This is a very simple and easy to make temperature controller and heater to be attached to a xtal (crystal). The xtal is normally used to clock a microcontroller (PIC or ATMEL etc).
Normally xtals provide an accurate clock to 50 or 100 PPM (parts per million) making them useful for real-time clocks, like in your wristwatch. However their frequency output changes with temperature.
This circuit keeps the xtal at a constant temperature – commonly called a “xtal oven” also called an “Oven Controlled Xtal Oscillator” or “OCXO”. Hence the xtal error is reduced to 1 – 10 PPM and the clock will keep almost perfect time.
This circuit can be built with very common cheap parts and means you don’t have to find or buy an expensive xtal oven.
Coupled with my 1-sec PIC timer algorithm HERE you can use ANY value of xtal to build a very high accuracy clock.
Via the comments. Thanks Nerd Ralph!