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Keh-Chang Gee,Yuan-Chun (Martin) Lanelectronic circuits; they are so fast that they see contacts make and break thousands of times per second. If you ignore this aspect of input controls in your design, you could wind up with dysfunctional custom controls for your Pi.

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Éva Erdős,Zoltán Nagy,Zoltán Vargae when many of the alternate functions are being used (SPI, I2C, UART, PWM, and so on). One way to cost effectively expand the number of Pi inputs is to attach one or more 74HC165 CMOS chips. These are available from digikey.com for as little as a dollar. Figure 9-1 illustrates an example of the 16-pin dual inline package (DIP) chip.

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Éva Erdős,Zoltán Nagy,Zoltán Vargaocks, or simple data input keypads. Normally these have their buttons arranged in rows and columns so that they can be scanned by the host computer, reducing the number of I/O ports necessary. You’ll do the same in this chapter but using the DIP form of the PCF8574 I/O chip to avoid using up precious GPIO ports.

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Éva Erdős,Zoltán Nagy,Zoltán VargaLooking through online forums, I often see posts from people struggling with how to interface between 3V and 5V logic systems. Advice varies widely, ranging from “connecting it directly seems to work for me” to other dubious methods.

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Éva Erdős,Zoltán Nagy,Zoltán VargaOne of the peripherals missing in the Raspberry Pi is an analog-to-digital converter (ADC). One low-cost solution is an 8-bit ADC peripheral based upon the PCF8591 chip. You can purchase the bare chip on eBay for about $1.50, but for about 10 cents more, you can get the assembled YL-40 PCB.

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