A Tried and True Thermometer

By Marshall Moore

Back in 1980, I purchased a Digital Thermometer Kit, model JE300, from Jameco Electronics for $39.95. For its time, the general description was fairly impressive:

• Dual sensors – switching control for indoor/outdoor or dual monitoring applications (2N2222 transistors TO-18)
• Sensors can be placed more than 100 feet from case
• Intersil 7107 CPL, 40-pin A/D converter chip circuitry
• Continuous reading of three bright .8" high HP3400 displays
• Settings for either Fahrenheit or Celsius degrees
• Range of -40°F to 199°F, -40°C to 100°C
• Accuracy of ±1% nominal (depending on range and calibrating conditions)
• Powered by a 12VAC wall transformer (AC 250)

Exterior of Marshall's Digital Thermometer
The unit was housed in a "simulated walnut" wooden case with a red bezel display window, measuring 3.5" x 6.63" x 1.38". I always enjoyed this unit because of its compact size, the 100-foot sensor range and the fact that I could easily read it from more than 25 feet away. It served me well and operated continuously for close to 10 years – until I moved.

It got packed away and was eventually replaced with an updated battery operated LCD unit. Over the years I've owned several of these thermometers but they have always been difficult to read because the readouts are small, the ambient light illuminating the display must be just right and trying to read those displays from 25 plus feet away is challenging, even with the right conditions.

With all the cold weather in Ohio, I was motivated to try and find my trusty old Jameco Digital Thermometer unit. I eventually located it at the bottom of my junk parts box. It looked pretty sad. The power transformer had been snipped off and was missing. So too was one of the sensors. I rummaged through my wall transformers and found a 12VAC replacement, hooked it up and powered on the unit.

Interior of Marshall's Digital Thermometer
After nearly 25 years of being dormant, it sputtered back to life with a very erratic display reading. I took some voltage readings and re-soldered the circuit board. It settled down and displayed a solid reading that gradually changed as I held the sensor between my fingers. Eureka, it still works!

I've since calibrated the unit and once again it's working flawlessly. Unfortunately, finding another 2N2222 sensor transistor has been a bit of a problem. I could use any NPN transistor with the same thermal characteristics, but I would like to maintain the unit's authenticity by using the same transistor supplied in the kit.

This experience speaks volumes when it comes to the quality, dependability and longevity of Jameco's kits and component offerings. No doubt, I will be checking the temperature with this same digital thermometer from 1980 long into the future thanks to them.

I'm confident more high-quality and durable kits will continue to be offered, too. With the debut of Club Jameco in 2012, a DIY electronics projects hub that is entirely hobbyist driven, electronics enthusiasts – like me – and designers are able to discover, discuss, develop, evaluate and purchase electronics kits through a central website. It's exciting, and I look forward to many more similar kit experiences in the future.

Editor's Note: The JE300 was one of Jameco's best selling kits. It has since been retired but we're always open to the idea of rebooting a classic. You can check out the Redesigning and Resurrecting the Classic Jameco Clock Kit story to see a Jameco kit that's in the process of being redesigned and resurrected.

What classic Jameco kit would you like to see revamped and ready to ship again? Share your ideas at our community forum.

If you have an electronics story or project you'd like to share, please email [email protected].
Marshall Moore lives in North Royalton, Ohio and is a retired computer scientist/engineer. His interests include home automation, antique radio restoration and fine cabinetmaking. Marshall has been a Jameco customer since 1980.