Sensors: the 1-Wire chips behind Midon Design's kits

A 1-Wire device talks to its bus master over a single data line plus a ground return. Many devices share that line, and each one is told apart by a 64-bit ROM ID laser-set at the factory: an 8-bit family code that identifies the chip type, a 48-bit serial number and an 8-bit CRC that lets the master check the address was read correctly. Because the family code is part of the address, an interface such as the TEMP08 1-Wire serial interface can work out what kind of sensor it has found without being told.

Many 1-Wire chips can also run on parasite power. Instead of a separate supply pin, the chip charges an internal store while the data line sits high and runs from that charge while the line is pulled low. Parasite power keeps wiring to two conductors, but tasks that draw more current, such as a temperature conversion, leave less margin, so long networks often provide a local 5 V supply on a spare pair instead.

Nearly all the chips on this page were designed by Dallas Semiconductor, which became part of Maxim Integrated, which in turn became part of Analog Devices. Datasheets and product pages now sit with Analog Devices. On a modern computer the same chips are read by OWFS, the Linux w1 kernel driver or the Home Assistant 1-Wire integration.

The DS2438 was designed as a battery monitor, but its combination of a temperature sensor and a voltage A/D converter made it the standard way to put analog sensors on a 1-Wire bus. A humidity element that produces a voltage proportional to relative humidity is wired to the DS2438's voltage input, and the bus master turns that voltage and the chip's own temperature into a humidity figure. Barometers and general voltage monitors use the same trick with a different analog front end.

The MD3020E humidity and temperature sensor

Midon Design's own humidity board, the MD3020E, paired a DS2438 with a sealed HIH4021 humidity sensor and added a DS18S20 thermometer. An on-board regulator converted up to 30 VDC down to the 5 VDC needed by the humidity sensor and by any locally powered sensors further down the line. A jumper, J3, selected parasitic (P) or local (L) power for the DS18S20. The DS2438 and humidity sensor always needed either +5 VDC or the 9 to 30 VDC input. When the DS18S20 ran on parasitic power, the matching sensor option also had to be set to P in the interface. The board measured 1.9 by 2.1 inches and connected through an RJ-12 jack. It was withdrawn before the company closed. The 1-Wire humidity sensor guide explains the measurement in more depth.

DS2423 counter

The DS2423 combines memory with two 32-bit counters that increment on external pulses. A rain gauge's tipping bucket, the reed switch of an anemometer or a lightning detector each produce pulses, and the bus master reads the running total and works out the rate. Weather stations read by the TEMP05 used the DS2423 for wind speed.

DS2450 quad A/D

The DS2450 has four analog inputs with selectable resolution. AAG used it in the Version 3 weather station to replace the older DS2401-based wind direction sensing, and the LOG08 loggers could record DS2450-based voltage sensors.

DS2408 and DS2406 switches

The DS2408 offers eight programmable I/O channels, each usable as an input or an open-drain output. It is the heart of the 1WIO multi-purpose sensor, which uses it for relays, opto-isolated inputs and LED displays. The DS2406 is a smaller addressable switch with built-in memory. In its TO-92 form, which Midon Design supplied as MD3023, only a single input/output channel is available, and that part was RoHS compliant.

DS2430 EEPROM

The DS2430 stores 256 bits of EEPROM. It measures nothing, but it is useful for keeping identification or calibration data on a sensor assembly. Midon Design supplied it in TO-92 form as MD3011.