Notes on experimenters board - updated August 22.


I received my component order -- so I now have (almost) all the required parts. Due to a mistype, I order LMC555CN rather than LMC555CM (the 8-dip rather than the SO8). Argh! I need to find a local store that carries surface mount parts. [It seems ridiculous to pay $10 shipping&handling for $1 of parts!]

It also turns out that I have the relay layout mirror imaged, so that is a good find.

I've fixed the parts list and the spreadsheet.


All sockets are mounted on the same edge of the board.  The board has a single power supply section, one pressure and one counter section.

The off board connections will use screw terminals. There is a power jack for an unregulated supply and an RJ-45 socket for a connection to the 1-wire network.

The board will be double sided with a VCC plane on one side (actually I'm now not fond of this idea as it will make the soldering rather more difficult) and the Gnd plane on the other (with a nice gap to make cutting the board in half easy). There is no solder mask or silk screen. Crucial items will labelled in the metal layer, but assembly is encouraged with a printout of what the silk screen would say. Pin 1 (or the marked pin) is sometimes marked with a different shaped pad. Otherwise you have to know that all the chips point the same way! Also all the resistors and capacitors are 1206 form factor. While this is more expensive that 0803, they are easier to work with -- and the few cent savings will not even buy a cup of coffee!

There are many options -- bits of the circuit do not have to be built.

I'm using Eagle PCB to design and layout the board. This is not ideal, but it is free for this size of project.

The current version of the schematic (200k PDF file) includes the functionality below. The corresponding top and bottom layers for the PCB are also available (actually I lie, these are from the version 1 layout). These are double scale. They are each around 100k bytes.

I have also made up a parts list that corresponds to the this version of the board.

There is now an Excel spreadsheet that calculates which components you need for which options. I don't guarantee that it is right, but it looks as though it works. It also does approximate pricing on most of the parts -- except the MPX4115A and the parts from Dallas.

I have now ordered my parts for four boards -- in various combinations. You can see what I am doing in the Excel spreadsheet above.

Power supply section

This contains a RJ-45 socket which is wired up compatibly with the 1-wire hub. I.e. regulated power (and maybe unregulated power) is supplied.

There are also four holes for Vcc, VccUn, DQ, Gnd. These can be used for attaching wires directly or using screw terminals. The Vcc, DQ and Gnd are also available on another three holes for daisy chaining.

There is fairly comprehensive transient protection. This consists of a zener on the data, and a zener on the unregulated power. Note that not all these need to be populated.

There is a undershoot protection diode and a parasite power takeoff diode. There is a jumper that chooses whether power comes from the RJ-45/power jack or whether parasite power is to be used.

There is a capacitor on the power supply.

There is a regulator that can be used to provide 5Volts from an unregulated supply.

Pressure Sensor

This consists of the MPX4115A and a DS2438. For maximum sensitivity, the output of the 4115 is taken directly into one of the current sense inputs, and the other current sense input goes to an adjustable voltage source.

The adjustable voltage source can either be a 10-turn pot, or a 1-wire digital pot (DS2890). Given that Dallas will provide two samples of anything, I'd go with the digital pot. This approach of using the current sense improves the resolution at the expense of range -- but the range still exceeds any atmospheric pressure change (at a specific altitude).

In the circuit below, either the pot (10-turn) or the DS2890 should be populated -- but not both.


An IButton clip (DS9094F) can be mounted for providing access to the hygro ibutton, or any other iButton.

Water Depth Sensor

This operates on the principle of measuring the capacitance of a bunch of capacitors, some of which are under water (and hence are [effectively] shorted out). The capacitors form the C part of a 555 timer circuit. This oscillates at a frequency proportional to the number of capacitors out of the water.

The frequency is measured by a DS2423 (counter). Actually, the pulses are counted and the host determines the frequency.

The capacitor chain is connected from two holes in the board. A CMOS 555 is used so that the circuit can run on parasite power. This uses channel A of the counter.

Generic Counter

A three pin header provides power, gnd and data out from a sensor. Essentially this couples directly into a 2423 counter.

A jumper is provided that selects either the unregulated or regulated power (5V) input. A position for a pullup resistor is provided if one is required. A jumper block is provided so that the generic counter can be attached to either channel of the counter.

Optically Isolated Counter

This provides some level of isolation between the count signal and the 1-wire circuitry. A two pin header is provided for Gnd and Signal. This must be suitable to drive an LED via a resistor. A zero-ohm resistor can be substituted. This uses channel B of the counter.

Voltage/Current Source

A digital pot (DS2890) driving (as an emitter follower) a darlington pair together with a DS2438 to monitor the voltage and current being fed to the external device. In my case, this will connect to a scheme to keep my underwater camera free of algae.

An alternative is provided in the form of an opamp that will act as a constant current source. The DS2438 can be used to measure both the current and the voltage.


A DS2409 is provided that can isolate the 1-wire on the board from the main 1-wire. The control output is used to drive an LED or to drive a transistor that drives a relay. The relay can handle 5A non-inductive load at 125VAC. A seperate set of screw terminals is provided for the NO and NC connections.


  1. Mounting is always a bear -- it is not clear what the form factor should be. I picked out a box from the digikey catalog and the board should slot into that.
  2. Some people want alternative pressure sensors. If it is just a case of a second set of pads, then that is OK. I'm nervous about signal conditioning circuitry.


Currently the board is 4 inches by 2 inches. It is not very tightly packed. For straight double sided -- no masks -- this would cost me around $60 + $5/board. Therefore think $10 per board.



The original version (suprise).


Added the relay and DS2409 for power control and also to be able to isolate the 1-wire.


Added two blocking diodes by the TVS on the 1-wire network. It turned out that the TVS has an apallingly high capacitance, and it would have killed the 1-wire network. By adding two low capacitance diodes, the overall capacitance of the suppression is under 30pF.

I also updated the spreadsheet with some more useful items -- like a power supply transformer.


Small changes -- replaced the 1N4004 with the DL4004 as it is smaller and matches the other parts used on the board. Also replaced the relay driver transistor with a MOSFET as that will drive the relay better. Also I split the lightning counter connector apart from the other connector. This improves the isolation. I'm hoping to get to around 0.2in gap between the floating side and the 1-wire side. This is an airgap of 5kV -- which is what the optoisolator can handle anyway.


Send me email at the address below.

Philip Gladstone