PERSONAL STEREO

   In the output stages of most broadcast receivers and some amplifiers,there is a limit up to which maximum power can be developed without distortion. In the widely accepted output circuit, two output transistors are connected in series between the positive and ground and biasing is adjusted so that each transistor gets half the supply voltage.

   The circuit presented here is a simple audio amplifier for a personal stereo system. In this, supply voltage to each transistor can be enhanced to produce a larger output. The audio driver transformer drives the transistors adequately. A 9V-0-9V, 300mA transformer has been used in the set-up. Out of the four diodes (D1 through D4), two are used for developing the positive voltage rail (+9V) and the other two are used for developing the negative voltage rail (–9V). In the push-pull amplifier, each transistor (T2 or T3) gets double the voltage when activated.

   Connect the low audio signal from the stereo system at input terminals A and B of the audio amplifier and provide mains AC to activate the circuit. During the first half cycle of an AF cycle, transistor T2 conducts and the current flows from positive rail to ground rail (centre tap of transformer X1) via the loudspeaker coil (connected between the emitter of transistor T2 and ground) in one direction. While in the second half cycle, transistor T3 conducts and the current flows from ground rail to negative rail via the loudspeaker coil (connected between ground and the collector of transistor T3) in a direction opposite to the previous

flow. Transistors T2 and T3 of the pushpull audio amplifier should be matched correctly. If these transistors get heated, change the bleeding resistor pairs (R3 and R4 for transistor T2 and R5 and R7 for transistor T3) so that the acceptable output without overheating is obtained. You can also replace these transistors with another pair of suitable high-power transistors. For driving transistors T2 and T3, a 9V audio driver transformer having six leads is used. It is readily available in the market and reasonably matches the output and input impedances of the preceeding and succeeding stages.

   To test the quality of the audio output, connect the stereo’s outputs to the respective terminals A and B. Now increase the volume level of the stereo slowly. If you get a high-level, high-quality sound across loudspeaker L1, the amplifier is working well. If the sound quality is not good, decrease the volume level until the audio amplifier gives good results. Note that this audio amplifier works well for low-level audio signals.

PC Multimedia Speakers

      This circuit of multimedia speakers for PCs has single-chipbased design, low-voltage power supply, compatibility with USB power, easy heat-sinking, low cost, high flexibility and wide temperature tolerance. At the heart of the circuit is IC TDA2822M. This IC is, in fact, monolithic type in 8-lead mini DIP package. It is intended for use as a dual audio power amplifier in battery-powered sound players. Specifications of TDA2822M are low quiescent current, low crossover distortion, supply voltage down to 1.8 volts and minimum output power of around 450 mW channel with 4-ohm loudspeaker at 5V DC supply input. An ideal power amplifier can be simply defined as a circuit that can deliver audio power into external loads without generating significant signal distortion and without consuming excessive quiescent current. This circuit is powered by 5V DC supply available from the USB port of the PC. When power switch S1 is flipped to ‘on’ position, 5V power supply is extended to the circuit and power-indicator red LED1 lights up instantly. Resistor R1 is a current surgelimiter and capacitors C1 and C4 act as buffers. Working of the circuit is simple. Audio signals from the PC audio socket/headphone socket are fed to the amplifier circuit through components R2 and C2 (left channel), and R3 and C3 (right channel). Potmeter VR1 works as the volume controller for left (L) channel and potmeter VR2 works for right (R) channel. Pin 7 of TDA2822M receives the left channel sound signals and pin 6 receives the right-channel signals through VR1 and VR2, respectively. Amplified signals for driving the left and right loudspeakers are available at pins 1 and 3 of IC1, respectively. Components R5 and C8, and R6 and C10 form the traditional zobel network. Assemble the circuit on a medium-size, general-purpose PCB and enclose in a suitable cabinet. It is advisable to use a socket for IC TDA2822M. The external connections should be made using suitably screened wires for better result.

Battery-level indicator

      Normally, in mobile phones,the battery level is shown indot or bar form. This lets youeasily recognise the battery level. Herewe present a circuit that lets you knowthe battery level of a device from thenumber of LEDs that are glowing. Ituses ten LEDs in all. So if three LEDsglow, it indicates battery capacity of 30 per cent. Unlike in mobile phones where the battery-level indicator function is integrated with other functions, here only one comparator IC (LM3914) does it all. The LM3914 uses ten comparators, which are internally assembled in the voltage divider network based on the current-division rule. So it divides the battery level into ten parts. The circuit derives the power supply for its operation from the battery of the device itself. It uses ten LEDs wired in a 10-dot mode. The use of different coloured LEDs makes it easier to recognise the
voltage level on the basis of the calibration made. Red LEDs (LED1 through LED3) indicate battery capacity of less than 40 per cent. Orange LEDs (LED4 through LED6) indicate battery capacity of 40 to less than 70 per cent and green LEDs (LED7 through LED10) indicate battery capacity of 70 to under 100 per cent. The brightness of the LEDs can be adjusted by varying the value of preset VR2 between pins 6 and 7. Diode D1 prevents the circuit from reverse-polarity battery connection. The tenth LED glows only when the battery capacity is full, i.e., the battery is fully charged. When the battery is fully charged, relay-driver transistor T1 conducts to energise relay RL1. This stops the charging through normally-open (N/O) contacts of relay RL1. For calibration, connect 15V variable, regulated power supply and initially set it at 3V. Slowly adjust VR1 until LED1 glows. Now, increase the input voltage to 15V in steps of 1.2V until the corresponding LED (LED2 through LED10) lights up. Now the circuit is ready to show any voltage value with respect to the maximum voltage. As the number of LEDs is ten, we can easily consider one LED for 10 per cent of the maximum voltage. Connect the voltage from any battery to be tested at the input probes of the circuit. By examining the number of LEDs glowing you can easily know the status of the battery. Suppose five LEDs are glowing. In this case, the battery capacity is 50 to 59 per cent of its maximum value. Assemble the circuit on a generalpurpose PCB. Calibrate it and then enclose in a box.