Outdoor Temperature Monitor

A project log for LoFi

LoFi is a very low cost ($5), small, auto-transmitting module. Preassembled and preprogrammed. Simply attach to your appliances or projects!

david-cookDavid Cook 07/20/2014 at 03:132 Comments

In the previous logs, I've shown LoFi in the remote corner of the basement and in the garage. Now, LoFi ventures outside to the garden to measure temperature and light levels.

A 'My M&M' silver favor candy tin (intended for weddings or parties) is the perfect size container for LoFi, the AM transmitter, a solar panel, coin cell (underneath the PCB), and various sensors. A clear lid permits light but blocks bugs, dirt, and hopefully rain. The transmitter antenna wire sneaks out the side through a #54 drill hole.

A foam liner was added to the candy tin to avoid electrical shorts. Even though the container has a non-conductive coating to avoid contaminating the candy, I've been burned by scratched or worn areas in the past.

When there is adequate sunlight, the solar panel powers the device. This solar panel was chosen because it fits so well, and provides the maximum voltage with good current. At full sunlight, the panel likely exceeds the ATtiny 5.5 V operating limit, but will stay under the absolute maximum of 6 V and has some voltage dropped by a Schottky reverse-protection diode.

The ATtiny internal temperature sensor and voltage reference are 'no-cost' inputs. An added photocell provides brightness data, and was selected to be sensitive to conditions after the solar panel yielded to the backup battery.

With wireless, it is nice to get immediate readings rather than logging to a flash as I have had to do in past experiments:

This was not my first choice of locations for LoFi, but I discovered that a spot of land was already claimed by a toad in a hole.


David Cook wrote 09/07/2014 at 20:42 point
Hi Keegan,

Wow. I just realized that there was a comment attached to this log entry. I wish it was displayed by default. So, I apologize for not getting back to you sooner.

With the transmitter, PCB, and parts (but no coin cell holder or batteries), it is $3.24 for quantity 100 and $2.42 quantity 1000. Pasting a table doesn't really work, but here it is anyway:

Part Number / Description 100 1000
PCB 1 square inch $1.00 $0.50
Capacitor 10 uF CC1206ZKY5V7BB106 $0.07 $0.04
Diode Schottky 10V PMEG1030EH,115 $0.19 $0.14
Microcontroller Attiny ATTINY84A-SSUR $0.74 $0.74
Transmitter eBay FS1000A $0.50 $0.50
Socket 6 female PPPC061LGBN-RC $0.51 $0.37
LED red/green LTST-C155KGJRKT $0.20 $0.11
Resistor 220 ohms AC1206JR-07220RL $0.02 $0.01
Resistor 100 kiloohms $0.01 $0.01
$3.24 $2.42

Note that I assume you can negotiate with the eBay sellers to get a bunch of transmitters for $0.50 each, rather than with the receiver for $1 for the pair. I suppose you could ditch the socket, diode, and LED to save almost a dollar each.

I agree with you that the key advantage of LoFi is the low price point. As I approach the next deadline, that will be one of the points I'll be focusing on.

Thanks for your support,


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Keegan Reilly wrote 07/28/2014 at 19:41 point
Hi David, just wanted to say I think your project is awesome and I'm looking forward to getting a bunch modules myself whenever they are available!

I think the price point you are aiming for ($2-$5) is key for the real IoT. Just before I saw this project I was thinking about distributing several dozen temperature sensors in my house. Using the standard arduino-wifi solution would have cost way too much (>$1000). This wouldn't be feasible for me (or most people) unless the price per module was made as low as possible, but LoFi puts it within reach!

Mine is an old house (1925) with warm and cold rooms, and I'd like to model the thermal gradient characteristics of the various exterior walls. Obviously getting a decent IR camera could show me spots where insulation is missing, but even a sophisticated instrument like that couldn't give me a quantitative measure of how much $ in energy I could save with specific changes.

With an accurate thermal model informed by dozens of temperature sensors linked to local weather reports, and a handful of insolation sensors I could predict with some precision whether adding more insulation or shutters to block sunlight would be most efficient, and how long it would take an attic vent fan or a new window to pay for itself in saved air conditioning costs.

Of course the applications for LoFi are huge, this is just something I would love to do with it. You mentioned that your goal is less than $5, and also that it could be less than $2 in quantity. Is that $2 figure still potentially feasible? I know $3 isn't that big a difference for one device, but for 30 to 50 it makes a significant difference. Just curious what the latest cost estimates are. Thanks for sharing your progress, I'll be following closely!

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