IR MUSIC TRANSMITTER AND RECEIVER

Using this circuit, audio musical notes can be generated and heard up to a distance of 10 metres.

The circuit can be divided into two parts: IR music transmitter and receiver. The IR music transmitter works off a 9V battery, while the IR music receiver works off regulated 9V to 12V. Fig. 1 shows the circuit of the IR music transmitter. It uses popular melody generator IC UM66 (IC1) that can continuously generate musical tones. The output of IC1 is fed to the IR driver stage (built across the transistors T1 and T2) to get the maximum range.

                                                              Fig. 1: Transmitter circuit


Here the red LED (LED1) flickers according to the musical tones generated by UM66 IC, indicating modulation. IR LED2 and LED3 are infrared transmitting LEDs. For maximum sound transmission these should be oriented towards IR phototransistor L14F1 (T3).

The IR music receiver uses popular op-amp IC µA741 and audio-frequency amplifier IC LM386 along with phototransistor L14F1 and some discrete components (Fig. 2).

                           
                                                               Fig. 2: IR audio receiver circuit

The melody generated by IC UM66 is transmitted through IR LEDs, received by phototransistor T3 and fed to pin 2 of IC µA741 (IC2). Its gain can be varied using potmeter VR1. The output of IC µA741 is fed to IC LM386 (IC3) via capacitor C5 and potmeter VR2. The melody produced is heard through the receiver’s loudspeaker. Potmeter VR2 is used to control the volume of loudspeaker LS1 (8-ohm, 1W). Switching off the power supply stops melody generation.

Intercom Using LM386

Keep in touch with your family members from one room to another and also from outside areas such as the garage, using this intercom circuit for bidirectional communication. The advantage of this circuit is that there is no talk/listen switch as many intercoms have. The circuit is built around two low-power LM386 audio amplifiers. 

Fig. 1 shows the block diagram of the intercom system. Fig. 2 shows the circuit. It has two simple and identical channels—unit 1 and unit 2. As shown in Fig. 2, the gain of both the amplifiers (built around IC1 and IC2) is about 200, which is usually enough to work with the condenser microphones. The circuit also works with carbon microphones or other high-level and low-impedance microphones.

As both units are identical, working of only the first unit is described here. When you speak in front of the microphone (MIC1), the low-level signal is amplified by the amplifier built around transistor T1. Transistor T1 (BC547) is low-noise and high-gain type.

The value of resistor R11 is selected such that the voltage between the collector and emitter of T1 is approximately half the power supply voltage. Resistor R12 should have a minimal value. For example, if the emitter of T1 is connected to the ground, the value of R12 could be as low as 1 kilo-ohm (usually, the range is 680 ohms to 4.3 kilo-ohms). Use of capacitor C18 is optional. The preamplifier may exhibit instability at very high frequencies. So minimal appropriate value is chosen for resistor R1 in order to make the input circuit less vulnerable to electromagnetic noises.

Signal from the microphone is filtered by the combination of resistor R2 and capacitor C1. Capacitor C2 blocks the DC component but allows AC signal to pass. The volume is adjusted by potentiometer VR1. The potentiometer can be replaced with a preset of the same value because the volume need not be adjusted frequently. The signal is amplified by IC1 and fed to the loudspeaker (LSP2).

The second unit works in the same way as the first unit.

Assemble the circuits for units 1 and 2 on separate general-purpose PCBs and enclose in suitable cabinets. As shown in the block diagram, place microphone MIC1 and loudspeaker LSP1 in the first room, and microphone MIC2, loudspeaker LSP2 and the entire electronics block plus the power supply in the second room. Keep the interconnection cables as short as possible—preferably shorter than 10 metres. These should be kept away from the cables of the mains power supply and other sources of electromagnetic interference. Use a shielded cable for connecting the microphone to the circuit.

It is important that there is no acoustical feedback between the microphone and the loudspeaker on both the sides. So the microphones and the loudspeakers should not face each other. As the gain of each channel is not very high, the probability of acoustical feedback is low. In case of acoustical feedback, lower the volume of the amplifier and change the positions of the microphones and loudspeakers.

The preferred power supply for the circuit is 6V or above but the circuit also works with regulated 5V from IC 7805 (not shown in the diagram). If a higher dynamic range is needed, the power supply should be 9V or even 12V. For power supply, you can use an AC-DC wall adaptor, dry batteries or rechargeable batteries.

The Light Listener

As with anything new, listening to light might sound like just a bunch of noise, but over time a trained ear could distinguish the difference between hundreds if not thousands of sources. Eventually, you could detect the subtle differences between a TV and fluorescent room lights, or between a flashlight and car headlights.

Some light sources are completely invisible to the human eye (infrared and ultraviolet) and can be converted easily to sound. Infrared remote controls give off coded light signals that make easily discernable tones for each different function of the remote. Everything from LEDs to fireflies make a sound with the light that they emit.By building this simple circuit you can listen to the light that you encounter on a day to day basis.

Listening to Natural Lights

Lightning flashes produce sharp clicks and pops. These sounds are picked up from the flashes of light and have no direct relationship with the sound of thunder that is heard much later after the lightning strike. Nighttime is the best time to listen to sounds produced by the light from lightening. It’s also possible that you will be able to hear sounds from lightning flashes that can not be seen by the human eye. Obviously, being outside in a lightning storm isn’t the safest course of action. If you intend to “listen” the lightning flashes, do it from inside a house or car.

Flames produce many different sounds. When the air is still, a soft rushing sound is heard. When the flame is disturbed by moving air, crackles and pops are heard. These sounds are seperate from the sounds emitted by a the fuel such as wood or wick that is reacting as it is burned.

Sounds can also be detected from winged insects that are flying in sunlight. When their wings reflect light to the detector, a buzz or hum will be heard. At dusk, a nearby firefly will produce a soft click for each flash. These sounds are independent from other physical sounds created by insects such as the hum of their wings moving air.

Sunlight can also produce sounds that can be detected by the listener as the sunlight is filtered through leaves, reflected off of windows, and passed through the gaps in a picket fence; resulting in a pop, pop, pop sound.

Listening to Artificial Lights

Flashlight beams, when swept across the light listener at different speeds, will create different sounds ranging from a soft swoosh to sharp pops. Tap the flashlight with a pencil and a ringing sound will be heard as the filament vibrates.

The headlights of cars, trucks, and motorcycles will produce a distinctive ringing sound when the vehicle is moving on a rough or bumpy road. The various rings can sometimes create what some describe as singing.

Electronic Displays are usually powered by rapid pulses of current. The flashes are merged into continuous light by the slow response of the eye. They can, however, be heard clearly as a buzz or a hum with the listener.

The displays of older televisions and computer monitors are formed by sweeping an electron beam across a phosphor coated screen. The light listener will transform that pulsating phosphor into a recognizable pulsating buzzing sound.

With a light listener circuit and a little time on your hands, you can listen to the sounds of the visible, and sometimes invisible, world around you as broadcast by light.

LM386 Based Low Power Audio Amplifier

LM386 is a low power audio amplifier IC designed for use in low voltage consumer applications. You can use this 8-pin IC to amplify the output signal from your mp3 player or computer audio output. The inputs are ground referenced while the output automatically biases to one-half the supply voltage. The quiescent power drain is only 24 milliwatts when operating from a 6 volt supply, making the LM386 ideal for battery operation.