The simplest lamp dimmer circuit one can think of is to add a potentiometer in series with the lamp and adjust it to the required brightness. Such types of linear regulators are awfully inefficient and lot of power is wasted across the potentiometer (and quite often a high wattage type is required), which can be deleterious to the batteries powering them. Moreover, in such potentiometers, the contacts are likely to get damaged on the long run, when the value is adjusted by moving the slider. Such kind of linear controls give the circuit an overall efficiency of less than or equal to only a mere 50%. This wastage of power can be avoided and hence an increase in efficiency can be achieved if one uses pulse width modulation which can be made to control an electronic potentiometer. The following circuit describes such a principle.

Gate A1 and its associated components constitute an oscillator producing approx. 200Hz with a pulse width of approx. 0.1mSec. This is fed to a transistor T1 for level shifting. At the output of this transistor is a potentiometer with which a DC component can be added along with the pulses emerging from T1. By adjusting this potentiometer/trimmer, one can have a good linear control of the lamp brightness adjustment from fully OFF to 100% ON. The signal is inverted by gate A2 and fed to the MOSFET.

The gates A1 and A2 are from a 74HC14 integrated circuit. This IC provides 6 inverting buffers with schmitt trigger action. They are capable of transforming slowly changing input signals into sharply defined jitter free output signals. They are usually used as wave and pulse shapers, astable as well as monostable multivibrators. The 74HC14 possesses high immunity and low power consumption of standard CMOS IC’s along with the ability to drive 10 LS-TTL loads. The quiescent current drain of this chip is extremely low amounting to approx. » 40m A.

In the accompanying circuit, loads upto 24W can be connected to the MOSFET source and +12V_cc without using a heatsink. The loads can even be DC motors, miniature heating elements, etc. If one uses a low R_{DS (ON)} MOSFET, a higher efficiency can be achieved. By using the following components, an efficiency of approx. 96% was attained. The flexibility of the design makes it possible to change MOSFET with similar ones in case of non-availability in certain regions. The circuit by itself does not draw much current when the load is disconnected. Ensure proper ESD protection while handling the MOSFET to prevent partial as well as full damage.