Description

Here is a simple and humble 2 Watts mini audio amplifier circuit suitable for small pocket radios and other portable audio gadgets.The circuit is based on Phillips Semiconductors IC TDA 7052.The amplifier can be run even from a 3V Mercury button cell.This makes it ideal for battery operated gadgets.

The IC TDA7052 is a mono output amplifier coming in a 8-lead DI package (DIP). The device is mainly designed for battery-operated portable audio circuits. The features of TDA 7052 include ,no external components needed,  no switch-on or switch-off click sounds , great overall stability ,very low power consumption(quiescent current 4mA) , low THD, no  heat sinks required and short-circuit proof.

The gain of TDA 7052 is fixed internally at 40 dB. . To compensate the reduction of output power due to low voltage supply  the TDA7052 uses the Bridge-Tied-Load principle (BTL) which can provide  an output  of around 1 to 2 W  Rms(THD = 10%) into an 8 Ohm load with a power supply of 6 V.

In the circuit the potentiometer can be used to control the volume. Capacitor C1 and C2 are meant for  filtering the supply voltage if a battery eliminator is used as supply source. For operations using a battery C1 and C2 are not necessary.

Mini Audio Amplifier Circuit Diagram with Parts List:

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Simple Rain Alarm

GIVES BEEP WHEN WATER IS IN CONTACT WITH THE WIRE

Water is a conductor of electricity. When water is in contact with the probe then there is a flow of current which reaches to the base of Q1. Transistor Q1 is a NPN transistor which conducts. With the conduction of Q1 electron reaches to Q2 which is a PNP transistor .Q2 also conducts and current flows through the speaker. In a speaker there is inductive coil which causes motion in one direction and also produce induce current which is in opposite direction to the flow of current this induce current in the form of pulse flows through a capacitor, resistance and switches off Q1 and relax .this process repeats again and again till probe is in contact with water or we can say there is a oscillation in the circuit thus speaker diaphragm vibrates and gives a tone. Frequency of the circuit depends on the value of Speaker Coil impendence, Capacitor and Resistance Value.

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Simple 9 V Battery Replacement

This circuit was originally designed to power a motorcycle intercom from the vehicle supply system. This type of intercom, which is used for communication between driver and passenger, generally requires quite a bit of power. In order to improve intelligibility there is often elaborate filtering and a compander is sometimes used as well. The disadvantage is that a battery doesn’t last very long. You could use rechargeable batteries, of course, but that is often rather laborious. It seems much more obvious to use the motorcycle power supply instead. 

9-V Battery Replacement Circuit Diagram

A 9-V converter for such an application has to meet a few special requirements. For one, it has to prevent interference from, for example, the ignition system reaching the attached circuit. It is also preferable that the entire circuit fits in the 9-V battery compartment. This circuit meets these requirements quite successfully and the design has nonetheless remained fairly simple. In the schematic we can recognise a filter, followed by a voltage regulator and a voltage indicator. D1, which protects the circuit against reverse polarity, is followed by an LC and an RC filter (C3/L1/L2/C1/R1/C2). This filter excludes various disturbances from the motorcycle power system. Moreover, the design with the 78L08 and D3 ensures that the voltage regulator is operating in the linear region. The nominal sys-tem voltage of 14 V can some-times sag to about 12 V when heavy loads such as the lights are switched on. 

Although the circuit is obviously suitable for all kinds of applications, we would like to mention that it has been extensively tested on a Yamaha TRX850. These tests show that the converter functions very well and that the interference suppression is excellent.

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Simple Water Level Detector

It is easy make a simple water level detector circuit. To monitor the filling of a bath, a water-tank, or a swimming pool, or to warn when a gully is overflowing, here’s a very simple water level detector built around a CD4011 CMOS quad NAND chip. Gates IC1.A and IC1.B are wired as an astable multivibrator. The oscillator frequency is determined by C1, R2 and preset P1.

Water Level Detector Circuit Diagram :


When quiescent, resistor R1 pulls the input to gate IC1.A down to logic low, which there-fore by default blocks the operation of the oscillator in the absence of water. When water is present between the e+ an d e−electrodes, IC1. A is taken high, enabling the oscillator. The output signal from gate IC1.B is shaped by IC1.C to obtain a rectangular waveform. Gate IC1.D inverts the signal so that transistor T1 is held of f in the absence of water, which avoids current flowing in the primary of transformer TR1 when the system is at rest. TR1 is a 12 V 1.5 VA AC power transformer wired as a step-up trans-former i.e. with the low-volt age winding connected to T1. The transformer’s step up ratio affords ‘passive’ amplification of the signal present at the drain of T1. The trans-former’s high voltage winding is connected to piezo sounder BZ1 (e.g. Murata; the ‘28’indicates the diameter) which produces the audible warning.

In order to optimise the sound output of the unit, you’ll need to adjust P1 so as to set the oscillator frequency to the resonant frequency of the piezo transducer; this setting can be done by ear. The electronics and batteries can be housed into a salvaged case (for example, the kind of oval box found inside giant chocolate ‘surprise’ eggs). The electrodes, formed from simple rigid copper wires, pass out through the case; the join is made watertight using epoxy adhesive.

Author : André Thiriot  - Copyright : Elektor

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Simple Security Door Electronic Key

It is a relatively simple circuit of electronic lock of safety with code of 7 digits. It should is given attention in the time that will be stepped the keys, that shape code and it does not exist it delays. With the right step of keys and if code is right then is activated exit Q7 for roughly 4 seconds, driving the transistor Q2, which with the line can drive one relay, for the opening of door, or any other circuit.

Security Door Electronic Key Circuit diagram :

With LED D we can have optical clue of activation. The code of circuit, as it has been given have been:1704570 but can change, if we change the connections between in the exits of IC1 and the switches.

Parts List :

• R1-7=4.7Kohm
• R8=15Kohm
• R9=1Mohm
• R10-13=10Kohm
• R11=100ohm
• R12=220Kohm
• R14=1.2Kohm
• C1-3=100nF 100V
• C2=4.7uF 25V
• D1-2=1N4148
• D3=RED LED 3mm
• IC1=4022
• Q1=BS170
• Q2=BD679
• S1-10=Push button or keyboard

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Simple Power Amplifier Circuit 2N3055

Simple Power Amplifier Circuit Diagram

Transistors:
Tr1 BCY70 (or BC 182L or BC212L or BC214L)
Tr2/3/4 BFY50/51
Tr5 BFX88
Tr6/7 2N3055

Risk of instability if no input connected. When testing, connect R (about 3k3). The Simple Power Amplifier Circuit needs well smoothed power supply of about 20 to 30 volts. Peak power is well over 10 Watts. Read More

Simple audio oscillator Circuit diagram

A very simple audio oscillator electronic project can be designed using two transistors and some other electronic parts . As you can see in the circuit diagram , this audio tone oscillator circuit require a 9 volts DC power supply .
R1 and C1 components can be a variable type . By modifying values of R1 and C1 will vary the tone .
 For this audio oscillator circuit you can use almost any transistor . To power this audio oscillator you’ll need to use a 9 volt battery or a 9 volt DC power supply .

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Simple Electrification Unit Diagram Circuit

The circuit is intended for carrying out harmless experiments with high-voltage pulses and functions in a similar way as an electrified fence generator. The p.r.f. (pulse repetition frequency) is determined by the time constant of network R1-C3 in the feedback loop of op amp IC1a: with values as specified, it is about 0.5 Hz. The stage following the op amp, IC1b, converts the rectangular signal into narrow pulses. Differentiating network R2-C4, in conjunction with the switching threshold of the Schmitt trigger inputs of IC1b, determines the pulse period, which here is about 1.5 ms. The output of IC1b is linked directly to the gate of thyristor THR1, so that this device is triggered by the pulses.

The requisite high voltage is generated with the aid of a small mains transformer, whose secondary winding is here used as the primary. This winding, in conjunction with C2, forms a resonant circuit. Capacitor C3 is charged to the supply voltage (12 V) via R3.When a pulse output by IC1b triggers the thyristor, the capacitor is discharged via the secondary winding. The energy stored in the capacitor is, however, not lost, but is stored in the magnetic field produced by the transformer when current flows through it. When the capacitor is discharged, the current ceases, whereupon the magnetic field collapses. This induces a counter e.m.f. in the transformer winding which opposes the voltage earlier applied to the transformer.

Circuit diagram:

Simple Electrification Unit Circuit Diagram

This means that the direction of the current remains the same. However, capacitor C2 is now charged in the opposite sense, so that the potential across it is negative. When the magnetic field of the transformer has returned the stored energy to the capacitor, the direction of the current reverses, and the negatively charged capacitor is discharged via D1 and the secondary winding of the transformer. As soon as the capacitor begins to be discharged, there is no current through the thyristor, which therefore switches off. When C2 is discharged further, diode D1 is reverse-biased, so that the current loop to the transformer is broken, whereupon the capacitor is charged to 12 V again via R3. At the next pulse from IC1b, this process repeats itself.

Since the transformer after each discharge of the capacitor at its primary induces not only a primary, but also a secondary voltage, each triggering of the thyristor causes two closely spaced voltage pulses of opposite polarity. These induced voltages at the secondary, that is, the 230 V, winding, of the transformer are, owing to the higher turns ratio, much higher than those at the primary side and may reach several hundred volts. However, since the energy stored in capacitor C2 is relatively small (the current drain is only about 2 mA), the output voltage cannot harm man or animal. It is sufficient, however, to cause a clearly discernible muscle convulsion.

Author: P. Lay
Copyright: Elektor Electronics

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Simple VGA to BNC Adapter Converter Circuit

There are monitors which only have three BNC inputs and which use composite synchronization (‘sync on green’). This circuit has been designed with these types of monitor in mind. As can be seen, the circuit has been kept very simple, but it still gives a reasonable performance. The principle of operation is very straightforward. The RGB signals from the VGA connector are fed to three BNC connectors via AC-coupling capacitors. These have been added to stop any direct current from entering the VGA card. A pull-up resistor on the green output provides a DC offset, while a transistor (a BS170 MOSFET) can switch this output to ground. It is possible to get synchronisation problems when the display is extremely bright, with a maximum green component.

In this case the value of R2 should be reduced a little, but this has the side effect that the brightness noticeably decreases and the load on the graphics card increases. To keep the colour balance the same, the resistors for the other two colors (R1 en R3) have to be changed to the same value as R2. An EXOR gate from IC1 (74HC86) combines the separate V-sync and H-sync signals into a composite sync signal. Since the sync in DOS-modes is often inverted compared to the modes commonly used by Windows, the output of IC1a is inverted by IC1b. JP1 can then by used to select the correct operating mode. This jumper can be replaced by a small two-way switch, if required.

 

  

This switch should be mounted directly onto the PCB, as any connecting wires will cause a lot of interference. The PCB has been kept as compact as possible, so the circuit can be mounted in a small metal (earthed!) enclosure. With a monitor connected the current consumption will be in the region of 30 mA. A 78L05 voltage regulator provides a stable 5 V, making it possible to use any type of mains adapter, as long as it supplies at least 9 V. Diode D2 provides protection against a reverse polarity.

LED D1 indicates when the supply is present. The circuit should be powered up before connecting it to an active VGA output, as otherwise the sync signals will feed the circuit via the internal protection diodes of IC1, which can be noticed by a dimly lit LED. This is something best avoided.

Resistors:
R1,R2,R3 = 470Ω
R4 = 100Ω
R5 = 3kΩ3
Capacitors:
C1,C3,C5 = 47µF 25V radial
C2,C4,C6,C7,C10 = 100nF ceramic
C8 = 4µF7 63V radial
C9 = 100µF 25V radial
Semiconductors:
D1 = LED, high-efficiency
D2 = 1N4002
T1 = BS170
IC1 = 74HC86
IC2 = 78L05
Miscellaneous:
JP1 = 3-way pinheader with jumper
K1 = 15-way VGA socket (female), PCB mount (angled pins)
K2,K3,K4 = BNC socket (female), PCB mount, 75Ω    . 

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Simple Precision full wave Rectifier Circuit Diagram

The circuit provides accurate full wave rectification. The output impedance is low for both input polarities, and the errors are small at all signal levels. Note that the output will not sink heavy current, except a small amount through the 10K resistors. Therefore, the load applied should be referenced to ground or a negative voltage. Reversal of all diode polarities will reverse the polarity of the output

Since the outputs of the amplifiers must slew through two diode drops when the input polarity changes, 741 type devices give 5% distortion at about 300 Hz.


Precision full wave Rectifier Circuit Diagram

 Sourced By: http://circuitsstream.blogspot.com/2013/07/precision-full-wave-rectifier-circuit.html

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Simple 12 Volt 30 Amp PSU Circuit Diagram

Using a single 7812 IC voltage regulator and multiple outboard pass transistors, this power supply can deliver output load currents of up to 30 amps. The design is shown below:

Simple 12 Volt 30 Amp PSU Circuit Diagram

Notes:
The input transformer is likely to be the most expensive part of the entire project. As an alternative, a couple of 12 Volt car batteries could be used. The input voltage to the regulator must be at least several volts higher than the output voltage (12V) so that the regulator can maintain its output. If a transformer is used, then the rectifier diodes must be capable of passing a very high peak forward current, typically 100amps or more. The 7812 IC will only pass 1 amp or less of the output current, the remainder being supplied by the outboard pass transistors. As the circuit is designed to handle loads of up to 30 amps, then six TIP2955 are wired in parallel to meet this demand. 

The dissipation in each power transistor is one sixth of the total load, but adequate heat sinking is still required. Maximum load current will generate maximum dissipation, so a very large heat sink is required. In considering a heat sink, it may be a good idea to look for either a fan or water cooled heat sink. In the event that the power transistors should fail, then the regulator would have to supply full load current and would fail with catastrophic results. A 1 amp fuse in the regulators output prevents a safeguard. The 400mohm load is for test purposes only and should not be included in the final circuit. A simulated performance is shown below:

 Circuit Diagram

Calculations:
This circuit is a fine example of Kirchoffs current and voltage laws. To summarise, the sum of the currents entering a junction, must equal the current leaving the junction, and the voltages around a loop must equal zero. For example, in the diagram above, the input voltage is 24 volts. 4 volts is dropped across R7 and 20 volts across the regulator input, 24 -4 -20 =0. At the output :- the total load current is 30 amps, the regulator supplies 0.866 A and the 6 transistors 4.855 Amp each , 30 = 6 * 4.855 + 0.866. Each power transistor contributes around 4.86 A to the load. The base current is about 138 mA per transistor. A DC current gain of 35 at a collector current of 6 amp is required. 

This is well within the limits of the TIP2955. Resistors R1 to R6 are included for stability and prevent current swamping as the manufacturing tolerances of dc current gain will be different for each transistor. Resistor R7 is 100 ohms and develops 4 Volts with maximun load. Power dissipation is hence (4^2)/200 or about 160 mW. I recommend using a 0.5 Watt resistor for R7. The input current to the regulator is fed via the emitter resistor and base emitter junctions of the power transistors. Once again using Kirchoffs current laws, the 871 mA regulator input current is derived from the base chain and the 40.3 mA flowing through the 100 Ohm resistor. 871.18 = 40.3 + 830. 88. The current from the regulator itself cannot be greater than the input current. As can be seen the regulator only draws about 5 mA and should run cold.

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Simple Ignition Timer Schematic

This circuit is a tester for flywheelbased ignition systems in small aeroplane engines. Basically the same ignition coils are also seen in other small combustion engines used in/on mopeds and lawn mowers  in brief, engines without a battery. The part to be tested comprises a primary coil in parallel with the contact breaker. The timing of this contact breaker has to be adjusted correctly.

Since the coil’s primary has a very low resistance it is difficult to determine whether the contact breaker is open or closed.  However, you can determine that reliably with this circuit, using an LED and a beeper. The circuit is implemented twice because aviation engines (Cessna, Piper and similar) always have two ignitions in parallel to increase reliability. For two-cylinder engines, well the purpose is obvious.

Ignition Timer Circuit Diagram

The circuit consists of a 555 and a few transistors. The 555 supplies a square wave of about 3000 Hz. This signal goes to power transistors T1 and T2; these can supply quite a bit of power and are robust enough to withstand the voltage transients from the big coils. The test connection (K2 and K3 respectively) are connected in parallel with the contact breaker to be tested, which itself is in parallel with the ignition coil. The frequency of 3000 Hz is either short circuited by the contact breaker or if the points are open  is amplified somewhat by the resonance of the coil itself.

This allows you to reliably detect the difference bet ween a closed and open contact breaker, despite the low resistance of the coil, which is in parallel with it. When the contact breaker is open the amplified pulses will turn on T3 and T4 respectively, so that the relevant LEDs turn on and the buzzer will sound. 

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Simple Sensitive Vibration Detector Alarm Circuit

Whether it is the hi-fi next door, the cat purring quietly, or a knock at the door, the detector described here does not miss a thing. Whenever it picks up a sound or vibration, it emits an ear-piercing tone.
The circuit is based on the use of an 8 S2 loudspeaker as microphone/ loudspeaker. As the signals from this microphone are very small, they are I amplified in A1 and rectified. The, resulting DC signal is then compared with a reference voltage in.A2. When a noise or vibration is picked up by the microphone, the voltage at the inverting input of A2 (pin 6), rises suddenly to about 4 V and then slowly decays to 0 V. The decay time depends on the time constant W R6/C3.

 The voltage at the non-inverting input of A2 (pin 5) is held constant at 0.7 V by R3/R4. When the input at pin 6 rises above 0.7 V, the output of A2 (pin 7) instantly switches to -4 V, which causes the squarewave oscillator A3 to start. The frequency (tone) of the oscillator can be adjusted by preset potentiometer Pl. The oscillator output (pin 8) is fed to amplifier stage Tl which drives the loudspeaker. The oscillator will continue to run however, so C3 charges steadily and will keep the output at pin 7 of A2 negative. As this is not the purpose of the circuit, the incoming signal must be interrupted somewhere in the chain. To do this, an FET, T2, is used as a switch. As soon as the output of the comparator becomes negative, D3 conducts, T2 is cut off and the incoming signal is interrupted. When C3 has discharged to the extent that the voltage across it drops to below 0.7 V, the output of A2 (pin 7) becomes positive, D3 is cut off and T2 conducts.

This should, however, not happen too rapidly, otherwise there is the risk that a false alarm may be given. Therefore, the gate (drive input) of T2 is connected to earth via capacitors C2 and C8. The consequent delay ensures that the circuit is not reactivated before half a second after the loudspeaker has gone quiet. The earth potential is fixed by the voltage divider R9/R10 and im- pedance converter A4, which derive a symmetrical supply of i 4.5 V from the 9 V battery. When T1 conducts, the supply voltage will drop a little because a battery cannot deliver energy as well as a mains power supply. It can therefore happen that the output signal of A3 is superimposed on the supply voltage. This undesired feed- back should be prevented by C5 and C6. lf in spite of these capacitors diffi- culties are encountered, it may be beneficial to increase the values of R5, C2 and C8 by trial and error. lf that fails to improve matters, increase the value of capacitors C5 and C6.

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Simple Frequency Doubler Circuit

This is a simple three transistor frequency doubler circuit to raise an audio frequency by a factor of two i.e., one octave.
O1 is connected as a phase splitter with anti-phase signals·’ appearing at its collector and emitter. These signals are fed to two emitter followers Q2 and O3, which have a common emitter resistor, and thus add the two anti-phase signals. A degree of distortion is inevitable as shown in Fig. 2, but is acceptable for speech and soloists and produces a sound similar to the Chipmunks or Pinky and Perky. 

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Simple Light Activated Alarm Circuit

1. Working of second and third sections is known to many of the hobbyists- second is merely a multivibrator, `whereas third is an audio amplifier.
2. This device comprises a triggering section, an oscillating section, and an amplification section.
3. SCR] may be triggered either by bringing the gate of SCRI in contact with positive pole of the battery momentarily or by flashing a light on the light dependent resistor (LDR).
4. By adjusting potentiometer. VRI, the sensitivity of the instrument can be controlled. To switch off the alarm the battery will have to be disconnected momentarily.
5. This signal is amplified by the third section and produces an audio alarm through the speaker. This circuit works on 9V.
6. Even a light from a cigar can trigger the silicon controlled rectifier. On power (negative voltage) being applied, the second section produces oscillations.
7. When light falls on the LDR, its resistance is reduced, thereby a current flows through the gate of SCR]. This gate current triggers SCR] and the oscillating and amplification sections of the alarm get negative voltage.
8. These two sections commonly get negative voltage when SCRI is triggered. 

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Simple 100 watt Inverter Citcuit Using IC 4049

You should not install the transistors straight onto the heat sinks. Make use of mica isolation kit to prevent immediate exposure and short circuiting of the transistors together as well as the ground.

Clamp the heat sink set up to the of a nicely ventilated, durable, heavy gauge metal enclosure.
Additionally attach the power transformer beside the heat sinks employing nuts and bolts.
Now attach the suitable areas of the constructed circuit board to the power transistors on the heat sinks.
Lastly connect the power transistor’s outputs to the supplementary winding of the power transformer.
Complete the building by fixing and interconnecting the exterior electrical “fittings” such as fuses, sockets, buttons, mains cord along with the battery inputs.
An alternative individual power source circuit using a 12V/3Amp. transformer might be included inside to recharge the battery the moment needed (see diagram).




You may further discover how to construct a simple 100 watt inverter circuit by focusing on the following examination method:


To better know how to construct an inverter, you will need to find out how the circuit features by means of nthe following actions:
Gates N1 and N2 of IC 4049 are configured as an oscillator. It carries out the major operation of providing square waves to the inverter part.
Gates N3... N6 are utilized as buffers to ensure that the circuit is not load dependant.
Alternating voltage from the buffer phase is applied to the base of the current amplifier transistors T1 and T2. These particular transistors execute in line with the applied alternating voltage and amplifies it to the base of the output transistors T3 and T4.

All these output power transistors oscillate at a complete swing, dispensing the full battery voltage into the every half of the secondary winding alternately.

This secondary voltage is brought on in the main winding of the transformer which is stepped-up into an effective 230 volts (AC). This voltage is employed to power the output load.
Testing Procedure

You can further understand how to build an inverter by focusing on the following testing procedure given in a comprehensive manner below:



Start out the testing method by attaching a 1 hundred watt bulb at the output socket of the inverter,

Add a 15 Amp./12V fuse inside the fuse holder,

At last connect a 12V car or truck battery to the battery inputs of the inverter.

If all the contacts are proper, the 100 Watt bulb must right away light brightly.

Continue the inverter ON for 60 minutes and let the battery discharge through the bulb,

Then transfer the given toggle switch to the charging mode, verify the meter reading,

The meter need to suggest the charging current of the battery.

The digital meter reading should certainly slowly die down to nil after a period of time, making sure that the battery is entirely charged and geared toward the subsequent action.

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Simple Solar Charger Wiring diagram Schematic

I suppose this schema is a very useful schema diagram for you all.Because this schema shows how to connect a Solar panel with a schema and a battery.You have to use a battery according to the power of the solar panel.here I have used 1N5817 diode to protect the panel and to control the power of the schema diagram.Keep it in your memory If the two connections of the solar panel touched together it will damage your panel.

Note 

 # Dont connect your panel to the schema with out diodes 
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Simple Sound Generator Wiring diagram Schematic

This is a simple Run-Down Clock or Sound Generator schema diagram. Used in electronic roulette or dice games, this schema produces a clock signal that initially is several tens of kHz (depending on C2) and gradually decreases to zero in about 15 seconds, as CI discharges through R4.

Simple Sound Generator Circuit Diagram

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Simple Fluorescent Without Transformer Wiring diagram Schematic

This schema caught my attention by using cold cathode fluorescent lamp and especially do not use any processor or oscillator. The schema is powered by the AC mains of 110/127 and apparently the lighting power decrease significantly, but it is interesting to test.

This simple schema Fluorescent Lamp without transformer, operates from 120V AC, uses cold cathode fluorescent lamp and uses an RC schema to limit the current. Caution! This assembly can be dangerous and cause shocks! We are not responsible for damages and malpractice in the assembly.

Simple Fluorescent Without Transformer Circuit Diagram

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Simple 0 30 Volt Laboratory Power Supply

The linear power supply, shown in the schematic, provides 0-30 volts, at 1 amp, maximum, using a discrete transistor regulator with op-amp feedback to control the output voltage. The supply was constructed in 1975 and has a constant current mode that is used to recharge batteries. 

With reference to the schematic, lamp, LP2, is a power-on indicator. The other lamp (lower) lights when the unit reaches its preset current limit. R5, C2, and Q10 (TO-3 case) operate as a capacitor multiplier. The 36 volt zener across C2 limits the maximum supply voltage to the op-amps supply pins. D5, C4, C5, R15, and R16 provide a small amount of negative supply for the op-amps so that the op-amps can operate down to zero volts at the output pins (pins 6). 

A more modern design might eliminate these 4 components and use a CMOS rail-to-rail op-amp. Current limit is set by R3, D1, R4, R6, Q12, R10, and R13 providing a bias to U2 that partially turns off transistors Q9 and Q11 when the current limit is reached. R4 is a front panel potentiometer that sets the current limit, R22 is a front panel potentiometer that sets the output voltage (0-30 volts), and R11 is an internal trim-pot for calibration. The meter is a 1 milliamp meter with an internal resistance of 40 ohms. Switch S1 determines whether the meter reads 0-30 volts, or 0-1 amp. 

0-30 Volt Laboratory Power Supply Circuit diagram

 A more modern schema might use a single IC regulator, such as the MC78XX, or MC79XX series, immediately after the half wave rectifier, to replace approximately 30 components, or at least a high precision zener diode to replace D10 as the voltage reference. The LM4040 is one such voltage reference and has excellent stability over temperature. IC regulators such as the MC78XX series may eventually become obsolete as newer IC regulators are designed, however, discrete transistors, op-amps, and zeners are more generic, have a longer production lifespan, and allow the designer to demonstrate that he understands the principles of linear regulated power supply operation.Link

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