This idea demonstrates three uses for the humble LED. The circuit in Figure 1a forms a simple light detector that latches and turns on an LED when the ambient light exceeds a preset limit determined by potentiometer P₁. LED D₁ is both the indicator and the light detector. All junction diodes exhibit some degree of photosensitivity. Light-emitting diodes are photovoltaic in that they generate a small voltage in response to light with suitable spectral content. Provided that they have light loads, some LEDs can generate more than 1V in adequate light conditions. The Q₂-Q₃ differential pair acts as a comparator. At low light levels, the photovoltage that D₁ generates is lower than Q₃'s base voltage, V_B3, and Q₁ and Q₂ are both off. When the light falling on D₁ exceeds the threshold that P₁ sets, Q₂ begins to conduct, thereby biasing the second LED, D₂, which acts as a voltage reference (the third function of an LED). Q₁ now turns on, sourcing current to D₁, which illuminates.

Regenerative action around Q₁ and Q₂ ensures that the circuit makes a rapid transition into the latched state. The circuit stays latched, and D₁ remains illuminated even if the light level falls below the trip point. You can reset the circuit by short-circuiting D₁. Because Q₁ is off in the unlatched state, D₁'s only load is R₂ and Q₂'s base current. Provided that Q₂ has adequate beta and R₃ is fairly large, the loading is negligible. You can use R₂ and C₁ to reduce D₁'s sensitivity and provide a degree of filtering. You may need such filtering in noisy environments, such as monitoring the light from fluorescent tubes. With adequate forward current, most green LEDs drop approximately 1.7V, so, in the latched state, approximately 1V appears across R₁. A value of approximately 330Ω sources 3 mA into D₁, providing adequate brightness. For obvious reasons, you should not expose the voltage-reference LED, D₂, to light. If necessary, you can omit D₂ by modifying the circuit (Figure 1b). For battery-powered applications, the values of R₄, R₅ , and P₁ should be fairly large to ensure minimal current drain in the unlatched condition. If you need a broad threshold range, you should select the values to provide a range of V_B3 from approximately 0.6 to 1.4V.

You need to experiment to find the optimum LED to act as photodetector. Tests on more than 50 LED color samples produced a range of unloaded output voltage varying from a low of 135 mV to more than 1V in overcast sunshine. A green LED with a clear lens produces almost 1.5V on a dull afternoon. The circuit can work from supply voltages as low as 3.3V, guaranteeing operation from three cells. In the off state, current drain is minimal. The circuit in Figure 1a consumed just 88 µA in the unlatched state. Although simple and effective, the circuit suffers the disadvantage that its trip point varies with changes in supply voltage. Also, the trip point's lower limit cannot be less than the 600 mV or so to bias Q₁ and Q₂. The circuit in Figure 2a remedies these problems. The circuit uses IC₁'s internal bandgap reference to generate a stable threshold. Because the MAX921 (www.maxim-ic.com) comparator's input-voltage range includes the negative rail, you can set the trip point at 0 to 1.18V (the value of the internal reference), thus allowing D₁ to work over a wider range of light levels.

Power requirements are minuscule. The circuit operates from supplies as low as 2.5V (the MAX921's minimum supply voltage), and, in the off state, the only current drain comes from P₁ and IC₁'s quiescent supply current. The prototype's total off-state current was just 7.8 µA. You can use the comparator's rail-to-rail output voltage at V_OUT as a digital signal and feed it to other systems to indicate light conditions. The comparator output can source more than 10 mA with little reduction in high-level voltage, thus allowing you to slightly simplify the circuit (Figure 2b). However, in this version, you must accommodate the voltage drop across D₂, so the minimum supply voltage becomes approximately 3V. Alternative devices you can consider for IC₁ include the Linear Technology (www.linear-tech.com) LTC1440 and the Telcom (www.telcom-semi.com) TC1031.

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