Introduction: Solar Personal AC & Phone Booster

Based on a promising MIT research project for augmenting the way people perceive temperature, this wrist mounted device uses a peltier cooler to trick the body into thinking that it's cooler than it actually is.

Additionally, it features a phone mount with USB charging cable and a Bluetooth speaker with control pad, all of which is of course solar rechargeable.

Step 1: Find Components

As the idea of personal temperature augmenting is still an emerging science, with not even the touted Wristify electronic bracelet widely available yet, there is as of yet no quote-en-quote one "correct way" to create one, and by extension no wrong way. Thus, I would suggest not taking this guide as a must-follow but rather as an illustrated process that can be modified to suit what you think such a device should have included and look like.

At it's core the concept needs three things:
1. 5V 1A peltier cooler & heats sinks (Update: 5V fan recommended too)
2. Power bank and micro controller
3. Some sort of frame* + wrist brace to hold it all together

Additionally, other items, out of several possibilities added were:
1. Solar panel, never be without power :)
2. Phone holder & USB Type-A Jack
3. Bluetooth audio and control pad

When getting components, I highly recommend first ordering the cooler + sinks and then by manually connecting to a battery, with alligator clips, use a bit of low stick tape to test where on your wrist you find the cooling effect of 5 seconds on 10 seconds off is most prevalent. For me it was on top of my wrist where my watch normally goes, though in the MIT study they found the bottom of the wrist worked better.

It's important to note the relative polarity of the peltier cooler as reversing polarity will make the skin-side of the unit heat up rather than cooling down. Which depending if you live a cold climate and want to be comfortable in a house with the heater on low, might be just what you want to do.

*I commonly use a shin guard for a frame of which I should note I used a smaller one than other prototypes, to achieve a sleeker look and subsequently had a notably harder time getting all the components to fit together.

Step 2: Wiring Diagram & Component Prepping

Suffice to say, if you are not familiar with basic wiring diagrams and soldering I would not recommend trying to create one of these. However, compared to other wiring projects this is more straightforward as it largely involves using existing circuitry with longer wire leads added.

As with all electrically based projects, the best first step is a comprehensive wiring diagram, for which it should be noted that the diagram included here shows the circuit with an additional 5V fan added as I discovered, after the initial build, that the heat dissipation of the cooler, and thereby it's effectiveness, is limited in hot weather without one.

For these wiring diagrams you will need the following electrical components in addition to extra wire, electrical tape, and solder.
1. TIP41C Transistors
2. 5V regulator
3. Momentary button

1. Status LED
2. EL & JST connectors (for all lower-to-upper connections)

The assembly of the electronics in this case actually starts with the disassembly of the pocket juice and the Bluetooth speaker. Of which you will want to superglue the rubber control pad of the Bluetooth to the associated circuit board so it doesn't shift while being installed. The battery pack you will want to carefully split open, remove the top, hacksaw off the portion that holds the control circuit, and then reassemble as a separate explosion resistant battery*, and control unit.

*Flat 3.7v Li-ion batteries not recommended, the metal casing of round 18650 type cells combined with a plastic enclosure is the safest near-body Li-ion option.

Step 3: Preparing the Frame

Use a razor knife to separate the foam from the plastic on the brace, and then using a hacksaw/jigsaw and drill. First cut out a rectangular slot to hold the solar panel with the saw, drill a 1-1/2" hole with the hole saw to make a place for the Bluetooth control pad to go, and then use a 5/8" forstner drill bit to make an access hole for the main power button.

Once all holes are cut, glue the foam back on (hot glue used) and then use an X-Acto knife to cut matching holes in the foam.

Step 4: Create the Lower Assembly

Starting with the wrist brace remove the grey Velcro over strap and metal insert, locate where the cooler needs to go, a task for which I found a watch worked quite nicely, and then cut a cooler sized square hole.

Insert the cooler with heatsinks attached into the hole and leave it sticking out 1/4" below the brace on the inside, and hot glue in place. Optionally you can use thermal epoxy to attach a copper pad to the bottom of the cooler, which is something I did as copper is softer than the ceramic of the cooler against the skin and naturally works as an antimicrobial surface. Either way round off each 1/4" protruding edges of the underside of the cooler with hot glue to eliminate any sharp edges that can potentially scratch the wearer.

Next take the phone holder apart by poping out the suction cup section, then glue the ring that is left on around the cooler heatsinks, glue the speaker on in front of it, and then the battery on the back side of both of them. Of note, is that hot glue won't always stick to plastic so if after the glue cools any components start to wiggle loose, use super glue, to glue the plastic parts to the hot glue. Also make sure to use excess hot glue between components as to make a semi-ridged assembly.

Next upload the Arduino program shown below into the micro controller, of which you will need a program load adapter if using the shown Pro Mini (a 5v Trinket Pro is a simpler option FYI). Also per the micro controller fitting so close to the conductive aluminum heatsinks I do recommend coating all the active pins in hot glue so that they don't cross interfere with each other.

Finally, slide the finished lower assembly onto your arm and make sure that everything is secure if you shake your arm up and down rapidly.

int pinOnButton = 10;
int pinRunPeltier = 11; int pinRunFan = 12; int pinStatusLED = 13; int iRun = 0; int iCountRuns = 0; //Two timer control variables int iCountSubs = 0; //for quicker response times void setup() { // put your setup code here, to run once: Serial.begin(9600); // set up Serial library at 9600 bps Serial.println("Debug Mode"); pinMode(pinOnButton, INPUT_PULLUP); pinMode(pinRunFan, OUTPUT); pinMode(pinRunPeltier, OUTPUT); pinMode(pinStatusLED, OUTPUT); iRun = 0; } void loop() { // put your main code here, to run repeatedly: if ((digitalRead(pinOnButton) == LOW) && (iRun == 0)) { iRun = 1; Serial.println("Turning On"); digitalWrite(pinRunFan, HIGH); delay(500); } else if ((digitalRead(pinOnButton) == LOW) && (iRun == 1)) { iRun = 0; Serial.println("Turning Off"); digitalWrite(pinRunFan, LOW); digitalWrite(pinStatusLED, LOW); digitalWrite(pinRunPeltier, LOW); delay(1000); } //5 seconds on 10 seconds off cycle if (iCountSubs iCountSubs += 1; } else if ((iRun == 1) && (iCountRuns iCountSubs = 0; iCountRuns += 1; Serial.println("Power On Pulse"); digitalWrite(pinStatusLED, HIGH); digitalWrite(pinRunPeltier, HIGH); } else if ((iRun == 1) && (iCountRuns iCountSubs = 0; iCountRuns += 1; Serial.println("Power Off Pulse"); digitalWrite(pinStatusLED, LOW); digitalWrite(pinRunPeltier, LOW); } else { iCountRuns = 0; } delay(100); } //End Main Loop

Step 5: Create the Upper Assembly

The Upper assembly is notably simpler as it simply requires hot gluing on the solar panel, main power circuit, and Bluetooth into their respective holes, some X-Acto foam trimming required, and then wiring them all together.

Though you can solder to the control board directly, I have found that it's always far easier to use the power-only USB cable, that comes with the pocket juice, to attach input and output power leads.

Worth noting is that the 5V voltage regulator to keep the solar panel from overvolting the system is the most compact solution, but not necessarily the most efficient. Thus a proper solar charge controller is recommendable if you want to get the most out of the solar panel. Speaking of charging, cut the Mini-B USB port off of the Bluetooth speaker before you wire the solar charger in its place, and then attach it to the frame somewhere you can access it (right under the solar panel is where I put the one shown) and correspondingly wire it to the input of the pocket juice so that the system can be recharged with a standard USB cable.

Step 6: Testing and Finally Assembly

Even if you have already looked at it a dozen times already, once all components are soldered in place, look over the wiring diagram once more as electric wiring mistakes are always easier to correct before you let the smoke out of the components ;)

Next connect all the lower to upper connectors, per the wiring diagram, power every thing on, and verify that all components are working as they should. Also plug in the short USB cable to the USB jack and verify that it is both polarity correct, as not to fry your phone, and that it is also able to charge your phone.

Once verified, power unit back off and place the upper over the lower while tucking the wires inside, and then hot glue the edges of the upper to the battery pack casing and the stem of the phone holder. While not the most permanent adhesive, hot-glue as apposed to epoxy, allows for taking the upper back off, with a bit of wiggling & twisting, to either test different configurations, or adding components, as I found when adding in a subsequent cooling fan. Thus once you test the fully assembled unit for a while and are confident with how it works, going back with epoxy or other permanent adhesive is recommendable.

With assembly complete operate the unit by first turning on the main power button to recharge a phone, and then taping the blue button to activate the cooler and once more to turn it back off. Bluetooth turns on/off independently from the main power button via the power button on the control panel. Solar charging is always active anytime the panel has sufficient sunlight, and the auxiliary power jack works great for overnight charging.

While just an MK1 prototype for a concept I intend to further develop, as I have the energy to do so, I have already found a nitch application for it that is personally a must-have in the form of a superior GPS phone mount when traveling. Able to play my Audible travel mix at boosted volumes with a super convenient control pad; what's truly invaluable is that the phone being mounted to my wrist, allows me to look at my GPS while also looking straight ahead at all times by simply lifting my arm up to eye-level, and pivoting the mount to the desired viewing angle. Thus eliminating the tacky suction rings, sudden pop-offs, and view blocking inconveniences of a windshield suction mount. And of course the "AC" peltier cooler functionality proved to be noticeably "nice" when I turned the actual AC off on a Texas highway in the middle of a cloudless windless day, while in a standstill traffic jam, to put it to the ultimate test. Best of all, rather than arriving to my destination with a phone that was mostly drained, as is the case with other wrist mounts for phones, I arrive with a fully charged one!

Speaking of the core concept of the device to be a personal AC, it should be noted that the popular Wired Article with the headline "MIT wristband could make AC obsolete" and similar reports are misleading as even someone wearing two of these devices in a room heated by summer sunshine to 90degF would still feel quite hot & sweaty. Likewise someone using the concept for the effect of feeling warmer, would still feel quite cold if they did not otherwise bundle up in zero degree weather.

What the core concept can do however, is take advantage of the fact that people are actually "comfortable", without realizing it, in a fairly wide range of temperatures above and below common room temperature. Specifically, the range of temperatures where one is neither sweating nor shivering. So while you won't be able to turn your AC off if you live in Texas, you just might be able to turn it down a few notches, which will actually save on your power bill exponentially. Furthermore, as most people in any given office environment typically prefer the thermostat to be at as many settings as there are people in said office; my testing thus far of the device seems to indicate that a more compact version of the device, with various smart phone features added for further utility, could potentially allow an office thermostat to be set to the economy setting year round and everyone then personally adjust how they "feel".

At any rate further development and testing of the concept is needed to determine just how much these devices can augment the temperature range people feel comfortable in. So if you build and/or test one feel free to drop a comment about what you think, good, bad, or otherwise. Also suggestions for increasing device utility are welcome, of which I should note that incorporating a goTenna Mesh is on the to-do list.


WannaDuino (author)2016-11-27

WannaDuino!!! ,likes iT.

I got some Peltier's also, must look which i got, but this is a nice addition to use it in a very AMAZING IBLE.

put it in 1 of our contests.


jbain6 (author)2016-10-10

ive thought about toying around with a personal peltier cooler. have you looked into stacking 2 of them to boost the cooling effect?

Mechtechnal (author)jbain62016-10-10

Can’t say I have, in fact in this
project I ran the cooler off of 3.7v despite it being rated for 5V as I did not
want it to get too cold as peltiers, if sufficiently boosted, can get cold
enough to cold-burn skin.

Though from what I know you can stack
them to boost the cooling effect but only if you then at least double the
heatsinks on top of the uppermost one, with tripling being recommend to due to
the exponential heat transfer factor.

However, the simplest thing to do for
max cold is to use a more powerful 12V one, with both a massive heatsink and
cooling fan, such as this Adafruit one