A light-sensing circuit that consumes very little power can serve as an automatic backlight sensor in portable instruments (Fig. 1). This function is easily implemented with a logic gate or Schmitt-trigger inverter, but those approaches draw considerably more supply current.
A logarithmic graph of supply current versus supply voltage illustrates a comparison (Fig. 2). As one expects for CMOS circuits, the 74HC inverter and 74HC14 Schmitt-trigger inverter draw very little current (less than 1 µA) when their inputs are near the supply rails. Near mid-scale, however, the 74HC04 at 5 V draws more than 10 mA! The 74HC14 is better, but still draws more than 0.5 mA at mid-scale. These currents are a problem because the mid-scale condition in a light-sensing circuit can persist for a long time.
Even though 3-V power supplies lower the supply current by an approximate factor of three, the current is still significant. Adding hysteresis also helps somewhat, but there will remain a point just above or below the switching threshold at which these CMOS devices draw excessive class-A supply currents.
The lowest curve, representing the supply current for IC1 of Figure 1, varies only slightly over the signal range and never exceeds 7 µA. The external light sensor and bias resistor draw a maximum supply current of 3 µA with a 5-v supply. Therefore, the circuit’s total supply current, independent of light level, is less than 10 µA. Unlike the other approaches, this circuit compares the light level (represented by a voltage on resistor R1) with a fixed reference voltage rather than a loosely specified logic-switching threshold.
Supply voltage can range from 2.5 to 11 V, with supply current measuring several microamperes at 11 V. IC1 comes in an open-drain version (MAX836) whose output (tied to a pull-up resistor) can exceed the supply voltage in a mixed-voltage system. If minimum power consumption is more important than size, choose the MAX931 comparator/reference IC. It comes in a shrink SO-8 package called µMAX (versus the MAX837 SOT package), but its maximum supply current is only 3 µA.
