12V 20W Compact High Performance Stereo Amplifier Circuit project with schematic and explanation

Amplifiers which run from 12V DC generally don’t put out much power and they are usually not hifi as well. But this little stereo amplifier ticks the power and low distortion boxes. With a 14.4V supply, it will deliver 20 watts per channel into 4-ohm loads at clipping while harmonic distortion at lower power levels is typically less than 0.03%. This is an ideal project for anyone wanting a compact stereo amplifier that can run from a 12V battery. It could be just the ticket for buskers who want a small but gutsy amplifier which will run from an SLA battery or it could used anywhere that 12V DC is available – in cars, recreational vehicles, remote houses with 12V DC power or where ever.

20W Stereo Audio Amplifier Image:

Because it runs from DC, it will be an ideal beginner’s or schoolie’s project, with no 240VAC power supply to worry about. You can run it from a 12V battery or a DC plugpack. But while it may be compact and simple to build, there is no need to apologise for “just average” performance. In listening tests from a range of compact discs, we were very impressed with the sound quality. Long-time readers might recall that we presented a similar 12V power amplifier design back in May 2001. It was a similar configuration to this one but it is now completely over-shadowed by the much lower distortion and greatly improved signal-to-noise ratio of this new design. In fact, let’s be honest: the previous unit is not a patch on this new design. It used two TDA1519A ICs which resulted in distortion figures above 1% virtually across the board and a signal-to-noise ratio of only -69dB unweighted.

20W Stereo Amplifier Circuit:

 

However, by using the TDA­7377 power amplifier IC and making some other improvements, the THD (total harmonic distortion) of the new design is about 50 times better than the older unit (see performance graphs for details). The bottom line is that the THD under typical conditions is around just 0.03% or less. It is also able to deliver more output power due to the improved output transistors in the new power amplifier IC. In addition, its idle power consumption is low – not much more than 1W. As a result, if you don’t push it too hard it will run cool and won’t drain the battery too quickly. And because the IC has self-protection circuitry, it’s just about indestructible. It will self-limit or shut down if it overheats and the outputs are deactivated if they are shorted.

20W Stereo Amplifier Circuit Diagram:

With a 12V supply, the largest voltage swing a conventional solid-state power amplifier can generate is ±6V. This results in a meagre 4.5W RMS into 4O and 2.25W RMS into 8O, without considering losses in the output transistors. Even if the DC supply is around 14.4V (the maximum that can normally be expected from a 12V car battery), that only brings the power figures up to 6.48W and 3.24W for 4O and 8O loads respectively – still not really enough. There are three common solutions to this problem. The first is to boost the supply voltage using a switchmode DC converter. This greatly increases the cost and complexity of the amplifier but it is one way of getting a lot of power from a 12V supply. However, we wanted to keep this project simple and that rules out this technique.

Parts layout:

There are variations on the boosting method, such as the class H architecture used in the TDA1562Q IC featured in the Portapal PA Amplifier (SILICON CHIP, February 2003). It is able to achieve 40W/channel but with >0.1% THD. In that case, the amplifier output itself provides the switching for a charge pump. The second method is to lower the speaker impedance. Some car speakers have an impedance as low as 2O, which allows twice as much power to be delivered at the same supply voltage. However, we don’t want to restrict this amplifier to 2O loudspeakers.

Author: Nicholas Vinen - Copyright: Silicon Chip

Class-A Headphone Amplifier Circuit project with schematic and explanation

This circuit is derived from the Portable Headphone Amplifier featuring an NPN/PNP compound pair emitter follower output stage. An improved output driving capability is gained by making this a push-pull Class-A arrangement. Output power can reach 427mW RMS into a 32 Ohm load at a fixed standing current of 100mA. The single voltage gain stage allows the easy implementation of a shunt-feedback circuitry giving excellent frequency stability.

Class-A Headphone Amplifier Circuit diagram:

The above mentioned shunt-feedback configuration also allows the easy addition of frequency dependent networks in order to obtain an useful, unobtrusive, switchable Tilt control (optional). When SW1 is set in the first position a gentle, shelving bass lift and treble cut is obtained. The central position of SW1 allows a flat frequency response, whereas the third position of this switch enables a shelving treble lift and bass cut.

Note:

• Before setting quiescent current rotate the volume control P1 to the minimum, Trimmer R6 to zero resistance and Trimmer R3 to about the middle of its travel.
• Connect a suitable headphone set or, better, a 33 Ohm 1/2W resistor to the amplifier output.
• Connect a Multimeter, set to measure about 10Vdc fsd, across the positive end of C5 and the negative ground.
• Switch on the supply and rotate R3 in order to read about 7.7-7.8V on the Multimeter display.
• Switch off the supply, disconnect the Multimeter and reconnect it, set to measure at least 200mA fsd, in series to the positive supply of the amplifier.
• Switch on the supply and rotate R6 slowly until a reading of about 100mA is displayed.
• Check again the voltage at the positive end of C5 and readjust R3 if necessary.
• Wait about 15 minutes, watch if the current is varying and readjust if necessary.

Parts List :

P1          : 22K  Dual gang Log Potentiometer 
R1          : 15K 
R2          : 220K
R3          : 100K
R4          : 33K 
R5          : 68K 
R6          : 50K 
R7          : 10K 
R8,R9       : 47K 
R10,R11     : 2R2 
R12         : 4K7
R13         : 4R7
R14         : 1K2
R15,R18     : 330K
R16         : 680K
R17,R19     : 220K
R20,R21     : 22K
C1,C2,C3,C4 : 10µF/25V 
C5,C7       : 220µF/25V
C6,C11      : 100nF
C8          : 2200µF/25V
C9,C12      : 1nF
C10         : 470pF
C13         : 15nF
D1          : LED
D2,D3       : 1N4002 
Q1,Q2       : BC550C 
Q3          : BC560C  
Q4          : BD136   
Q5          : BD135   
IC1         : 7815
T1          : 15CT/5VA Mains transformer
SW1         : 4 poles 3 ways rotary Switch 
SW2         : SPST slide or toggle Switch

Save Your Ears - A Noise Meter Circuit project with schematic and explanation

‘Hello… HELLO! Are you deaf? Do you have disco ears?’ If people ask you this and you’re still well below 80 , you may be suffering from hearing loss, which can come from (prolonged) listening to very loud music. You won’t notice how bad it is until it’s too late, and after that you won’t be able to hear your favorite music the way it really is – so an expensive sound system is no longer a sound investment. To avoid all this, use the i-trixx sound meter to save your ears (and your neighbor's ears!).

With just a handful of components, you can build a simple but effective sound level meter for your sound system. This sort of circuit is also called a VU meter. The abbreviation ‘VU’ stands for ‘volume unit’, which is used to express the average value of a music signal over a short time. The VU meter described here is what is called a ‘passive’ type. This means it does not need a separate power supply, since the power is provided by the input signal. This makes it easy to use: just connect it to the loudspeaker terminals (the polarity doesn’t matter) and you’re all set.

The more LEDs that light up while the music is playing, the more you should be asking yourself how well you are treating your ears (and your neighbours’ ears). Of course, this isn’t an accurately calibrated meter. The circuit design is too simple (and too inexpensive) for that. However, you can have a non-disco type (or your neighbors) tell you when the music is really too loud, and the maximum number of LED lit up at that time can serve you as a good reference for the maximum tolerable sound level.

Although this is a passive VU meter, it contains active components in the form of two transistors and six FETs. Seven LEDs light up in steps to show how much power is being pumped into the loudspeaker. The steps correspond to the power levels shown in the schematic for a sine-wave signal into an 8-ohm load. LED D1 lights up fi rst at low loudspeaker voltages. As the music power increases, the following LEDs (D2, D3, and so on) light up as well. The LEDs thus dance to the rhythm of the music (especially the bass notes).

 Noise Meter Circuit Diagram:

This circuit can easily be assembled on a small piece of prototyping board. Use low-current types for the LEDs. They have a low forward voltage and are fairly bright at current levels as low as 1 mA. Connect the VU meter to the loudspeaker you want to monitor. If LED D2 never lights up (it remains dark even when LED D3 lights up), reverse the polarity of diode D8 (we have more to say about this later on). In addition, bear in mind that the sound from the speaker will have to be fairly loud before the LEDs will start lighting up.

If you want to know more about the technical details this VU meter, keep on reading. Each LED is driven by its own current source so it will not be overloaded with too much current when the input voltage increases. The current sources also ensure that the final amplifier is not loaded any more than necessary. The current sources for LEDs D1–D6 are formed by FET circuits. A FET can be made to supply a fixed current by simply connecting a resistor to the source lead (resistors R1–R6 in this case). With a resistance of 1 kΩ, the current is theoretically limited to 1 mA. However, in practice FETs have a especially broad tolerance range. The actual current level with our prototype ranged from 0.65 mA to 0.98 mA.

To ensure that each LED only lights up starting at a defined voltage, a Zener diode (D8–D13) is connected in series with each LED starting with D2. The Zener voltage must be approximately 3 V less than the voltage necessary for the indicated power level. The 3-V offset is a consequence of the voltage losses resulting from the LED, the FET, the rectifier, and the over voltage protection. The over voltage protection is combined with the current source for LED D7. One problem with using FETs as current sources is that the maximum rated drain–source voltage of the types used here is only 30 V.

If you want to use the circuit with an especially powerful fi nal amplifier, a maximum input level of slightly more than 30 V is much too low. We thus decided to double the limit. This job is handled by T7 and T8. If the amplitude of the applied signal is less than 30 V, T8 buffers the rectified voltage on C1. This means that when only the first LED is lit, the additional voltage drop of the over voltage protection circuit is primarily determined by the base–emitter voltage of T8. The maximum worst-case voltage drop across R8 is 0.7 V when all the LEDs are on, but it has increasingly less effect as the input voltage rises.

R8 is necessary so the base voltage can be regulated. R7 is fitted in series with LED D7 and Zener diode D13, and the voltage drop across R7 is used to cause transistor T7 to conduct. This voltage may be around 0.3 V at very low current levels, but with a current of a few mili-amperes it can be assumed to be 0.6 V. Transistor T7 starts conducting if the input voltage rises above the threshold voltage of D7 and D13, and this reduces the voltage on the base of T8. This negative feedback stabilizes the supply voltage for the LEDs at a level of around 30 V. With a value of 390 Ω for R7, the current through LED D7 will be slightly more than 1 mA.

This has been done intentionally so D7 will be a bit brighter than the other LEDs when the signal level is above 30 V. When the voltage is higher than 30 V, the circuit draws additional current due to the voltage drop across R8. The AC voltage on the loudspeaker terminals is half-wave rectifi ed by diode D14. This standard diode can handle 1 A at 400 V. The peak current level can be considerably higher, but don’t forget that the current still has to be provided by the fi nal amplifier.

Resistor R9 is included in series with the input to keep the additional load on the fi nal amplifi er within safe bounds and limit the interference or distortion that may result from this load. The peak current can never exceed 1.5 A (the charging current of C1), even when the circuit is connected directly to an AC voltage with an amplitude of 60 V. C1 also determines how long the LEDs stay lit. This brings us to an important aspect of the circuit, which you may wish to experiment with in combination with the current through the LEDs.

An important consideration in the circuit design is to keep the load on the fi nal amplifi er to a minimum. However, the combination of R9 and C1 causes an averaging of the complex music signal. The peak signal levels in the music are higher (or even much higher) than the average value. Tests made under actual conditions show that the applied peak power can easily be a factor of 2 to 4 greater than what is indicated by this VU meter. This amounts to 240 W or more with an 8-Ω loudspeaker.

You can reduce the value of C1 to make the circuit respond more quickly (and thus more accurately) to peak signal levels. Now a few comments on D8. You may receive a stabistor (for example, from the Philips BZV86 series or the like) for D8. Unlike a Zener diode, a stabistor must be connected in the forward-biased direction. A stabistor actually consists of a set of PN junctions in series (or ordinary forward-biased diodes). Check this carefully: if D2 does not light up when D8 is fi tted as a normal Zener diode, then D8 quite likely a stabistor, so you should fi t it the other way round.

Source: Elektor Electronics 12-2006

Stereo Power Amplifier Using IC 7905

For a few pounds you can buy a kit from any automotive accessory shop that will allow your car to be fitted with a central-locking door system. Such a kit essentially comprises a number of motors. There is also a control unit that enables the whole system to function. Here we show an example of such a unit. There are 5-wire motors and 2-wire motors. The 5-wire version is used in doors that have a key-lock.

Car Central Locking System Project Image: 

There are 2 connections for the motor itself and 3 connections for the sensor part (an ‘open’ and a ‘close’ contact). These sensors determine whether the door is to be unlocked or locked. If there is no key lock in the door, these sensors are superfluous and a 2-wire motor can be used.

The polarity of the motor determines whether the locking mechanism goes up or down. By making a circuit that simply reverses the polarity of the motor, the door can be either locked or unlocked. The winding of the motor is connected between M1 and M2 in the schematic. When relay Re1 is energised, all motors will, for example, rotate anti-clockwise. By activating Re2 the motors will rotate clockwise. This depends on the actual polarity of the motor, of course.

The sensors are connected to R1 and R10. Here you have to pay careful attention. If Re1 causes the door to unlock, then Re1 has must obviously be connected to the ‘open’ contact. In that case, Re2 is for locking the doors and R10 is then connected to the ‘close’ contact. The R/C-combinations R16/C3 and R15/C4 ensure that the relays are energised for a certain amount of time (obviously this can be changed if this time is too short or too long for your doors).

Car Central Locking System Circuit Diagram:

This time has to be just long enough to lock or unlock the doors. The third wire of the sensors is the common and has to be connected to +12 V. The RC circuits at the inputs Sopen en Sclose ensure that the motors are driven only once when the door is locked or unlocked.  In addition, there is a provision to allow the unit to be connected to a car alarm. There are two types of alarm available, with positive or negative control. In order to make the unit universally applicable, both types of alarm can be used. The circuit around T3 and T4 makes this possible. The diode inputs (D3 and D8) react to a rising edge, R6 and R8 react to a falling edge. An RC time constant is used here as well to ensure that both relays are energised only once.

Maybe this is stating the obvious: a motor unit has to be built into each door. All motor wires and sensor wires are connected in parallel to the electronics. The actual type of relay is not critical. The type indicated has the following proper-ties: coil 12 V/400 Ω; max. switching current 12 A (AC), max. switching power 1200 VA.

PCB Layout:

Finally, a car is a hostile environment for electronics. Ensure good connections, use automotive connectors and crimp these on the wires using the appropriate crimping tool. Solder connections in wires are best avoided. They have the tendency to break where the wire transitions into the solder connection when the wire is subject to vibration. Fasten the wires at regu-lar intervals.

Steam Whistle Circuit project with schematic and explanation

This circuit consists of six square wave oscillators. Square waves are made up of a large number of harmonics. If six square waves with different frequencies are added together, the result will be a signal with a very large number of frequencies. When you listen to the result you’ll find that it is very similar to a steam whistle. The circuit should be useful in modelling or even in a sound studio. This circuit uses only two ICs. The first IC, a 40106, contains six Schmitt triggers, which are all configured as oscillators. Different frequencies are generated by the use of different feedback resistors.

Steam Whistle Circuit Diagram :

The output signals from the Schmitt triggers are mixed via resistors. The resulting signal is amplified by IC2, an LM386. This IC can deliver about 1 W of audio power, which should be sufficient for most applications. If you leave out R13 and all components after P1, the output can then be connected to a more powerful amplifier. In this way a truly deafening steam whistle can be created. The ‘frequency’ of the signal can be adjusted with P2, and P1 controls the volume.

Author: Gert Baars - Copyright: Elektor Electronics 2004

Simple Dag-gerboard Position Detector Circuit Schematic with Explanation

It is easy to build Dag-gerboard Position Detector Cirucit.  In sailing regattas it’s handy to have a dag-gerboard that can be raised and lowered vertically. As the winding handle or positioning motor needs to rotate the spindle of the lifting device some 100 to 150 times throughout its full range it would be extremely handy to have a quick idea of its current position. An electronic count of the number of revolutions would be ideal. Thank goodness most sailors now have a 12-V supply available!

To get this to work you need to apply white and black markings to the spindle, each covering half of the circumference. Next, mask off two electric eye devices (reflected light sensors) next to one another (approximately 10 mm apart). For secure detection both sensors should be positioned not more than 5 mm from the paint markings.

Dag-gerboard Position Detector Circuit Diagram :

The markings to be read by the sensor should be displaced laterally, so that the direction of rotation can be recognised in addition to the number of revolutions counted. At the heart of our circuit is a PIC16F628 from Microchip, which as usual can be bought ready programmed from Elektor or you can do this bit yourself by downloading free firmware (for details of both see [1]).

At pins 1 of the two reflected light sensors IC3 and IC4 we need to ‘see’ more than 2.0 V from the white segment and less than 0.8 V from the black mark (with an operating volt-age between 4.5 and 5.5 V). The two signals detected are taken to plug connector along with the operating voltage and ground. It’s convenient if you also provide a connector from the microcontroller as well, so that the sensor and the controller board can be linked by a test lead.

The multiplexing of the three seven-segment displays is programmed at a rate of 100 Hz. 
Acceptable values for the revolution count are between 0 and 140. If the count exceeds or falls below these limits, then the counter is not incremented. The RESET key S2 sets the counter back to zero. Jumper K2 enables you to reverse the direction of counting. The count is retained if the operating voltage is removed and is loaded again when next pow-ered up.

The source code can also be downloaded from the website mentioned above, making it possible (for instance) to define alternative counter limit values (the maximum value is defined in the line #define max 140). For compiling the code you can use the CC5X compiler, of which there is a free version (www.bknd.com/cc5x).

Author : Hermann Sprenger - Copyright : Elektor

DETECTOR VIBRATIONS FOR CAMERA OBSCURE circuit schematic with explanation

DETECTOR VIBRATIONS FOR CAMERA OBSCURE

SCHEMA ELECTRONIC SENSOR VIBRATIONS:

It is an instrument of great precision and sensitivity, realized with very modern concepts. I / element
sensitive is is a photo resistance that is able to reveal smaller changes in light thanks to two transistors Tl and T2 connected in Darlington. Vindication of dimming is performed by a series of 1 6 LEDs.

The connection between resistance and picture circuitry is printed with a cable effeclue blind. For the adjustment, you must proceed as follows:
1) Install the photo resistance in terms of the enlarger (sensitive part to haul)
2) Adjust the lens to a diaphragm of 5.6 or 8.
3) Adjust the enlarger for the maximum amplitude (maximum height). At present, adjust the trimmer Tl to the ignition point of the LED number 16. By lowering the enlarger, note the extinction of the aforementioned LED and I Ignition other depending on the distance of the light from the plane. Each LED corresponds to a number, just done the user, by precedent trials will build a table oil make it correspond to the numbers data, exposure times; and, whenever it will make prints it will have the exact indication of exposure time for any expansion.

It is to be noted that for the operations of adjustment, and for normal use of the exposure meter, insert the film in the enlarger. Laboratory tests were performed realized by using an enlarger DURST B30 model with an objective NEOTAR 1: 3.5 / 50 and a lamp of 75 watts. Remember also that you should not perform tests with different values diaphragm, because, like any amateur photographer knows, for each additional diaphragm must double the time of exposure while for each diaphragm and less time is halved. Ie device must be powered with a stabilized 12 volts ; For mounting of components, follow the scheme practical implementation carefully.

LIST OF ELECTRONIC COMPONENTS:
All resistors are
of 1/4 watt unless stated
otherwise.
R = 1 Kohm
R2 = 1 Kohm
R3 = 8,2Kohms
R4 = 560Kohms
R5 = 5,6Kohms
R6 = 39Kohms
TR1 = 4.7 ohm trimmer
Cl = 0,22uF100Vpol.
DL1 = red Led
DL2 = yellow LED