The moment I discovered Peltier units I was amazed. First it was at the fact of how cold something could get just by applying a current. As I extended my research, my focus turned to the other effect – or SeeBeck Effect – as it’s called, which is the process or generating a current or voltage through temperature differentials. I know this technology has been used for some time, but the average person is not aware of what electricity, or energy for that matter, really is. Every time I show someone what the peltier unit is able to do just by putting my hand on it, they are amazed. I wanted to design something that would be able to find wasted heat energy from my body and transform it into usable electrical energy. A very simple proof of concept.
I naturally run hot. My body temperature is usually about a degree warmer than anyone else. You can ask my girlfriend, who frequently uses me as a heater. I started to wonder how many more extra calories I was burning during the course of a day, just because of this fact of being overall more hot; from there I thought about how much of that was just being wasted. I wear a ring on my hand, one day I picked it up off my hand and realized how hot it was to the touch. From there the idea for the ring began to build. What if I could create a self contained circuit with an LED that would run (or blink) whenever enough charge was built up? I wasn’t thinking flashlight or any of that sort. Just a very faint blink to create a proof of concept and be able to explain that the led will basically run for as long as I’m alive and hot(in a cold room haha).
I first started by playing with a few cheap peltier units, measuring voltages I could get with the palm of my hand, and the other side on a heat sink. I was amazed to find that when the heat sink was room temperature I was about to get between 0.15V to 0.3V for quite a long time. Keep in mind this was a rather large peltier module, and I was in a air conditioned room.
Now let me just say, I am in no way an electrical engineer. Before this project I had never designed a PCB ever. With that said, the next step for me was to look into voltage upscaling. How could I get a few volts from < 0.3V? Is this even possible? Am I asking too much?
Through a search on SparkFun(I always start here) I looked for something that was possible of boosting that much voltage. I came across a Lipower Boost Converter which stated it could take as little as 0.3V to upscale to 5V. I was amazed that this existed and was glad I didn’t have to make my own circuit, I could just (hopefully) rework this one to fit on a ring. When it finally arrived, I tested it countless times, but I could never get it to work with the peltier. I believe it was because it had a higher start up voltage. Non the less, I could use it for many of the li-po batteries I had, but this meant my search would continue further than my favourite electronics supplier.
As I was searching, a new term was starting to flow in and out of my browser, “energy harvesting”. It began the basis of my searches. I finally found something amazing and perfect for what I needed for my circuit; it was the Linear Technology 3108 Ultralow Voltage Step-up converter and power Manager. It was tiny and inexpensive, but there was one small problem. The chip was surface mount, and though I am just fine soldering just about anything through hole; at this time I had not yet mastered surface mount soldering. I figured this would be a good place to start as any! I ordered a couple of the SSOP16 versions, some adapter boards for SSOP16 and a week or so later they arrived. I also had been going over the data sheet for the Linear 3108 IC and looking at various circuits, from which I really began to understand what was required to make what I needed. I have had classes in electrical type engineering, but it was never the direct focus, and more so on programming. Everything in the circuit I had ordered except for one. I had no idea what the 1:100 coil was or where to get one. I understood that it was a 1:100 windings coil, but to find one to order online was proving difficult. That was however until I read the small print in the datasheet for the 3108 (doh!). It specifically said that its a Coilcraft 1:100 coil. I found the Coilcraft website and ordered a couple(after realizing they had samples, so I got the samples also and they arrived the same day haha).
It was now time for me to begin my adventures in SMD soldering. I ordered all the components in SMD as well just for the practice. I decided to just make a 2.35V circuit with a simple output. I started with soldering the SSOP16 3108 to the adapter board. I was amazed at how it turned out for the first time.
Next I started working on the circuit. Slowly adding components, and soon afterward I had a circuit completed. The circuit had a button and an led that you could press when enough voltage was present; it was kind of like a check button. Button has since broken off and the bottom LED is not connect, who knows why. One side you add the input voltage, and the other is the 2.35V
It was the moment of truth, I hooked up the peltier unit to my first homemade circuit build from using a data sheet. Hooked up my voltmeter to the output pins and put my hand on the peltier unit. I was watching the voltage jump around. From 0V to 0.3V to 0.1V to 0.5V etc.. it seemed to be oscillating up a little then down a little then up a little higher and so on. Finally it got to 2V and suddenly jumped to 2.35V and was rock steady. I couldn’t believe it. It was actually working like it was supposed to, and I didn’t let the magical smoke out of anything. I hit the button on the circuit and the LED does one blink and goes very dim. The initial voltage made the LED fully glow, and then the current draw from it drops the voltage down to very little. Once the LED is off the voltage takes very little time to jump back up to 2.35V.
So the circuit was done, the concept worked. Now how can I make the led blink forever after building up enough juice? I discovered a pin called PGD which turned high(2.2V) when the voltage on the Vout was within 7% of the desired voltage. This was perfect for a transistor. I figured that as the voltage built up it would get to that 7%, fire the transistor making the led glow for a second, which would make the voltage drop and make the transistor turn off to repeat the cycle. I soldered a transistor I had into my circuit and… It didn’t work, not sure why but everything stopped working. I think it may have constantly been drawing power not allowing anything to charge up. I thought it was broken for good, but I removed it and all was better. I decided maybe my messy circuit was best left in working order as a first prototype so I decided to build the first PCB in Fritzing with the ability to remove the Linear 3108 in case I fried one.(I later realized I did not implement a resistor on the transistor).
The first PCB designed in Frizting was very simple, but I decided to make it slightly adjustable as to make it any output (2.35V, 3.3V, 4.25V and 5V) you wanted. It also had the built in transistor and two LEDs, one to signify voltage present and the other to show full voltage from the supercap. The coilcraft coil had to rig up a couple LED solder pads to make it work, and I was hoping it would…This is what I came up with.
I ordered these through OSHPark.com and they arrived in very little time, the cost was great! Purple boards were cool, and the best part; I soldered them together and they worked flawlessly, seemed to charge up even better than my prototype.
Now that I had a real working concept(not for a ring but for just pure electronics end) I decided to shrink the board even further. Could I do it? After some reworking and realizing that OSHpark will do a way smaller trace size! This time I ended up with a board that is 33mm x 13mm. Tiny! At least I thought so. I also upgraded some components such as a few capacitor that I found smaller versions of, and a real super cap that I should be able to squeeze onto the board.
The next article will have the ring design and further development in some very tiny/expensive/powerful TEG peltier units. This project is coming along pretty nicely.