Description.
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.
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. 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. 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.

Circuit diagram.

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.

Description.
This circuit is a simple approach for converting a loud speaker into a microphone. 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. 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. 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. The voice quality of this circuit will not be as much as a conventional microphone but quite reasonable quality can be obtained. 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.

Circuit diagram with Parts list.

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.

Description.

Here is the simplest scheme for converting a CD ROM drive of your computer to a Audio CD player.The minimum requirement for the player is that, it should have a audio output and skip button. Luckily most of the Cd ROM’s are equipped with both of these.

The CD ROM drive needs two voltages ,12V & 5V for its operation.So the main objective is to build a suitable power supply for the CD ROM drive.The IC1 (7812) together with associated components produce a regulated 12V DC.The IC2 (7805) together with associated components produce a regulated 5V DC.These voltages as well as ground can be connected to the corresponding voltage pins of the CD ROM drive using a male type CD ROM drive power connector.The 12V can be connected to the yellow wire of connector, 5V to red wire and GND to black wire as shown in figure1.Now the power supply is ready.

Testing.

Make the circuit as shown in the circuit diagram.Power up the circuit after connecting the power connector to the CD ROM drive. Now the power LED of the drive will glow.Insert the audio CD.Now the music will be available at the audio output socket of the drive.It can be heared using a headphone.The skip button of the drive can be used to play next song.

By connecting the audio output to a power amplifier you can enjoy the music in a greater wattage.Click Here! to get the amplifier circuits suitable for this project.

Circuit Diagram with Parts list.

Notes.

• For car stereo applications you don’t need the transformer,rectifier and the 7812 regulator.12V will be available from battery.You just need to produce a 5V from it using a 7805 based regulator. Connect the corresponding voltages to the connector as shown in figure 1 and connect the connector to drive.Done.

• The amplifier for the car audio CD player must be one operating from 12V.

• Do not connect the voltages to CD ROM drive in wrong polarity.Double check the voltages using a multimeter. Wrong polarity could easily damage the drive.

Introduction.

Here is the circuit diagram of a effective light to frequency converter circuit that can be used for variety of applications such as light intensity measurement,fun etc.

The circuit is based on TLC555, the CMOS version of famous timer IC NE 555. A photo diode is used for sensing the ligt 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.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.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.

Circuit Diagram & Parts List. 

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.