This is my second four-channel temperature logger and my last one based on a PICAXE chip. It's based on the PICAXE 28X1.

Click to enlarge...

What's really great about this unit is that it's completely standalone. It even has it's own 8.4V 3000mAh battery pack which lasts about a week.

The connectors are all on the rear of the unit this time and are of higher quality too.

Features include:

• Two backlit LCD screens, this allows the temperatures have their own display, separating them from other information like memory and battery capacity. The back-light can be switched between a level that is suitable for when you are dark adapted or not.
• The Battery capacity is displayed as a percentage of power left and as an absolute voltage level, by pressing the Battery Check switch.
• 2MB EEPROM to save 29 hours data from each sensor (simultaneously). The lower LCD displays the percentage of memory that has been used. Once the memory is full the unit continues reading temperatures (the previous data logger stopped at this point). The temperatures are still sent to the top LCD and the computer (if connected), but the data is no longer recorded in the EEPROM.
• The data logging can be re-started from the beginning of the EEPROM without turning the unit off and on again. Just press the Restart Data Log button.
• When downloading the temperature data from the EEPROM to a computer, the download can be stopped at anytime by pressing the Stop Download button. The unit then returns to normal data logging (the previous data logger would continue to download until it finished downloading all the data or it was hit with a hammer, ie. turned off).
• 4 Digital Temperature Sensors with 0.1 degree resolution.
• Handles positive or negative temperatures.
• Samples taken every 6 seconds. The time the last sample was taken is displayed on the lower LCD.
• Can operate connected to a computer or standalone.
• 8.4V 3000mAh low self discharge NiMH battery back. Made from seven 1.2V Enerkeep SUB C cells. Charging takes a 3 or 4 hours and allows the unit to operate for about one week.
• Dedicated battery charger that monitors the temperature of the cells to avoid overcharging.

The next image shows the relative size of the unit compared to the first data logger I made.

The firmware files and code description are available in SCPedia as well, here's the link.

Also here are the pinouts I used for the PICAXE 28X1 chip:

If you are using a 28X you must connected a resonator to pins 9 and 10 as listed above. Since I used the newer 28X1 which has an inbuilt resonator I left these pins not connected. A PICAXE 14M was used as an interrupt controller and connected to pins 12 and 13 on the 28X1. A description of the 14M and how it monitors the input buttons is included on the code page.