Linear potentiometer provides nonlinear light-intensity control

-April 28, 2005

The human eye's highly nonlinear response to light levels poses problems for designers of adjustable lighting. Simple hardware or software linear-control methods compress most of the apparent intensity variation into a relatively small portion of the adjustment range. A strongly nonlinear control characteristic is necessary. Such a characteristic spreads the intensity adjustment over a wider range and offers a more natural feel. This Design Idea shows how to use an inexpensive linear potentiometer to develop a satisfactory hardware technique. In an experiment in a darkroom, one of the room's corners was too dark because a fixed barrier shielded safe light. Using spare parts from a junk box, you could assemble a simple red LED-based auxiliary safe light, but if the light level were adjustable, you could balance the light levels and minimize the risk of fogging the printing paper. However, the experimenters in this case lacked an audio-taper intensity-control potentiometer and wanted to avoid paying for one.

Figure 1 shows a simplified version of the technique. Diode-connected transistor Q1 and an AD589 1.235V reference, IC1, produce a reference voltage of 1.235V+VBE(Q1) at Node A. Connected between Node A and Q2's emitter, linear potentiometer R2 and resistor R3 cause Q2's emitter and collector current to vary as 1.235V/(R2+R3). The relationship isn't exact because the VBE voltages of Q1 and Q2 vary slightly as you adjust the potentiometer, but, in practice, this nonlinear—if not logarithmic—characteristic works well.

Transistor Q2's collector current generates the control voltage across R4, and, whereas Q2 always operates close to saturation, the components limit Q2's collector-base forward bias to an acceptable 200 mV. When you set R2 to its minimum resistance for maximum light intensity, resistor R3 limits LED current, and, when you set R2 to its maximum resistance for minimum intensity, R1 limits the current through IC1.

The reference voltage produced at Q2's collector drives a standard integrating servoamplifier comprising an AD8031 rail-to-rail op amp, IC2; an IRFD010 low-power MOSFET, Q3; R5; R6; and C2. The servo sets the current through R5 to R4/R5 times the current through R4. Resistor R7 isolates Q3's gate capacitance to prevent load-induced instability in IC2. A 12V-dc module supplies power to the circuit and allows the use of four LEDs per string, for a total voltage drop of approximately 8V across each string. To prevent current hogging and provide a maximum of approximately 20 mA for each series-connected LED string, resistors R8 through R11 divide Q3's drain current into four. Voltage drop across each resistor is 1V, leaving Q3 to support a 3V drain-source voltage and an approximately 250-mW power dissipation. If you increase the number of LEDs or the power-supply voltage, you may need to replace Q3 with a higher dissipation MOSFET.

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