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TEMP05 Introduction
TEMP05 Frequently Asked Questions

FAQTEMP05 owner answers

Common TEMP05 questions on wiring, sensors and setup

The TEMP05 generated the same questions again and again: how parasitic power works, how far a sensor can sit from the board, why a reading turns into question marks and which firmware reads which chip. The answers below are grouped by topic and drawn from the kit documentation that accompanied the board.

First published April 6, 2002 · Text rewritten 2026

Isometric illustration of a garden with a greenhouse, shed and planter, each fitted with a wired temperature sensor (AI-generated image)

Parasitic power and the 1-Wire bus

Parasitic power is the name Dallas Semiconductor gave to the way its 1-Wire devices can run without a dedicated supply line. The DS18S20 thermometer used with the TEMP05 is one of those devices. The name 1-Wire is slightly generous: there is one data line shared by every device on the bus, plus a ground return, so two conductors in practice.

Power comes from the data line itself. Whenever the bus sits at a logic 1, each device on it charges a small internal store. As long as logic 1 appears often enough, that stored charge is sufficient to keep the device running while the line is pulled low for communication. The idle state of the bus is logic 1, so in normal operation the line spends most of its time high and the devices stay charged.

Parasitic power keeps the wiring simple, but it has a cost. Temperature conversions draw more current than idle chatter, and long cables or many sensors make the margin thinner. That is why the wiring rules later on this page insist on grounding both outer pins of a parasitically powered DS18S20, and why several Midon boards offered a local 5 V supply on the connector as an alternative.

Which sensors and firmware worked together

A weather station was never required. The TEMP05 read any combination of the supported 1-Wire devices (only one weather station per bus), and it could even run with no sensors at all as the serial driver for a RELAY05 relay board. Support for some chips depended on the firmware version in the board's processor:

Device	Role	Firmware needed
DS18S20, DS1820	Temperature	Any version
DS1920	Temperature (identical to DS1820)	Any version
DS18B20, DS1822	Temperature	V4.21 or later
DS2438 humidity sensor	Humidity	V4.14 or later
Weather station V1 (Dallas)	Wind and outdoor temperature	V4 only (dropped in V5.00)
Weather station V2 and V3 (AAG)	Wind and outdoor temperature	V4 and V5
Sensirion SHT11/71	Humidity	Not supported

DS1820 or DS18S20?

Buyers who ordered DS18S20 sensors sometimes received parts marked DS1820. Dallas/Maxim did not print the S on the newer part, probably because the original DS1820 had been discontinued (production stopped around May 2000) and confusion seemed unlikely. The only visible difference is the package: the old DS1820 came in a PR-35 package, while the DS18S20 uses a standard TO-92 package. Electrically, the DS18S20 has better temperature accuracy and tolerates a wider supply voltage range.

Why not the SHT11/71?

The Sensirion SHT series was evaluated and rejected. Its two-wire interface is meant for short, local connections and does not suit remote sensing over long cable runs, which is exactly what the 1-Wire bus is good at.

Wiring sensors around the house

Sensors connect to terminal J2 over twisted pair cable, normally CAT-5 or better. A home run layout gives each sensor its own cable back to the board, with all the ends tied together at J2. A daisy chain uses one or more long runs, with each sensor tapped into the run where it is needed. Daisy chaining needs no terminator at the far end, although the open end of the run must not be allowed to short. With good quality CAT-5, a sensor can sit up to 300 ft (100 meters) from the board.

The sensors themselves are normally soldered onto the cable. Builders who preferred not to solder had success crimping sensors into RJ-11 plugs and installing RJ-11 sockets at each location. Midon Design also made small circuit boards that held a DS18S20 with either an RJ-11 jack or a two-screw terminal. When running on parasitic power, both outer pins of the DS18S20 must be connected to ground or readings become unreliable.

Star wiring and question marks

If readings turn into three question marks right after a new cable leg is added, the likely cause is a star layout creating reflections on the bus. The documented cure is a 100 ohm resistor in series with the data (DQ) line of each leg of the star, and only on that line. The 1-Wire network wiring guide covers layouts in more detail.

Sensors in water

Sensors can measure pools, fish tanks and spas, provided the leads and joints are fully waterproofed. Silicone RTV or epoxy both do the job.

Switch chips

A DS2405, DS2406 or DS2407 all share the same basic hookup: the PIO pin can read a switch or contact closure, or act as an output driving an LED, a relay or another device.

Identifying sensors and talking to the board

Every 1-Wire device carries a unique 64-bit serial number, and the TEMP05 stores the numbers it has found in EEPROM. Matching a number to a physical location is the part that trips people up. The recommended routine:

Connect one sensor at a time. Attach the new sensor and leave the rest as they are.
Run INI. The INI command searches the bus and adds any new serial number to the next free EEPROM slot, usually the last one. There is no need for ERA, which erases the whole list, unless old sensors should be forgotten.
Check with DIS. The DIS command lists the stored serial numbers. The newest entry is the sensor just attached, so note where it is installed.
Turn on SID ON. With this setting the serial number appears next to each reading in the output.
Find strays by elimination. On an existing installation, disconnect sensors one by one and watch which reading changes from a temperature to question marks.

With firmware V5 and later, the board announces sensors in real time as they are disconnected and reconnected, as long as they were first registered with INI.

Serial connection

The board connects to a PC serial port through a straight-through cable, not a null modem cable. Terminal settings are 9600 baud, 8 data bits, no parity, 1 stop bit and no flow control. Flow control must be set to none, otherwise the session stays silent. The board was also tested with a Belkin F5U103 USB serial adapter.

Software

HomeSeer was a popular host, with script and plug-in interfaces to the serial port, but any program that reads a serial port works. To switch a RELAY05 relay from a HomeSeer script without the TEMP05 plug-in, the script sends the text rly x on to the COM port, where x is the relay number. With the plug-in installed, relays appear as buttons on the HomeSeer web page and can also be switched as X10-style devices.

Weather station and rain gauge

Dallas Semiconductor stopped making its 1-Wire Weather Station. Tecnologia Aplicada (AAG), a Mexican company, reproduced the design, and Texas Weather Instruments offered a similar product. Midon Design verified that both the AAG and the original Dallas stations worked with the TEMP05.

Version 1, 2 and 3 stations

The Version 1 station is the original Dallas unit. AAG's Version 2 is electrically equivalent. AAG then replaced the DS2401-based wind direction sensing with a DS2450-based design and sold the result as Version 3. Firmware V4 reads all three versions, while V5.00 and later no longer support Version 1.

Wind direction shows question marks

On firmware V4.19 or later, the NOR command calibrates the wind direction sensor and stores the result in the EEPROM of one of the DS1820 thermometers. If that particular thermometer is removed, or the sensor list is wiped with ERA and rebuilt with INI, the calibration can no longer be found and wind direction reads as question marks. Running NOR again usually fixes it. The problem does not occur with V5 firmware.

Rain gauges and counters

Rain gauges, lightning detectors and general purpose counters on this bus are built around the DS2423 counter chip, and the board reads several of them at once. More on station hardware is in the 1-Wire weather station guide.

Cutaway illustration of a rain gauge with a funnel feeding a tipping bucket mechanism (AI-generated image)

Humidity, barometers and outdoor accuracy

Firmware V4.14 and later reads humidity from multiple DS2438-based humidity sensors, including those from Midon Design, AAG and Hobby Boards. Dallas/Maxim originally offered such a sensor and later discontinued it. Midon Design's version was part number MD3020E. The AAG humidity sensor only works in parasitic power mode. Barometers and plain voltage monitors on the TEMP05 bus use the same DS2438 chip. The 1-Wire humidity sensor guide explains how a DS2438 turns a humidity element's voltage into a reading.

Outdoor readings that run high

Direct sunlight makes an outdoor thermometer read high. The standard fix is a radiation shield, often called a pagoda: a stack of spaced plates that shades the sensor while letting air move past it. Plenty of construction write-ups exist for home-built versions.

Power supply

Almost any wall adapter works. Without a RELAY05, look for at least 9 V (AC or DC) and at least 100 mA. With a RELAY05 connected, the adapter must supply 12 to 15 V and at least 800 mA.

A small starter setup

Reading three rooms needed one assembled TEMP05, three DS18S20 sensors, a 12 V AC or DC adapter and a straight-through serial cable. Because one DS18S20 is already on the board, a room where the TEMP05 itself sits needs no extra sensor. The board had no dedicated enclosure at the time, and a redesign to fit a PacTec CM5-125 case was planned.

Isometric illustration of a vented sensor housing mounted under the eaves of a house, with clouds nearby (AI-generated image)

RJ11 and RJ12 pin assignments

Different makers wired their modular jacks differently, which caused many connection problems. Pins are numbered 1 to 6 across the connector; the comparison of RJ11 versus RJ12 connectors shows the numbering.

Device	Pin 1	Pin 2	Pin 3	Pin 4	Pin 5	Pin 6
Dallas/Maxim wiring standard	+5VDC	GND	DQ	GND	N/C	DC supply
Midon MD2004 TEMP05	N/C	+5VDC	DQ	GND	N/C	N/C
Midon MD2104 TEMP08	DC supply	+5VDC	DQ	GND	N/C	N/C
Midon MD3009 temperature sensor	N/C	+5VDC	DQ	GND	N/C	N/C
Midon MD3020 temperature and humidity	DC supply	+5VDC	DQ	GND	N/C	N/C
Midon MD3021 relay and LED sensor	DC supply	+5VDC	DQ	GND	N/C	N/C
AAG TAI8550 combo switch	+5VDC	GND	DQ	GND	N/C	N/C
AAG TAI8520 temperature sensor	+5VDC	GND	DQ	GND	N/C	N/C
AAG V3 weather station	N/C	+5VDC	DQ	GND	GND	N/C
AAG TAI8540A humidity sensor	N/C	N/C (GND?)	DQ	GND	N/C (+5VDC?)	N/C
AAG TAI8555 latch relay	N/C	GND	DQ	GND	N/C	N/C
AAG TAI8585 counter kit	N/C	N/C	DQ	GND	N/C	N/C
AAG DS9097U-S09-X adapter	N/C	GND	DQ	GND	+5V	N/C

DQ (pin 3) and ground on pin 4 are the only assignments every device agrees on. Check the supply pins before joining equipment from two makers, and see the TEMP05 introduction for the board's own connectors.

Further reading: DS18B20 product page (Analog Devices) · OWFS, the 1-Wire file system

Questions and answers

Is a Dallas weather station required to use the TEMP05?

No. The TEMP05 reads any mix of supported 1-Wire devices, including DS18S20, DS18B20 and DS1822 thermometers, DS2438 humidity sensors and barometers, DS2423 counters and DS2405, DS2406 or DS2407 switches. Only one weather station is allowed per bus, and the board can also run with no sensors as a serial driver for the RELAY05.

Can temperature sensors be daisy-chained on the TEMP05 bus?

Yes. A sensor can be tapped in anywhere along a cable run from the board, and no terminator is needed at the far end. Keep the open end from shorting. If a star layout causes question-mark readings, add a 100 ohm resistor in series with the data line of each leg.

How does a newly connected sensor get recognized?

Open a terminal session to the board and send the INI command. It searches the bus, finds the new serial number and stores it in the next free EEPROM slot. Avoid the ERA command unless you intend to delete previously registered sensors, because it clears the entire stored list.

Does the TEMP05 work with a USB serial adapter?

Yes. The board was tested successfully with a Belkin F5U103 USB serial adapter. Whatever adapter is used, the port must be set to 9600 baud, 8 data bits, no parity and 1 stop bit, with flow control turned off. A straight-through cable is required between the adapter and the board.

Why do outdoor TEMP05 readings run too high?

Direct sunlight heats the sensor package, so the reading climbs above the true air temperature. The usual fix is a radiation shield, often called a pagoda, made of stacked and spaced plates that shade the sensor while letting air flow past. Mount it away from walls and other surfaces that absorb heat.

Can TEMP05 sensors be used in a pool or fish tank?

Yes, as long as the sensor leads and every joint are fully waterproofed. Silicone RTV or epoxy both work for sealing. Any gap lets moisture reach the leads, which causes leakage on the bus and unreliable readings from that sensor and possibly others sharing the same cable run.