The Mini Pearl Logger is a powerful and inexpensive device for logging environmental data. It is based on an Arduino Pro Mini in combination with a real time clock and a microSD card reader. Soldering together these three common electronic components allows logging data from multiple sensors for multiple months. Some clever modifications of the components reduces the sleeping current between logging events and allows the logger to collect data unattended on small batteries (3 AAA) for two or more months. The total cost of parts for the Mini Pearl Logger can be under $15.00.

The DS3231 real time clock has an integrated temperature sensor and timekeeping is automatically corrected for temperature. Data from this sensor can be read and logged. The Mini Pearl Logger does not have any other sensors, but a wide variety of sensors (including I²C, 1 Wire, and Grove) can be used.

Above: These parts can be assembled into a data logger which will operate autonomously for months. The three blue PCBs are (left to right) 3 volt microSD socket, Arduino Pro Mini, DS3231 real time clock. Total cost can be under $15.00 if components are purchased carefully on eBay.

This logger is based on Ed Mallon's Cave Pearl Logger which uses a similar combination of components. Ed has done extensive testing to extend the field longevity to more than a year. We are still trying to replicate Ed's results, and have not yet managed to log data for more than two months on a few AAA batteries.

Above: An assembled Mini Pearl Logger. Clockwise from upper right: battery case (e.g., 3 AA), microSD card socket, DS3231 real time clock, Arduino Pro Mini.

Parts

3 Primary components:

• Arduino Pro Mini. Pro Minis are available on eBay from sellers in Asia for under $2.00. At US retailers prices are between $4.00 and $20.00. Pro Minis come in 3.3v and 5v versions, and the 3.3v version is required.
• DS3231 real time clock. This module is available on eBay for about $1.20 or from US retailers for $4.00 to $15.00.
• 3 volt MicroSD card socket. These modules are available on eBay for less than $1.00. There are many different microSD card modules, and some are better at low sleep current.

Also required:

• Battery or battery case for AA or AAA. The Mini Pearl logger is a 3 volt device. Three alkaline batteries (3 x 1.5v = 4.5v) are adequate or four NiMH (4 x 1.2v = 4.8v). LiPo or LiPoly batteries (3.7v) are also good.
• MicroSD card. Cards of 1 GB or smaller capacity are best because they tend to sleep better and at lower current. 128MB cards can allow 2 to 4 million data logging events, so larger cards are not needed.
• CR2032 coin cell battery for real time clock.

Kits with all the parts needed to solder together a Mini Pearl Logger are available at the KAPtery for $19.00.

Assembly

There are two documents with guidance about assembling and getting started with the Mini Pearl Logger at the KAPtery Guides page.

Software

An Arduino sketch which logs data from the integrated temperature sensor and displays some of it on a serial monitor can be downloaded at the KAPtery Guides page.

Below are some research notes about activities done with the Mini Pearl Logger. Some of these notes include the sketch which was used in the activity described.

Activities

Questions