BATTERY CHARGING INDICATOR CIRCUIT

1. This simple circuit can be used to monitor whether a battery is charging or not.
2. The voltage comparator IC LM393 is the heart of this circuit. The LED D1 will remain ON whenever there is at least 25 milli ampere current flowing to the battery.  
3. This particular circuit is designed for a 12V battery with charging current less than 1A.
4. By slightly modifying the component values, the charging current and voltage can be modified.

NOTES:

• Assemble the circuit on a good quality PCB.
• The IC1 must be mounted on a holder.
• Do not use this circuit with batteries has more than 1A charging current.

SOUND PRESSURE METER

1. The sound pressure meter is very useful in setting up home theater systems , if you need to test the sound pressure of each channels on different positions in the room..
2. The circuit is nothing but a non- inverting amplifier based on op-amp CA 3140 9 (IC1).
3. The sound picked by the condenser mic will be amplified by the IC1 and rectified by the bridge D1 to drive the meter M1.
4. The deflection on the meter will be proportional to the pressure of the sound falling on the mic.The switch S1 can be used as an ON/OFF switch.

NOTES:

• The bridge D1 can be a 1A bridge.If 1A bridge is note available,make one using four 1N4007 diodes.
• The circuit ca be powered by using a 9V PP3 battery.
• The capacitors must be rated 10V.
• Mount the IC1 on a holder.

24 V FLASHER CIRCUIT

1. The circuit shown here works from 24V DC and can flash two 24V bulbs alternatively. The circuit is nothing but an astable multivibrator operating at 1Hz and 50% duty cycle.
2. The lamp to be flashed is connected in place of the collector resistor (Rc) of a typical design.
3. 2N3055 transistors which are well known for their ruggedness are used here. They can handle collector currents up to 10A. 
4. So a wide range of lamps (in terms of wattage) can be connected to this circuit.

NOTES:

• Assemble the circuit on a good quality PCB.
• Use 24V DC for powering this circuit.
• Lamp L1 and L2 can be 24V/25W type.
• It is advised to fit the transistors with heat sinks.
• Flashing rate can be modified by adjusting the values of R1, R2, C1 and C2.

AUTOMATIC NIGHT LIGHT

1. The circuit will automatically switch on the AC lamp when night falls and the lamp will be automatically switched off after a preset time.
2. The working of this night light circuit very simple. An LDR is used as the sensor here. At day time the resistance of the LDR will be low and so do the voltage drop across it, the transistor Q1 will be in the conducting mode.
3. When darkness falls the resistance of LDR increases and so do the voltage across it. This makes the transistor Q1 OFF. Base of Q2 is connected to the emitter of Q1 and so Q2 is biased on which in turn powers the IC1.
4. NE555 is wired as monostable multivibrator that is automatically triggered at power ON. This automatic triggering is achieved with the help of capacitor C2. The output of IC1 remains high for a time determined by resistor R5 and capacitor C4.
5. When output of IC1 goes high transistor Q3 is switched ON which triggers triac T1 and the lamp glows. A 9V battery is included in the circuit in order to power the timer circuit during power failures.
6. Resistor R1, diode D1, capacitor C1 and Zener D3 forms the power supply section of the circuit. R7 and R8 are current limiting resistors.

NOTES:

• The circuit can be assembled on a vero board.
• Preset R2 can be used to adjust the sensitivity of the circuit.
• Preset R5 can be used to adjust the ON time of the lamp.
• With R5 @ 4.7M the ON time will be around three hours.
• The wattage of L1 must not exceed 200W.
• Heat sink is recommended for BT136.
• IC1 must be mounted on a holder.

LIGHT TO FREQUNCY CONVERTER CIRCUIT

1. The circuit is based on TLC555, the CMOS version of famous timer IC 555.
2. A Photo diode is used for sensing the light intensity.The timer IC is wired in astable mode.The leakage current of the reverse biased photo diode is proportional to the light intensity falling on it.
3. This leakage current charges the capacitance C1.When the capacitor voltage reaches 2/3 of the supply voltage the out put (pin 3) goes low.
4. As a result the capacitor discharges through photo diode .When the capacitor voltage reaches 1/3 the supply voltage the out put (pin 3) of IC goes high.This cycling continues and we get a frequency at pin 3  proportional to the light intensity falling on the photo diode.

NOTES:

• With the given components the frequency  varies from 1KHZ @ complete darkness to  24 Khz @  bright sunlight.The frequency range can be changed by using different values for C1.
• Use any general purpose photo diode for D1.

AUTOMATIC CHANGEOVER CIRCUIT

1. This circuit can be used for the automatic switchover of a load between a battery and a wall adapter.LTC4412 controls an external P-channel MOSFET to create a near ideal diode function for power switch over and load sharing. 
2. This makes the LT4412 an ideal replacement for power supply ORing diodes. A wide range of MOSFETs can be driven using the IC and this gives much flexibility in terms of load current.
3. The LT4412 also has a bunch of good features like reverse battery protection, manual control input, MOSFET gate protection clamp etc.
4. The diode D1 prevents the reverse flow of current to the wall adapter when there is no mains supply. Capacitor C1 is the output filter capacitor. Pin 4 of the IC is called the status output.
5. When wall adapter input is present the status output pin will be high and this can be used to enable another auxiliary P-channel MOSFET 

NOTES:

• Assemble the circuit on a good quality PCB.
• The wall adapter input can be anything between 3 to 28V DC.
• The battery voltage can be anything between 2.5V to 28V.
• Do not connect loads that consume more than 2A.
• Maximum continuous drain current of Q1 (FDN306P) is 2.5A.
• D1 (1N5819 is a 1A Schottky diode.
• Q1 (FDN306P) is a P-channel MOSFET.

SPEAKER TO MICROPHONE CONVERTER

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.