Room Management System - Input Stage part 1

As mentioned before, for our demonstration unit I am using push buttons S1 to S18 to simulate the sensor switches and the button to set a room to “priority Off”. For time being the needed 5V supply is coming from our Arduino board, feeding through the switches into the parallel input pins of our CD4024B shift registers with a 10K resistor parallel to ground. This pull down resistor is needed to drain the remaining current when the button is released not to give the shift register still the impression the switch is pressed while the remaining current stays floating or is not draining fast enough. The ground goes also back to 1 of the Arduino's GND pins. The first shift register is connected with the latch pin, clock pin and data pin to the digital input pins 2, 3, and 4 of our Arduino board.

You may have noticed that I am not connecting the shift register pins continuously to the switches. In fact the routing is as followed:

Shift register 1        Shift register 2        Shift register 3

PI 1 to S1              PI 1 to S9              PI 1 to S17

PI 2 to S2              PI 2 to S10            PI 2 to S18

PI 3 to S3              PI 3 to S11            PI 3 to R19

PI 4 to S4              PI 4 to S12            PI 4 to R20

PI 8 to S5              PI 8 to S13            PI 8 to R21

PI 7 to S6              PI 7 to S14            PI 7 to R22

PI 6 to S7              PI 6 to S15            PI 6 to R23

PI 5 to S8              PI 5 to S16            PI 5 to R24

Simple reason for is if we want to design our circuit board a little later we like to have as many direct routes as any possible to avoid a large amount of jumpers.

Enough theories for now, lets start putting it together:

We start with connecting our power supply. The Arduino 5V pin connects to the red bus on the breadboard and since we have three of them we bridge them all together. The same goes for the negative. We connect one of the Arduino GND pins to the blue bus. Here we need to bridge only the first two breadboards.

Next we are mounting the 18 push buttons and connect 1 pin of each button with 5V bus. Try to keep this short connections as short as possible since we will have quite a few jumpers running across the boards. Now we start with our input shift registers (3 x CD4021B).
To help a little to find the pins, I've included also a small image:

Not to get mixed up, please make sure you put all three chips the same way. In my example, I started at the bottom of the breadboard with pin 1 pointing away from me. Pin 8 of each shift register connects to the negative bus and pin 16 connects to positive.

The latch pin (pin 9 of the first shift register) connects to the digital input 2 of the Arduino. The clock pin (pin 10 of the first shift register) goes to digital Input 3 of the Arduino board and the data pin (pin 3 of the first shift register) connects to Arduino pin DI 4.

Lets connect the remaining two shift registers. The latch pin is just looped through, from pin 9 of shift register 1 to pin 9 of shift register 2 to pin 9 of shift register 3 and so is the clock pin, pin 10 of shift register 1 to pin 10 of shift register 2 to pin 10 of shift register 3. The data pin (pin 3 of shift register 2) connects to pin 11 (serial In) of shift register 1 and the data pin (pin 3 of shift register 3) connects to pin 11 (serial In) of shift register (2).

The next Part is coming soon...