Sorry Elon, I stole your flamethrower-name.
A wrist mounted device which helps you stay either cool or warm. The switch on top changes the polarity of the peltier-element inside, so the device either cools or heats you.
How it works:
The bottom of the device houses a peltier-element. When power is applied to this, its surface gets either hot or cold (depending on the polarity). By mounting this at the wrist, we take advantage of the body's circulatory system to spread the heat or cold through the body. The other side of the peltier is connected to an aluminum heatsink with a fan which blows air through it. When in cooling-mode, waste heat will get blown out of the sides of the device. When heating, cold air will be blown out. The box on the rear of the device contains electronics for supplying the appropriate voltages to the peltier-element and the fan, and also has a micro-USB socket.
- Powered by micro USB.You can use common phone chargers, USB power-banks, or even a computer USB-port.
- Low power usage: about 400mA @ 5V. Most powerbanks should be able to power this thing for hours.
- Currently better at cooling than heating. When heating, it's trying to take heat from air at maybe 20C and put it into skin that's about 30C, which is difficult for the peltier-element. When cooling this temperature difference works to our advantage instead.
- Input voltage from Micro USB is 5V.
- Uses an adjustable boost-converter to power a 12V fan. The fan is run at less than rated voltage to keep the fan quiet.
- Uses an adjustable buck-converter to power the peltier-element. The peltier is run at a really low voltage, 1.7V in my case (uses about 1 to 1.5 Watts of power), to keep efficiency high, and to keep the device touch-safe.
Instructions are being made.
Do no supply too much voltage to the peltier-element. The surface-of the peltier element might get too cold or too hot, which can result in frostbite or burns!
Suggestions, improvements or outright forks are welcome. I want this device to be as good as possible.
The "Lower electronics Box" part requires supports; all other parts can be printed without supports, as oriented. There are lots of 45 degree overhangs, and some long bridges, but it came out fine on my printer.
I recommend printing all parts with a brim.
- 40x40x10mm Fan: eBay Search, Example listing
- 40x40x20mm Heatsink: eBay Search, Example listing
- 40x40mm peltier element (TEC1-12303): eBay Search, Example listing
- DPDT switch (2 position): eBay Search, Example listing
- Mini 360 DC-DC buck converter: eBay Search, Example listing
- Micro USB breakout board: eBay Search, Example listing
- Thermal plaster: eBay Search, Example Listing
- Elastic band, 22mm wide: eBay Search, Example listing
- Boost converter: eBay Search, Example listing
This is how the nameless boost converter looks.
There are many different DPDT switches to be found. Mine looks like this.
Micro USB cable, preferably 1m long so you can keep the powerbank in your pocket.
And one of these:
- USB powerbank
- Computer with USB ports
- Regular micro usb phone charger (wall wart)
Active temperature control
An ESP8266, a thermistor and a mosfet can be used to read the peltier temperature, and turn off the heating/cooling when the temperature gets out of safe limits. This can let us put more power through the peltier, and get a larger effect, while still keeping temperatures inside safety limits.
Better thermal interface
The flat peltier against skin does not transfer heat all that well. Perhaps some other materials can be used in between to get better heat transfer.
I have been thinking of incorporating silicone heat sink pads like this into the wristband, to spread the heating/cooling around the wrist area. This could let us run the peltier at higher power levels.
Better wearing comfort
The device is currently pretty bulky. Perhaps a custom powerbank can be made, which houses both batteries and the voltage conversion electronics. This could enable the electronics to be kept in a pocket, while only the main housing needs to be worn on the wrist.
Another option is to save about 9mm of height by using a shorter heatsink (11mm tall instead of 20mm), though this will probably lead to a lower heating/cooling effect.
There exist 7mm thick fans on eBay, so about 3mm of height can be saved by switching to these,
Larger temperature operating range
The peltier element can only create about 10-15C temperature differential in relation to the air temperature. By stacking two peltier elements on top of each other, we can reach higher or lower temperatures at the cost of higher power usage. This requires the "active temperature control" improvement to be implemented first, since we can more easily reach unsafe temperatures with two peltier elements.
- Airflow control: Something to guide the exhaust air away from the body.
Attach heat sink to peltier element
The first thing you should do is to attach the heat-sink to the peltier-element. Use the thermal plaster for this, and let it set overnight. Make sure you keep the peltier-element and heat sink aligned, as misalignment will make it hard to insert the these two parts into the main housing.
Note the direction of the fins of the heatsink in relation to where the wires of the peltier-element come out. The fins need to point out to the sides like this, so that air can flow through the two vent holes in the main housing.
The thermal plaster is a pretty strong glue when spread across two large surfaces like this; the only way you are getting the peltier-element off the heatsink again is with a hammer and chisel.
Add fan to main housing
Place the fan into the main housing, there should only be one way that this will fit. Take note of the groove for guiding the wire from the fan out of the main housing.
Add peltier-element and heat sink to main housing
Slide the heatsink and peltier-element into the main housing. There should be some resistance, but not too much. If things stop sliding at the peltier-element, be very careful, as the ceramic peltier-element might crack. If this happens it will break the circuit inside, and the peltier element will just be a useless slab of ceramic. If it's hard to insert the peltier element, it's better to take it out and use a knife to cut some plastic off the main housing to make space.
Mount lower electronics box
Slide the "lower electronics box" onto the locking slide at the back of the main housing. Keep the wires in the correct slots while doing this (wires not pictured). Solder some wires to the + and - pads on the micro USB breakout board, then glue it in place at the bottom of the electronics box.
Mount buck converter
Fit the Mini-360 buck converter into the grooves above the micro USB breakout board. Glue it in place.
Add boost converter and DPDT-switch
Solder the boost converter and DPDT-switch into the circuit, then place them somewhat like in the picture.
Mount upper electronics box
Slide the upper electronics box onto the locking slides, like with the lower part of the box. Make sure the DPDT-switch goes into its slot, and the same with the blue boost converter. This part is pretty finicky. If you are unable to get the upper electronics box all the way in place, there is probably a wire caught between the upper and lower box somewhere.
There is nothing explicitly holding the DPDT-switch up in place, however the pressure from all the wires and stuff inside the box keept it firmly in place when i built mine.
I used a hot-glue gun to put a tiny dab of glue between the two boxes. Otherwise there was a small air-gap, as there is nothing mechanically holding the two box parts together.
- Solder the + and - from the micro USB board to the inputs of both DC-converters.
- At this point you should use a multimeter to check and adjust their outputs before connecting anything else! The blue boost converter should be set to output about 10V for now. You can adjust this voltage as you like later, depending on what noise-level you can tolerate. The tiny green buck converter should be set to output about 1.5-1.7V. Do not go above this voltage, as the peltier might cause heat/cold damage to your skin.
- Solder the fan wires directly to the outputs of the blue boost converter. Use the correct polarity.
- Solder the outputs from the green buck converter to two pins on the DPDT-switch. Solder the peltier to two other pins, and add some extra wires so that the switch reverses the polarity of the peltier when switched (TODO: Explain).