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Maxim > Design Support > Technical Documents > Application Notes > Miscellaneous Circuits > APP 1909
Keywords: tiny, light, sensor, logic output, Schmitt trigger, inverter, backlight, logic gate
APPLICATION NOTE 1909
Tiny Light Sensor with Logic Output Draws Less
Than 10µA
Apr 06, 1998
Abstract: This application note showcases how a low-power light sensor with logic output can be an
improved design for an automatic backlight sensor in portable instruments, compared to approaches that
use a logic gate or Schmitt-trigger inverter.
A light-sensing circuit that consumes very little power can serve as an automatic backlight sensor in
portable instruments. This function is easily implemented with a logic gate or Schmitt-trigger inverter, but
those approaches draw a considerable amount of supply current. The circuit IC1 (Figure 1) offers a
different—and better—solution.
Figure 1. This light sensor provides a low-to-high output transition at a light level determined by the
value of R1.
A logarithmic graph of supply current vs. supply voltage (Figure 2) illustrates a comparison. As expected
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