A Microammeter Thermometer

I wanted to familiarize myself with Microchip’s MCP9700 thermistor ICs for some upcoming projects that require temperature measurement / compensation.

I decided to build a simple circuit around the MCP9700 to convert an old analog micro-ammeter into a thermometer for the lab. It displays 1uA per degree Celsius, has an output of 1mV per degree Celsius for external temperature monitoring / data logging, and draws only 20uA from a 5V supply.

The analog thermometer reading 20 degrees Celsius in the lab

The analog thermometer reading 20 degrees Celsius in the lab

Not only is it a cool looking centerpiece (I’ve always found old analog meters absolutely beautiful), it’s quite useful too!

Analog thermometer schematic

Analog thermometer schematic

The circuit is very simple: the LCT1541 op-amp is configured as a differential amplifier, converting the output voltage from the MCP9700 into 1mV per degree Celsius. This is used to drive the micro-ammeter to display 1uA per degree Celsius.

Except for R2 and R7, all resistors used were precision .1% types; U2b is the band-gap voltage reference integrated into the LTC1541. A more thorough circuit description and demonstration is available in the video:

While this initial design works, there are definite shortcomings. Stay tuned for updates on design critiques and improvements!

References:

MCP9700 datasheet (microchip.com)
LTC1541 datasheet (linear.com)
AA battery discharge tests (powerstream.com)

9 thoughts on “A Microammeter Thermometer

  1. That’s a beautiful ammeter! I have one also and I might try to turn it into an thermometer, although my scale is not clean as yours, it has passed a lot of years since the ’60s 🙂

    • Thanks! Yes, I had a few beat-up ones sitting around too, so when I came across this one on ebay for a reasonable price, I just couldn’t pass it up. It’s made by “Conway Electric Enterprises”, though I couldn’t find much about the company or the meter, so I’ve no idea how old it is, but the previous owner kept it in very nice shape!

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  5. Good Man Craig,

    Please, Please, Please.

    Continue on your path and your ideas and your projects. I sincerely wish you all the best in future projects, keep the haters, help vampires, idea leeches and dumb-asses away. This is a great project and the way it is presented is perfect.

    MoFo David Attenborough of Analog. *Exploding Fist Pound*

    Can I hire you as a tutor?

    P.S. Ignore all electronic forum comments. Only answer questions if they crossed questions you posed yourself. Peace.

  6. Pingback: Developing a Thermometer from an Old Ammeter » Electro-Labs.com

  7. Hi,

    Nice detailed description. Should there be a ground connection from the bottom 100K potentiometer?

    Thanks,
    Sudhir Gupta

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