Keypad - PCF8574
I2C keypad - I2C - PCF8574
Fig. 1 I2C keypad
Description:
This board is 12 way keyboard with a led indicator (red or green), connected to a host with an I2C port. Like the I2C 8 bits board, it is based around a PCF8574 chip.
The board can be powered from 2.5 to 5V processor board.
Specifications:
Input voltage 2.5to 5V
InterfaceI2C
Schematic:
The SDA and SCK I2C lines are buffered by a PNP transistor, so it can run on longer lines if needed. The I2C address of the chip is set to 40h by keepeing lines A0 to A2 low.
D1 and C1 filter the power supply. Bit 0 drives the bicolor LED through 2 transistors. When bit 0 is low, Q3 is conducting and Q4 blocking. When bit 0 is high, Q3 is blocking and Q4 has its base pulled low through R5 and LED1.
The keypad is connected with bits 1 to 7. Column 3 is connected to bit 7, column 2 to bit 6 and column 1 to bit 5. The rows 1, 2, 3 and 4 are connected to bits 3, 2, 1 and 4 respectively. The signals are mixed to make it easier to design the PCB. The software will look after this.
K1 provides connections to the host, with power supply, the I2C lines (sda and scl) and the interrupt signal, if required.
Fig. 2 I2C keypad schematic
Construction:
The PCB fits on a small single sided board. The components are soldered on the top (copper) side, except LED1 and J1, which are on the bottom side.
Fig. 3 I2C keypad layout
Start with the wire link beside pin 1 of IC1, then continue with IC1. Finish with the higher components.
| I2C Keypad Component list |
|---|
| C1: 22 uF D1: 1N4148 R1, R2: 820 R3, R4: 4.7 K R5, R6: 560 LED1: Bicolor led, common cathode Q1 to Q4: BC557 IC1: PCF8574T (SO package) K1: Connector 5pins 5mm J1: header 7 pins male |
How to use / Testing:
To check the board, you need a processor board running the I2C keypad tutorial.
Files and links:
Eagle PCB and schematics files for I2C keypad.PDFs of PCB, layout and schematics of I2C keypad.
PIC Tutorials for I2C keypad.
NXP PCF8574 8 bits IO expander.