Sensor chips, connectors and support parts inside the kits

A small kit maker saves effort by reusing what already works. Midon Design built nearly every board around the same few families: 1-Wire devices for sensing and switching, the Motorola 68HC705 for control, a Maxim RS232 driver for the serial port, and an LM78L05 regulator for the logic supply. Once those parts are familiar, most of the schematics on the kit pages read quickly.

The company also sold a handful of these parts on their own, for hobbyists who wanted to add a sensor or build a small project: DS18S20 thermometers, DS2406 switches, DS2430A memories, a current sensor and the parts of the project case used by TEMP08. This page lists those alongside the parts that were only ever soldered onto boards. It is a reference for identifying and understanding components; none of them are offered here.

For the kits themselves, see the kit catalogue. For finished sensor assemblies, such as the MD3020E humidity and temperature board, see the sensors page.

Microcontrollers

The Motorola 68HC705C8 was the workhorse. It offers 8K bytes of EPROM, 304 bytes of RAM, 24 programmable I/O lines, 7 input-only lines, a serial communications interface, a serial peripheral interface, a 16-bit timer and a watchdog. Both one-time-programmable and windowed EPROM versions exist, and the company's PROG05 programmer handled both. The much smaller 68HC705K1, with 504 bytes of ROM and 32 bytes of RAM, ran the STATX10 X-10 display. The datasheet list also included the MC68HC908KX8 and MC68HC908JB16 from the later flash-based 68HC08 family. The documents folder held one more Freescale part, the MC9S08QG8 from the HCS08 family: an 8-bit flash microcontroller with 8K bytes of flash, 512 bytes of RAM, a 10-bit A/D converter and SCI, SPI and I²C serial ports, running from 1.8 to 3.6 V. No kit page on this site names the board it went into.

Memory

Loggers need storage. The datasheet list named several Microwire serial EEPROMs: the 93C56 and 93C66; the Fairchild FM93C86A, a 16-kilobit part organized as 1,024 words of 16 bits, or as 8-bit bytes when its ORG pin is tied to ground; and the STMicroelectronics M93S46, M93S56 and M93S66, 1-, 2- and 4-kilobit parts with a user-defined write-protected area. Alongside them sat the AT25256, a 256-kilobit SPI EEPROM. The 93C86 is the chip the TEMP08 firmware history lists for release 2.24 of September 2012 on the KX8 processor, the release that added large EEPROM support. The LOG08 and LOG08-II loggers kept their time-stamped samples in on-board EEPROM, which is what allowed stand-alone logging over long periods.

RS232 drivers

The MAX233 converts the microcontroller's TTL-level serial signals to RS232 levels using built-in charge pumps and needs no external capacitors, which saved board space on the programmer. The MAX232 does the same job with four small external capacitors.

Two entries on the old datasheet list belong to no kit page on this site, and they are described here for completeness.

MC14489 LED display driver

The Motorola MC14489 is a multi-character LED display and lamp driver. One chip drives up to five seven-segment digits with decimal points, or up to 25 separate LEDs, by multiplexing the segments itself, so the controlling microcontroller only sends it data. That data arrives over a three-wire serial link that is compatible with the SPI port of the 68HC05 family, and several drivers can be cascaded on the same link for longer displays. A single external resistor sets the segment current. For a board built around a 68HC705, it turns a handful of numeric LEDs into a job for three I/O lines.

Figaro TGS2610 gas sensor

The Figaro TGS2610 detects LP gas, that is propane and butane. Its sensing element is a tin dioxide semiconductor on a small heater: in clean air the element has a high resistance, and when combustible gas reaches the heated surface the resistance falls, the more so the higher the concentration. Placed in a voltage divider, it gives an analog voltage that rises with the gas level. That is the same kind of signal the humidity and pressure elements above produce, which is why an A/D input such as the DS2438 or DS2450 is the natural way to put an analog sensor like this on a 1-Wire bus. The heater needs its own steady supply and a warm-up period before readings settle.

Humidity sensing in the kits paired an analog Honeywell element with a DS2438, whose A/D input read the sensor voltage and whose temperature sensor supplied the correction. The datasheet list named the HIH-3610 and HIH-4000; the MD3020E board used the sealed HIH4021. DS2438-based barometers worked the same way with a pressure element, and the datasheet list included the MPXA4115 pressure sensor. Current sensing used a current transformer from the CR Magnetics CR3100 series, sold as the CR-3100 current sensor. The CR3110 is the model in that series that the company's datasheet and its TEMP08 application drawing were written for. A current transformer senses the alternating current in a conductor that passes through its core, so it measures a load without an electrical connection to it.

Project case parts

TEMP08 was sized for the Pac Tec CM5-125 case. The company listed the case, a red translucent bezel, a black panel, rubber feet and two kinds of screws, one pair to mount the board and one pair to close the case.