555Timer. - Here is Circuit Schematic Dew Heater Controller For Telescope using LM555. The strip can be fabricated using a Nichrome wire loop of specific length. Nichrome wire is an alloy of nickel, chromium and iron. It is strong, flexible, can withstand very high temperatures and is available in a whole variety of thickness with different resistances. For making the strip, take required length of duct tape and carefully place the pre-cut nichrome wire on its sticky surface. Next, cut another piece of duct tape to cover the nichrome wire loop with it, connect an RCA plug to the loop using suitable wires, and carefully wrap the whole assembly with duct tape one more time. Finally attach an adhesive velcro stripe to the finished Dew Heater strip.
The PWM is designed around one of the most popular ICs on the planet-the 555 timer chip! The 555 (U1) is configured as a single-channel pulse width modulator which runs at a frequency determined by resistors R1, R2, P1 and capacitor C3. The 555′s output directly drives a logic-level MOSFET (Q1) that switches the current through the heater coil connected at the output RCA jack (J2). The 47K Potentiometer (P1) controls the PWM operation and thus the dew heater strip temperature. The Red LED (D3) is a simple power status indicator.
The Original Article: "Dew Heater Controller For Telescope"