Microphone is added here for the very use it has, and by the ease of making one. It can be used as a good quality microphone. Here is a simple circuit that makes the condenser mikes work, which can also be used as a collar mike. Condenser mikes are rugged, have excellent frequency response, and they develop fairly high signal output. They can certainly be implemented to test later circuits. Make this and I am sure you will make many for many of your friends.

These ubiquitous devices are seen everywhere, in portable tape recorders to cell phones. These are small, sensitive, and rugged and have an excellent frequency response, and high signal to noise ratio. They deliver higher level of signal into preamplifier. It has an internal field effect transistor which needs power at less than half a milliampere. Condenser mike has two terminals; one terminal can be seen as connected to the body of the mike. The other terminal is the HOT terminal. It doubles up as an output terminal and power +ve terminal. A single pen cell can power it.

Circuit is shown in Schematic 35. Condenser mike is biased with a 3.3K resistor to power the internal FET. Output is also taken from this terminal to the Mike (MIC) input of an amplifier. There must be a capacitor at the input of the preamplifier to block the DC voltage from condenser mike. Preamplifiers generally have this capacitor at their input. Negative terminal of the battery is connected to the ground terminal of the mike. It is also connected to the ground of the amplifier through the shield of the connecting mike cable.

Connect 3.3 K resistor to the positive side of single cell battery holder with the other end of the resistor to the HOT terminal through a mike cable. Negative side of the battery holder is connected to the ground terminal of Mike: Solder a shield wire of required length to the

terminals of the mike. Central core wire goes to the hot or output terminal and the external shield goes to the ground terminal. The other end of the shield wire is connected to the appropriate connector suitable to the power amplifier. Use good quality shield wire and appropriate input jack pin suitable for the amplifier. Make two such mikes for stereo applications

Preamplifiers are used to amplify low level signals such as those from mikes, tape heads before they are fed into power amplifiers. Power amplifiers are generally less sensitive. Frequency response also can be suitably trimmed and modified at preamp stage. LA 3161 is one of those widely used in tape decks and amplifiers as a stereo preamplifier.

Block Diagram is shown in Figure 42. LA 3161 has two low noise preamplifiers with good ripple rejection on chip catering to stereo applications. External part count is low and Single In line (SIL))(Figure 43) package makes mounting easy. While the operating voltage is 9V, the IC can tolerate voltages up to 18V. Typical input resistance is 100K and output resistance is 10K with an open loop gain of78dB. Block diagram of the IC is given below. Input is given at Pin 1 and 8, output is taken at Pin 3 and 6, and negative Feedback is given at Pin 2 and 7. Power is at Pin 5 and Pin 4 is the ground terminal. There is an internal voltage regulator.

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Wouldn’t it be better if we have an adjustable but regulated power supply, which can cater to all the voltages we need? And we have such a wonderful device where you can continually adjust your output DC voltage. Well! It is short circuit protected, has only three terminals and all other good things. LM317

LM317T is an adjustable 3 terminal positive voltage regulator capable of supplying around 1.5 amps over an output range of 1.25 to 37 volts. It also has built in current limiting and thermal shutdown features, which makes it virtually blowout proof. This is an excellent startup project with low ripple. With an easy adjustment of regulated voltage, it can be used as power source for most of the applications in the next chapters. Pin out is given in Figure 13.

Circuit :

Rectifier part is same as shown in the earlier schematic of bridge rectifier. Transformer is changed to 18 V to get a better range of regulated voltages. 9-0-9 Transformer is used as it is difficult to get an 18 V transformer. Please change transformer rating as per your requirement and the capacitor also for higher voltage rating. It is preferable to use 2200 mfd. caps as larger value makes good, low ripple output voltage.

Pulsating DC output from the bridge is now filtered by the 2200uF capacitor and fed to TN’-put terminal (1) of LM317 regulator. The output of this regulator is varied via the ‘Adj’ pin(3) and the 5K variable resistance or preset pot meter (Rl) connected to it. The regulator uses an internal Zener diode to provide a fixed reference voltage of 1.2 volt across the external resistor R2. Hence the lower end of output voltage is limited to 1.2 volts. C2 is 47uF decoupling capacitor to filter out the transient noise. Metal tab of LM317 is connected internally to the ‘Output’ pin (2). The circuit diagram is shown in Schematic 5.

Construction

Use a small Vero board to fix all the components, cut the tracks where not required and solder the pins. Mount the LM317 regulator on a heat sink. However it can be screwed to the metal case of the enclosure box with the mica insulator and the nylon washer with the mounting screw as shown in Figure 12.

Use a little of heat sink compound on the metal tab and mica insulator as it helps to transfer heat between LM317 and case or heat sink.Use a metal box of suitable size and fix the veroboard and transformer. Mains wire is connected to the primary side of the transformer and taken out. Carefully insulate the mains joints. Mains switch is not shown in the schematic. You may add one if it is required. 5k linear potentiometer is fixed at the casing. Measure the voltages and mark them suitably on a dial fixed on the face of the casing with appropriate indication of the voltage. LED is also fixed on the casing to indicate the power supply.

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500mA transformers are generally used for these circuits. Connect the primary wires to the mains chord after carefully insulating the joints. This transformer has three wires on the secondary side. AC voltage across both end wires is 12 and voltage across any one end wire and center wire is 6. So if both end wires are connected as shown in the present schematic the output DC voltage will be 12 and if any one of the end wires and center tap are connected the voltage will be 6.

Take a Veroboard and solder all components on it except mains power supply. Solder IN4003 diode to one end of secondary winding making note of the cathode. Solder lOOOuf /25V capacitor. Please note the capacitor is polarized, which means that you should connect it one way only. Negative side of the pin is marked on the can. Add a light emitting diode to know that the power supply is on. Now use a 1K-1A watt resistor in series with it.

LEDs are also polarized and marked. LEDs will not light up if they are connected in reverse. Take two pen light cells and connect the ends of a Red LED to positive and negative and then turn the leads over and try. (Forget the cathodes and anodes for the time being.) You will know that it works only one way. That also explains how a diode works. Do not connect them directly to 12 V  LEDs cannot work beyond 5 V and their current capability is extremely limited. Although they are very rugged devices, they must always be used with a current limiting resistor.

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