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One way that I get reliable 1-wire communication is to use a power injector. This means that the along with the 1-wire signals, I inject a supply voltage into additional cable pairs.  Since  the most readily available commodity cable currently is 8 conductor Cat-5 cable, chances are that you also use this cable for 1-wire cabling and have spare wires going unused. Dallas called the 1-wire bus a MicroLAN, so I will continue to use their terminology here.

For short runs, where the total length of the MicroLAN is < 30 meters and the network only has maybe 6 to 10 slave devices on it, cable selection for use on the MicroLAN is reasonably simple, as even flat modular phone cable can work with a small numbers of 1-Wire slave devices. However, the longer the MicroLAN, the more important cable properties and therefore cable selection becomes more critical.  However, since I’m dealing with 30M or less, I’ll focus on that here and leave the research for  better cable techniques for when I run into a problem.

One of the remaining issues with using 1-wire driven from an Arduino pin directly is the fact that it is also providing power to all of the devices on the the 1-wire LAN when in parasitic mode.   While 1 or 2 devices are of little concern, I started to worry about building a reliable 24/7 full time  MicroLan configuration that would report weather data from 8 or so sensors, most of which were located at the garden bed outdoors.  It became clear that some of my devices were really not performing as desired  when “parasitically”  powered when I finally put all the pieces together.

Experimentation showed me that the signal from the Arduino pin was just not up to that task of powering and communicating with all the devices without some random miscommunication here and there.  I would get values like 185F and not the 70F I was expecting from a 1-wire temperature sensor, for example.

My solution was reasonably simple.  I would create an external “middle-man” device that would insert power into the CAT-5 cable after leaving the Arduino board.  Its really just a dual supply from typical commodity wal-wart power source.  The unmodified 12VDC supply will be fed into the cable on a spare pair and treated as “unregulated” and a 5V regulator will be added to another spare pair and considered to be “regulated”.  All code will now be written to comply with the “non-parasitic” mode of 1-wire.

The Design:

The PCB:

While looking at low cost pressure sensors in the Mouser Electronics catalog, I located the FREESCALE MPXAZ6115A as a possible sensor for my project. The sensor has the following statistics;

Device: MPX6115, MAX PSI 16.7, MAX kPa 115.

Since barometric pressure here hovers at around 100kPa or so, this sensor would do just fine. The analog output of the sensor is relative to the min/max pressure range of the sensor.

According to my initial tests, the sensor would output about 4.06 volts at 100kPa.

The built-in analog input on the Arduino would also keep the circuit simple and after a few tests I was able to determine the offset value I needed to get correct readings for the localized barometric pressure.
In my case… testing showed that the magic number is 0.13.

// Nominal Transfer Value:
// Vout = VS x (0.009 x P – 0.095)
// ± (Pressure Error x Temp. Factor x 0.009 x VS)
// VS = 5.1 ± 0.25 Vdc

float Vin;
float P;

void setup()
{
Serial.begin(9600);
}

void loop()
{
Vin = (5.0/1024.0) * analogRead(0);
Vin = Vin + 0.13;     // Offset Adjustment
 Serial.print(Vin);
 Serial.println(" Volts");

P=((Vin/5.0)+0.095)/0.009;
 Serial.print(P);
 Serial.println(" kPa");

Vin = (P * 0.2952999);
 Serial.print(Vin);
 Serial.println(" Inches of Mercury");
delay(2000);
}

I’m using a LADYADA Boarduino on a solder-less breadboard for testing. The sensor hookup is dead simple with only one exception that makes it tricky. The part I selected is designed to be surface mounted.

I decided to create a carrier board using the board layout software I prefer called SprintLayout from ABACOM in Germany. Other than 5V power and ground connections, the Vout from the carrier board goes directly to the Arduino Analog(0) pin.

To create the PCB board, I use the GOOTIE toner transfer method to apply the layout on the PCB for etching. (google search “gootie PCB” for more info)

Having developed a dislike for the chemical etchant that Radio Shack sells; Ferric Chloride, I have also adopted the etchant that
Gootie describes.

It is based on the swimming pool chemical Muratic Acid and Hydrogen Peroxide in a 1 to 2 ratio. It’s fast, non-opaque and does not require heating or excessive agitation.

Note: I also recently picked up a used GBC Creative Laminator at the local Goodwill for $14.00. It does an excellent job of applying the toner to the copper on the PCB to be etched. Using an hand iron was OK, but the results were not always predictable.

My method of making PCB’s at home.

Here is the Layout: