VR1: 10K Potentiometer Function as the motor speed control.
R9: Resistor as a determinant of minimum speed. Diseri with 10K VR, R1 for 1K will provide the settings range from 0 - 100% better used if the load used is a motor or a lamp. If R1 for 10K, will provide the range 5V - 12V is suitable if the load used is the cooling fan.
D1: Diode is used to prevent back-emf which usually occurs in the load inductor such as Fan or Motor. Back-emf can damage the transistor!
Description IC The LM124 Low Power Quad Operational Amplifier
The LM124 series consists of four independent, high gain, internally frequency compensated operational amplifiers which were designed specifically to operate from a single power supply over a wide range of voltages. Operation from split power supplies is also possible and the low power supply current drain is independent of the magnitude of the power supply voltage.
Application areas include transducer amplifiers, DC gain blocks and all the conventional op amp circuits which now can be more easily implemented in single power supply systems. For example, the LM124 series can be directly operated off of the standard +5V power supply voltage which is used in digital systems and will easily provide the required interface electronics without requiring the additional ±15V power supplies.
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