I like drinking tea, however it’s not unusual for me to make the tea then get distracted and forget until 20 minutes later; this results in warm, stewed cup of disappointment instead of the tea I’d intended. To help prevent such tragedies, I’ve made myself a teapot stand that reminds me to pour the tea once it’s sufficiently brewed:
For the top surface I used an old hard drive platter; they are shiny, waterproof and resistant to damage caused by heat or impact. Basing the stand on one will also make it match my coasters (other HDD platters). The only problem is the big hole in the middle; exposing the electronics would both look ugly and subject them to spilt water/tea sooner or later. I found that a 50p is a perfect size to fill this hole, with the 7 corners projecting enough to steady it (plus, if I’m attention-whoring on the internet, adding a nicely embossed picture of the queen to a teapot stand can’t hurt). I lined it up and taped it on using kapton tape. While it is generally reserved for transformer insulation and powder coating masking, I find it very good for general high temperature use as it is essentially sellotape that works up to about 400°C.
I’ve split the circuitry into three stages; the first of which handles the temperature measurements and simply gives out a binary ‘cold?’ value. It is made up of two op-amps, the first of which is configured as an inverting amplifier with a gain of ~1000 and a zero point of ~1.8V. These values are not massively critical, and will depend on the thermocouple used; you can get away with being fairly sloppy here as the circuit only needs to be able to distinguish ~20°C from ~95°C. The second op-amp functions as a comparator, with a potentiometer allowing a suitable threshold to be set by measuring the hat and cold voltages with a meter and choosing a value in between.
The second part of the circuit handles the timing features. The first thing the signal encounters is a simple pulse generator that turns a drop in voltage into a ~0.1s low pulse; this triggers the next step once when a fresh pot of tea is made then waits for the temperature to drop before it resets. The thermal mass of the platter means that it stays hot while pouring the tea so there is no reminder for subsequent cups. This then triggers a 555 configured as a monostable with a period of about 2 minutes; the time here is not massively critical but brewing tea for around 2-3 minutes gives good results (the time that the platter takes to reach trigger temperature also needs to be taken into account, for me this was about 30s). After the delay, the output goes low. I’ve added a pulse generator here, working in the same way as before but with a period of ~1s; this controls the length of the beep after the tea is brewed.
Finally, the last part of circuitry turns the short pulse into a beep; it consists of a 555 in astable configuration with an adjustable frequency in the kHz range. I also had to add a logic gate to buffer the input because the reset pin of a 555 draws enough current to disrupt the operation of the pulse generator. The piezo transducer is connected directly to the output; to get the best sound I tuned the frequency by ear until it hit a nice resonance. I did this by connecting the power to the 555 without connecting the reset input to the NAND gate; this gives a constant tone.
Building the circuit was fairly straightforward; I used a 556 instead of two 555s and a LM324 for the op-amps plus a 4011 for the buffer as it was the first suitable chip to come to hand (any NOT,NAND,NOR would do it). There was a slight hitch when it came to connecting the thermocouple: the particular alloys that are used cannot be easily soldered with standard flux. I didn’t have any appropriate flux about so had to come up with an alternative solution; it turns out that creating a mechanical connection to normal jumper wire by twisting then covering that in solder is messy but workable.
I decided to use USB as the power source; the stand will almost always be used on a desk next to a computer and I didn’t want to have to fiddle with batteries or worry too much about power consumption. I cannibalised a mini-USB lead that came with an old HDD; it was one of the ones with two USB plugs to get around the 500mA limit, I simply cut off the additional power-only lead at the main plug.
I taped a piece of polystyrene foam to the back to prevent any leads piercing tape and shorting onto the platter then super glued it to the back of the coin. Also notice that the transducer wires were too long, so have been cut for resizing later.