I like a good visual gag. I also don’t know what tea lights are for. I have never seen anyone use one at all, let alone in a way related to tea. They are so apparently popular though that discount stores often have LED based versions. Frankly the simple small candles are more visually appealing by far, but I suppose if you light a candle with your cup of tea nothing beats the convenience of just flick on an LED... seriously I don’t know why these exist. However, I had an idea; I could make it actually related to tea. The result:
Tea powered tea light. My new (very chunky) coaster is a TEC assembly. I saved this whole assembly from a scrap metal pile and was looking for a use for it. When a cup of tea is placed on it a temperature difference forms across the top plate and the bottom heatsink. The Seeback effect causes a small voltage across the TEC. This voltage is about 0.3v, this is far to low to power an LED. The next challenge is to make a boost converter that can start at such a low voltage. There are commercial ICs available like the LT3108 which claims to start at about 20mv (impressive), however I came up with a simple solution. My first thought was the classic “joule thief” circuit. This is a simple 1 transistor blocking oscillator with an autotransformer usually hand wound on a toroidal core. It is a classic hobby circuit often used to squeeze a little power out an otherwise dead alkaline cell (hence the name).
I found however that this circuit can’t start below ~400mv. This is due to the minimum voltage needed to start to turn on the base of the BJT transistor. I liked the idea of keeping it simple so I decided to make it work. I went on eBay and bought a NOS germanium transistor. These transistors have long been supplanted by the typical silicon transistors of today, but the germanium transistors do have a significantly lower threshold for forward biasing the base. After building a “joule thief” with this transistor, I found it operated down to about 200mv, and sure enough, when paired with my TEC and a hot cup of tea the light came to life. Despite only intending to make an odd art piece, I found the temperature at which the light went out was about the right temperature to drink the tea.
Obviously this looks very clunky, I was thinking of making some nice project case for it, however I decided that I like the contrast of the very mundane discount store tea light connected to the over sized obviously industrial TEC plate.
As an aside, I also tried this with a TEC I bought on Aliexpress. In a similar setup it had far lower output voltage. It took two in series to get near the 200mv to get the light started and it did not stay light long. Given the cheap TECs cost 1/10th what the one in my assembly cost originally you get good value and it is amazing you can buy a TEC at all for that price, but you need to know what you are buying. Aliexpress can be a boon for the electronic hobbyist, but caveat emptor. They are clearly either designed for ultra-low cost above all else, or are B grade factory rejects from the production of other branded TECs.
Overall this is silly, but fun, I consider this the one true “tea” light, since it makes light from tea!