As seen in the IR Wireless Headphone schematic, transmitter circuit is very simple here. The transformer assembled as a matching impedance, and low impedance windings are connected in parallel with the TV or radio speaker. IR diodes are commonly used. 10 ohm resistor that limits the current through the IR diode should be 1 mg. This 9Vdc powered transmitter that can be provided either by regular batteries or AC / DC adapter.

As the receiver is concerned, the same infrared light captured by the phototransistor, it is pre-amplified and amplified by transistors BC549C and then gives enough power to move the speaker of headset through the output transistor.

This receiver, like the transmitter is also powered from 9Vdc, but in this case must inevitably be provided by the battery, because with AC / DC adapter all this will be useless. Would look funny, right? When we use a wireless Headphone. But still use power supply with cable.

Remember that the audio is transmitted must have line of sight between the transmitter and receiver.

You can extend the range of the transmitter by placing more transistors BD140 with more IR LEDs.

Each time you press the doorbell generator creates a Ding-Dong weak audio signal with the sound of bells. The signal is high in volume by the amplifier and is reproduced by the speaker. The power supply circuit provides the voltage necessary to operate. The interface circuit to connect the rings fed centrally as buildings or intercom.

The circuit is powered through the point marked V + and ground. The heart of it is the integrated HT2811, developed by the firm koreana Holtek. For the pin 1 enters the trigger pulse, telling the chip to produce the sound “Ding-Dong”. Pins 2 and 3 are connected to sets RC establishing each of the sounds (2 = “Ding” / 3 = “Dong”). Altering these components is achieved by varying the sound of bells. Pin 4 is the mass. By the pin 5 leaves the audio signal which is amplified by a pair of transistors used in darlington configuration. The terminals 6 and 7 are connected to a 680K resistor which adjusts the gain of the internal preamplifier chip. Finally the terminal 8 enters the feed to the chip which is current limited by the resistance of 100 ohms and 3.3v stabilized by means of zener diode. The 100μF capacitor filters the possible ripple left in the power line.

In case of using this bell at departments or locations where you can not modify the wiring in the bell push is necessary to use this interface. It receives at its input an AC or DC voltage and rectifies through PR the bridge rectifier whose output is filtered by continuous capacitor 470μF and subsequently attacks the small coil of a reed relay. The key to this relay trips the main circuit as would a conventional switch. The rectifier bridge (PR) can be either formed by diodes 1A 250V or more. While the voltage of the relay coil must be the same as the voltage of the previous original buzzer ring (usually 12V). While the relay can be operated without rectifying the line or filter is not suitable because the alternating current would act to relay as a buzzer, opening and closing your key 50 times per second and this can cause some damage in the mechanism after a time.

This section of the circuit adapts the voltage from the power supply required by the household the equipment. It also enables battery power for all occasions when the power fails. The transformer reduces the voltage AC 4.5v. The bridge rectifier (PR) converts AC power continuously, which is filtered by the capacitor of 2200μF. The 1N4007 diodes act as source selector system by operating the mains or batteries as needed. The fuse protects the transformer section 220v. The bridge rectifier (PR) can be any voltage which is greater than 250V and whose current is not lower than 1A. Point + V represents the output of the source, while the batteries (4 in series) input by the points + Bat and-Bat.

Above diagram is the schematic diagram of mono power amplifier which use power IC of STK-1050 as the main component. Build two similar circuit for stereo amplifier circuit.

Download STK-1050 datasheet for circuit reference:
Download Link

The circuit built based SSM2211. I have never heard about this IC before, this IC may be quite difficult to find in the market, so before deciding to make this circuit, you better look for information about availability of this component in the nearest electronics store.

Here the datasheet of SSM2211 IC: datasheet

Above diagram is the schematic diagram of stereo amplifier, build based integrated circuit STK4141II. This stereo amp will deliver about 25W power output on each channel. Take a note that a heatsink must be attached to the power IC STK4141II to prevent overheating.

Download STK4141II datasheet:
Download Link

You should have the datasheet to define the pin connections.

Circuit diagram for one audio input/output channel:

Download Download the STA5100 datasheet:
Download Link

Circuit design:

PCB layout:

Download TDA2003 Datasheet

Above circuit is an amplifier circuit which built based on single power IC STK082. This is a very simple and easy electronic project to built, very nice for student’s project.

Download the STK082 datasheet for the reference/information about STK082

Schematic diagram:

PCB layout:

Download this Datasheet LM4756 for the project reference.

The circuit build according to common tone control circuit, by applying two transistors FCS9014 in each channel, so there’re four transistors for this 12v stereo tone control.

Take a note that you must connect the circuit prior to the amplifier circuit. The output of tone control should be connected to the input of the amplifier. Any small signal NPN transistor including BC547B, 2N3904, C828, or C945 could be put to use to replace FCS9014.