Why measure the temperature of the coffee? Well, coffee taste depends on two things. First, how strong the coffee is and second, how hot it is. The second criterium is quite easy since most of the coffee drinkers prefer to have it hot, that is, around 80 degrees centigrade. Since not everybody has a built-in thermometer in his tongue, the circuit featured here is very useful to save you from burning your tongue.
The coffee thermometer is composed of a voltage regulator, a temperature to voltage converter, a comparator and two LEDs. Roughly described, the circuit works this way: if the coffee temperature is not hot enough, the IC output is logical 0 and T1 does not conduct current. Transistor T2 at this time conducts and the red LED lights up. If the temperature is hot enough, the green LED lights up. IC2 measures the temperature and converts it into a voltage value. Because of this, IC2 must be dipped into the coffee. IC2 can also be installed inside an empty pen wich can serve as a probe.
The output of IC2 increases in proportion to the temperature by 10 mV per degree. Therefore, if you feel that by 800C the taste of the coffee is at its best, you must set the reference temperature at the minus input of the IC3 to 800 mV through P1. When the voltage level at the plus input of IC3 reaches 800 mV, the comparator output swings to logic 1 and T1 conduct currents. At this time T2 turns off and the red LED is off. Otherwise, the green LED lights up showing that the coffee tastes good. There are two PCB layout for this coffee thermometer circuit. Choose any of the two.