This circuit is capable of regulating 12V motors and DC light bulbs. No coolrib required for Q1 or D3 if the current does not exceed 2A. If it does, a sufficient coolrib for both the Schottky Diode (D3) and MosFET Q1 is required.
The regulation is obtained via PWM or Pulse Width Modulation. The output pin 3 of the 555 provides square-wave with an adjustable duty-cycle. What that basically means is that the pulse width changes the speed of the motor. The output from the 555 feeds the mosfet via current limiting resistor R2 of 47 ohms. Because the MosFET Q3 only "switches" and is not behaving like a pot, its energy-waste level is negliable, and also provides the motor more coupling at low rpm. The maximum current Q1 can provide (safely) is about 10A.

link to the circuit page<-------

Medium Power Linear and Switching Applications• Complement to BD136, BD138 and BD140 respectively
datasheet<----

New link in the calculators and converters page abot the hysterisis of an opamp as a comparator.
link here<-----

The circuit has been made for the creation of a symmetrical voltage that would come from a single power supply.
The circuit is providing the possibility the possibility of producing a symmetric voltage as required for some applications. This will always come from the existing single power supply of the circuit which is made possible by the use of IC1 L165. The voltage output is symmetrical in nature compared to the normal output.

The L165 is a monolithic integrated circuit inPentawattpackage, intended for use as poweroperational amplifier in a wide range of applica-tions, including servo amplifiers and power sup-plies. The high gain and high output powercapability provide superiore performance whereveran operational amplifier/power booster combina-tion is required.
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Here are the dimensions for smd marking codes