Energy-saving and environmentally friendly bicycle speakers

When we ride a bike to play or go to work, we usually wear headphones to listen to music, but this habit is not good because it will involve traffic safety issues. So we designed this bicycle speaker.

When you go out for a ride, you can’t carry too many things. Weight is a very important issue, which involves the weight of the battery. It can’t be too big, otherwise it will be inconvenient to use; it can’t be too small, otherwise the battery capacity will be reduced accordingly, which will affect the playback time. Therefore, saving power is a key point in designing bicycle audio, and how to make the sound loud is another key point.

The power amplifier chip used is TPA3122, which is a 15W Class D power amplifier chip from TI. It is a DIP package, that is, a direct plug-in type, and does not require a heat sink. This is a great convenience for people who like DIY, because if many power amplifiers are patch, it is quite troublesome to make a PCB. Its power supply voltage ranges from 10V to 30V. It has two connection methods, one is a dual-channel (SE) stereo connection method, and the other is a single-channel (BTL) bridge connection method. For us, it is enough to use one speaker, so the second connection method is used. For this connection method, a differential audio signal is required. As shown in Figure 1, two amplifiers are used to form a differential output, one of which works in a follower state and the other in an inverting state. In order to improve the sound quality, except for the filter capacitor, other capacitors should try to use CBB capacitors. The LM385-2.5 in the figure below is used as a reference voltage to increase the input mono audio signal to around 2.5V.

The preamplifier can be NE5532 or LM358. NE5532 has a static power consumption of 10mA, while LM358 only consumes about 1mA. Most people cannot tell any difference in sound quality. Therefore, LM358 is a more power-saving choice.

In order to protect the battery from over-discharge, some low voltage processing should be done. In the figure, an amplifier is used as a comparator. When working normally, the comparator output is high and TPA3122 works normally; when the voltage is too low, the comparator output is low, that is, the SD pin of TPA3122 is low. When this pin is low, TPA3122 does not work and the warning light LED is on. The alarm voltage value should be adjusted according to the actual battery voltage.

The battery can be a high-capacity lithium battery, with at least four connected in series, or a Ni-MH battery, with at least 10 connected in series. For lithium battery charging, there is a special four-series lithium battery charger. For Ni-MH battery charging, an adjustable voltage-stabilized power supply is used for current limiting charging. Of course, if there is enough space, more batteries can be connected in series to ensure sufficient playback time. After testing, 15 Ni-MH batteries with a capacity of 1600mAh connected in series can play for more than 7 hours, which is tested under high volume. In Figure 1, J1 is the charging interface, and J4 is the mono audio input terminal. The speaker can use 8 ohms and 5W to 15W. The volume is adjusted by adjusting the signal size at the input end, such as the volume adjustment of MP3.

In order to reduce power consumption, resistors with larger resistance values ​​are used as much as possible. The static power consumption of TPA3122 is about 30mA, and the overall static power consumption is less than 40mA.

In order to make it possible to install it on a bicycle, the speaker, circuit board, battery, etc. are installed in a pulsating beverage bottle, and a small and lightweight bicycle audio system is completed.