The adjustment control for the contrast of an LC-Display is typically a 10-k potentiometer. This works fine, provided that the power supply voltage is constant. If this is not the case (for example, with a battery power supply) then the potentiometer has to be repeatedly adjusted. Very awkward, in other words. The circuit described here offers a solution for this problem. The aforementioned potentiometer is intended to maintain a constant current from the contrast connection (usually pin 3 or Vo) to ground. A popular green display with 2×16 characters ‘supplies’ about 200 μA. At a power supply voltage of 5 V there is also an additional current of 500 μA in the potentiometer itself.
Not very energy efficient either. Now there is an IC, the LM334, which, with the aid of one resistor, can be made into a constant current source. The circuit presented here ensures that there is a current of 200 μA to ground, independent of the power supply voltage. By substituting a 2.2-kΩ potentiometer for R1, the current can be adjusted as desired. The value of R1 can be calculated as follows:
R1 = 227×10-6 x T / I.
Where T is the temperature in Kelvin and I is the current in ampères. In our case this results in: R1 = 227×10-6 x 293 / (200×10-6) R1 = 333 Ω Note that the current supplied by the LM334 depends on the temperature. This is also true for the current from the display, but it is not strictly necessary to have a linear relationship between these two. Temperature variations of up to 10° will not be a problem however. This circuit results in a power saving of over 25% with an LCD that itself draws a current of 1.2 mA. In a battery powered application this is definitely worth the effort! In addition, the contrast does not need to be adjusted as the battery voltage reduces. When used with LCDs with new technologies such as OLED and PLED it is advisable to carefully test the circuit first to determine if it can be used to adjust the brightness.