LEDs are commonly used for displaying information. Although not used frequently,modern LEDs can also work as photo-voltaic detectors. An LED under reverse bias condition can be modeled as a light dependent current source in parallel with a capacitor. More the incident light, larger is the current source value and that discharges the equivalent capacitor faster. Figure 1 illustrates the use of LED as a sensor. The LED such as Led display,Led manufacturer,Electronic signs is reverse biased, using two pins (pin1 and pin2 in the _gure) of a microcontroller,which charges the equivalent capacitor.
The circuit operates by _rst forward biasing the LED for a _xed period. It then reverse biases the LED, by changing the bit sequences applied to PB0 and PB1. In the next step, PB0 is then recon_gured as an input pin. An internal timing loop is used to measure the time it takes for the LED to change the logic voltage applied to PB0 from logic '1' to logic '0'. This time T, is inversely proportional to the ambient light incident on the LED. The LED is then ashed at a frequency inversely proportional to the time T. Thus for lower light levels, the LED ashes at a lower frequency. As the incident light intensity increases, the LED ashing frequency increases.
The ATTiny15 AVR Microcontroller is an 8-pin device. The circuit presented here uses only 3 out of the 6 I/O pins. The rest of the pins can be used to control external devices or for communication with other devices. Control program for the microcontroller was written in AVR assembly language and is enclosed.
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