.The transistor is leaky, replace it.
• S1 can be used as a power switch.

FAN SPEED CONTROLLER USING LM2941

1. The circuit diagram shown here is of 12V DC fan speed controller using the IC LM2941CT which is a low drop out 1A voltage regulator.
2. The IC has a dropout voltage as low as 0.5 and has also many useful features like power supply reverse protection, thermal protection, short circuit protection etc. The maximum output current the IC can source is 1A.
3. The 12V DC supply is connected between the Vin (pin4) and ground (pin3) of the IC.
4. he load, which is the fan, is connected across the Vout (pin5) and ground (pin3) of the IC. The network comprising of potentiometers R1, R2 and resistor determines adjust current (Iadj) of the IC. By varying the Iadj using the POT R2 we can adjust the output voltage of the IC and hence the fan speed.
    
    
    
    
   NOTES:

   • The circuit can be powered from 12V DC.
   • The maximum possible load current is 1A.
   • A heat sink is recommended for the IC.
   • POT R1 can be used to adjust the minimum fan speed.
   • POT R2 can be used to adjust the fan speed.
   
   
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
   
   

   
   

NIGHT SECURITY LIGHT

1. Here is a simple circuit switches on a light around 2 hours after midnight, the time at which most of the robberies taking place.
2. This simple circuit is build around a CMOS IC 4060 to obtain the required timing.
3.  During day time the LDR has low resistance and keeps the pin 12 of the IC1 high, preventing the IC1 from oscillating. When it is dark the LDR resistance becomes high and the pin 12 of IC1 becomes low and the IC starts oscillating, which indicated by the flashing of LED D3.
4. The values of the timing components R1, R2, C4 are so selected that the out put pin3 of IC1 goes high after 8 hours.
5.  That means the high output drives the triac to switch on the lamp around 2’O clock. At morning, the LDR resistance drops and the pin 12 of IC1 goes high and stops the oscillation, making the lamp OFF. The switch S1 can be used to manually ON the lamp. The capacitor C2 prevents false triggering.
   
   

NOTES:

• Assemble the circuit on a good quality PCB or common board.
• The LDR can be general purpose LDR.
• The light sensitivity can be adjusted using the preset R6.
• The IC1 must be mounted on an IC holder.