The DS1620, made by Dallas Semiconductor, is an integrated temperature sensor and thermostat. The chip measures its own temperature, sends out its reading through a digital interface, and gives off a signal when the temperature reaches a certain pre-set value.

Here it is in a typical application: measuring the temperature of a computer's processor. You can see the processor socket in the background (it's for a Pentium Pro, by the way), and the DS1620 is just situated right below the CPU's hot spot. This enables the motherboard to respond when the processor gets too hot, for example by activating a fan.

This is a composite image of the chip's internals. On the left, we find an area with memory cells. You can program this chip to respond to certain temperature levels, and these values are then stored in this bit of memory.

The centre and the top are taken up with digital circuits. This includes a D/A converter and all the stuff necessary to send and receive digital data.

The bottom part includes all the analogue circuitry. This is the stuff that actually measures the temperature and makes it into a nice, stable value to be processed by the rest of the chip.

A close-up of some of the analogue circuits. This is obviously four copies of the same circuit. It might be an amplifier or something. The little round things are transistors. Bipolar transistors are often made with round emitters, for a variety of reasons.

A cute little piece of chip-art. It speaks for itself, really.

There's even a little ad for Dallas's security chip business! There's also a long, winding resistor made of polysilicon in the background. 

And these are the guys/girls who've made it all possible.