This circuit of only a few components is powered by two AA batteries and features a mode that extends its power savings.
Imagine a smart light switch which not only automatically turns itself on when it gets dark, but also adjusts its level of illumination to compensate for the ambient level of darkness. For example if this switch were applied to car headlights, the headlights would automatically come on at dusk, and gradually get brighter as the car’s environment gets darker. The circuit shown in Figure 1 performs this function: it automatically adjusts the LED’s output light density in proportion to the ambient lighting conditions as sensed by the LDR.
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| Figure 1. | An automatic light sensitive circuit. |
The circuit employs an ADA4807, low power, low noise, rail-to-rail voltage feedback operational amplifier, an LDR, an LED, and a few passive components. The LDR, or light-dependent-resistor, is the main sensor of the circuit while the op-amp is a square wave generator and also serves as an LED driver because it can typically source or sink a reasonable ±40 mA linear output current. The LED, or light emitting diode, will turn on if there is more than 2 V of forward bias across it. The resistance of a photo-resistor or other LDR varies with light intensity. An LDR’s resistance is several mega-Ohms in darkness, decreasing to a few hundred Ohms in bright light (Figure 2). This allows the circuit to distinguish between direct sunlight and total darkness and everything in between.
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| Figure 2. | An automatic light sensitive circuit. |
Generally, the amplifier is connected as an oscillator; the positive feedback loop and the negative feedback loop causes the amplifier to oscillate automatically. When the capacitor reaches each threshold, the charging source is switched from the negative power supply to the positive power supply, and vice versa.
There is an RC circuit between the inverting input and the output of the comparator. Because of this, the inverting input of the comparator asymptotically approaches the comparator output voltage with a time constant RC. This time constant RC determines the frequency of the oscillation.
The duty cycle of the signal can be adjusted by changing the ratio of resistance of R1 to the resistance of the LDR. If those two resistances are equal the output is symmetrical (50% duty cycle). However, the resistance of the LDR is light sensitive; therefore, variations in ambient light will change its resistance value which directly affects the duty cycle of the oscillation. The frequency of the square wave oscillation is higher than that which human eyes can detect therefore one won’t see the LED flashing on and off. One only observes the brightness of the LED as a function of ambient light density as driven by the duty cycle of the square wave oscillation.
The output of the driver swings as close as 40 mV to the +/- rail voltage with a linear output current source of 50 mA, typically.
For energy savings when the circuit is not in use, it can be put into “hibernation mode” by pulling the DISABLE pin low which will shut down the op-amp within a few hundred nanoseconds. Once the circuit is disabled the remaining system operating current mostly comes from the resistor divider R1 and LDR, and as such, the supply current of the op-amp is reduced from 1 mA to 2 μA. The output enters a high impedance state but it takes less than a half micro second to enable it again.
When active, the op-amp’s power consumption is 3 mW. In hibernate mode, the typical supply current dramatically reduces to 2 μA and the power consumption decreases to 6 μW. This is a ratio in power savings of 500:1. The disable pins make switching between the two modes easy. With an extremely fast turn on and turn off time, there is virtually no waiting time in switching between the two operations.
Figure 3 shows three different stages of lighting conditions. When the environment is well lit, the output is low (0) as shown by the green trace, marked “Bright”, and of course, the LED is off. When the environment begins to darken as shown by the blue trace, marked “Little Dark”, the duty cycle is at about 15% and the LED’s output light is dim. When the environmental is very dark, as shown by the red trace marked “Dark”, the output is on longer than it is off and the LED’s output is very bright.
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| Figure 3. | Waveforms at output with different light densities. |
This type of circuit can be used in many lighting applications. As it exhibits low power consumption, has a rail to rail input/output, and features a useful disable mode, this circuit can be powered by just two AA batteries for use in very power sensitive applications. For higher-power lighting applications, one can add a power transistor for driving a heavier load consisting of an array of LEDs.
