SparkFun Forums

[e3b0c442]

Hi all,

As the title implies, I'm interested in utilizing the Artemis for a project which involves taking analog readings (temperature readings using thermistor probes). With my current MCU this is easy, as I can just use a high output from a digital pin, but that isn't really possible here.

I am concerned with power consumption, so I need to be able to turn the probe circuit on and off. Again, easy enough using the digital pin.

I'm fairly novice at this, and have a couple of general ideas, but would really like to hear from more experienced folks on best practices and theory.

The possibilities I've considered are:
1) Using a voltage divider, but the probe circuit is itself a voltage divider circuit so I'm not sure what the effect will be of chaining them, nor what the power considerations look like here
2) Using a BJT transistor as a switch from an external power source and then using an LDO or zener diode(s) to further adjust the voltage down as needed.

Thanks in advance for guidance and suggestions.

[El_Duderino]

Hi e3b0c442,

Either of those options could work but you might have some minor issues with both of them. I'm not entirely sure how a second voltage divider circuit would work with the sensor but you might have some stray current that affects your output voltage so you may get some errors in your data. With a LDO or zener diode, that will drop it but you'll need to make sure it's a smooth voltage or else you'll again get some errors in your data.

Another suggestion (albeit a bit excessive and also introduces other components/power draw) is to use an external ADC like this and then output the data to the Artemis. It's not ideal since you're adding another board (and also using a less accurate ADC) but it would help with the voltage difference.

I'll try and ask around here to see if other SparkFun folks have any other ideas on how to do this.

[e3b0c442]

Hey, thanks for the response.

I asked around on this and the general consensus seems to be to use an adjustable linear voltage regulator to hit the 2.0V target. I think that will work for me in this case; I could either power it directly from the power supply and use the digital out from the Artemis to control the enable/shutdown pin on the regulator, or if the digital out can handle the current (which it should, we're talking <100µa + the ground current of the regulator) then I can just feed the VIN of the regulator directly from the output pin and not have any consumption at all when the probe circuit is powered off. I'm OK with using the linear regulator because I was already designing the original circuit around 3.3v anyway, so I shouldn't see any real difference there.

That all having been said... if the folks on your side have a better idea I'm all for improvements.

[El_Duderino]

That's a really unique idea using a linear voltage regulator. I think that may be the best solution here if it is working in practice. It might be a bit power hungry when it is in use but if you are enabling and disabling it to reduce power consumption, that could certainly work.

A voltage divider would probably be the lowest impact solution here but dialing that in, especially considering the resolution of the ADC on the Artemis, is going to be tricky. You can start by measuring the actual resistance of your resistors in that voltage divider since you have a +/-1% error to really dial it in but that is a lot of trial and error.

Let us know if the adjustable linear voltage regulator works out because that is a really cool and unique idea to drop voltage on an analog signal. One final thought that was brought up when asking around and that I am going to try and do some more digging on is it may be worth researching how users were able to drop their analog input for the ESP8266 since that also has a unique analog input range (0-1V). Most of what I have found initially are just voltage dividers but there may be some other ideas around since the ESP8266 has been out for several years now and we're just in the infancy of Artemis.

[EarlyRiser]

Has anyone tried to lower the Artemis Vdd to 2 V? Maybe some sensors do not want to operate at 2 V. It has the promise to reduce the use of voltage dividers.