TAILIEUCHUNG - Practical Arduino Cool Projects for Open Source Hardware- P18

Practical Arduino Cool Projects for Open Source Hardware- P18: A schematic or circuit diagram is a diagram that describes the interconnections in an electrical or electronic device. In the projects presented in Practical Arduino, we’ve taken the approach of providing both a photograph and/or line drawing of the completed device along with a schematic. While learning to read schematics takes a modest investment of your time, it will prove useful time and time again as you develop your projects. With that in mind, we present a quick how-to in this section | CHAPTER 9 SPEECH SYNTHESIZER Figure 9-7. Shielded cable connected to female RCA line plug Once your shield is fitted to an Arduino and plugged into an amplifier you can go to the following section Speech Synthesizer Software to test it out. On-Board Amplifier Adding a small amplifier to the shield so you can drive a speaker directly is quite easy with the LM386 audio amplifier IC. LM386 chips are very common in small audio devices such as compact portable radios because they re very easy to use and produce reasonable quality sound at a good volume level. They certainly won t compete with the amp in your stereo system but for simple voice-level output in a portable device they re perfect. Small Outline Molded Mini Small Outline and Dual-In-Line Packages 149 CHAPTER 9 SPEECH SYNTHESIZER To build a simple audio amplifier with a gain of about 20 you can use the LM386 on its own with pretty much nothing else required. However with a few extra parts it can be configured to deliver a gain of about 50. If necessary it can be configured for a gain of up to 200 but that s pushing the limits of what it can do and since we re powering the amplifier circuit from the Arduino s 5V supply the current drain and noise on the supply rails could become a problem. Audio amplifiers pull a lot more power than the typical digital circuits you might be used to dealing with on an Arduino since amps equals watts divided by volts even a tiny 1 4W amplifier running on a 5V supply can suck down 50mA of current assuming it runs at an unrealistic 100 efficiency. Even worse than the current drain though are the fluctuations that can be induced on the supply rails since the current consumption isn t consistent but rather jumps around all over the place depending on the input signal. A gain of 50 gives a reasonable balance between a high output volume and low power consumption and won t strain the Arduino s power supply. If you want more volume you ll probably need to use a more substantial external

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