“Infrared is one of the commonly used media for wireless data transmission over limited distances. In this article, we’ll see how to build a simple wireless audio transmitter with infrared LEDs. Using this circuit allows your iPod, cell phone or computer to play music directly on external speakers without connecting them with audio cables. However, such a circuit is relatively limited, and the more perfect way is Bluetooth playback. Here is just a more interesting circuit to explain infrared transmission.
The working principle of this circuit needs to be explained by two separate circuits, one is the transmitter circuit and the other is the receiver circuit. The transmitter circuit will be connected to the 3.5mm audio interface for audio input, while the receiver circuit will be connected to the speaker for playing music. The audio signal is emitted through the infrared LED on the transmitter circuit; the photodiode on the receiver circuit is responsible for receiving the signal. But because the audio signal received by the photodiode is very weak, we need to use the LM386 amplifier circuit to amplify it and finally play it on the speaker.
It’s a bit like a TV remote, you point an IR LED at the TV and press a button, it sends a signal, which is then picked up by a photodiode (usually a TSOP), the signal is decoded and the TV learns which button you pressed . Whereas the signal transmitted here is an audio signal, and the receiver is an ordinary photodiode. This technology can also be implemented in ordinary LEDs and solar panels, and it has similarities with the Li-Fi technology we often talk about now.
IR LED x 2
3.5mm audio interface
100kΩ adjustable resistor
Fixed value resistance (1kΩ, 10kΩ, 100kΩ)
Capacitor (0.1uF, 10uF, 22uF)
The complete circuit schematic of this circuit is as follows:
The transmitter circuit consists of just a few IR LEDs and resistors, and connects directly to the battery and audio source. Where you might run into problems is putting the audio interface into the circuit. A normal audio interface will have three output pins, two for one left and one right channel, and the other as a ground for shielding. We only need one signal pin. You can use a multimeter to choose the most suitable pins. The pin format of the audio interface in the circuit here is shown in the figure below.
The principle of the transmitter circuit is relatively simple. The infrared light on the infrared LED acts as a carrier signal, and the intensity of the infrared light acts as a modulation signal. So if we drive the IR LED through an audio source, the battery will make the IR LED change intensity based on the audio signal. We used two infrared LEDs here to increase the range of the circuit;
The circuit can be driven by a Power supply of 5V to 9V. In the real world, a regulated 5V is used to replace the battery, so the 1kΩ current-limiting resistor in the circuit diagram is not used. The physical connection is as follows. IPod is used as the audio source here, but you can still use any device with an audio interface (of course, non-3.5mm mobile phones will not work)
The receiver circuit consists of photodiodes and is connected to the audio amplifier circuit. The audio amplifier circuit is composed of the common LM386, and the advantage of this circuit is that it requires few components. The power supply range of this circuit is 5V to 12V, you can use the voltage regulator module to output +5V so that you don’t need to use the 9V battery. The actual picture is as follows.
The pins of the LM386 are shown in the figure below
Pins 1 and 8: These two are gain control pins, the internal gain is set to 20, but can be increased to 200 by a capacitor between the two pins. We increased it to a maximum gain of 200 with a 10uF capacitor C3. Reasonable capacitance can allow this value to vary between 20 and 200.
Pins 2 and 3: These are the input pins for the sound signal. Pin 2 is the negative input pin and should be connected to ground. Pin 3 is the positive input pin and is also where the sound signal is input. in our circuit. The positive input pin is connected to the condenser microphone and the 100kΩ potentiometer RV1. Potentiometers can be used as volume controls.
Pins 4 and 6: Power supply pins of the LM386, pin 6 is for +Vcc and pin 4 is grounded. The driving range of the circuit is between 5V-12V.
Pin 5: This is the output pin and where we get the amplified audio signal. We connect it to the speaker with a capacitor C2 in the middle to filter out DC coupled noise.
Pin 7: This is the bypass pin. We can leave it blank, or ground it, or use a capacitor to improve stability.
After building the circuits, power them up separately and connect them to the audio source, and place the receiver circuit and the transmitter circuit in a symmetrical position about 10cm. If you don’t hear sound, try adjusting potentiometer RV1.
If the circuit works straight away, congratulations, because there are a lot of things that can go wrong with this circuit, and building an audio circuit on a breadboard in the first place is very susceptible to noise. So if the first time fails, you can follow the following ideas to troubleshoot.
1. After three points of the transmitter circuit, use your mobile phone camera to detect whether the infrared LED is flashing, which is easier to find in the dark room. It is also difficult for cameras to capture infrared light in brightly lit rooms.
2. After the receiver circuit is built, replace the photodiode with a 3.5mm interface, and then play a song. The music from your phone should be amplified and played on the speaker, if that doesn’t work keep adjusting potentiometer RV1. Replace the photodiode once it is working properly.
3. In addition to the above two steps, also ensure the distance between the two circuits, fix the transmitter circuit as much as possible, and adjust the angle of the receiver circuit until the signal is received.