Color Sensor from a Reversed LED and Op Amp

Posted on Feb 5, 2014

One of the first lessons that an electronics student learns is that an LED provides light from current flow. But, did you know that an LED put in backwards provides current flow from light Yes! It`s true. Hook up a high-quality ultra-bright red LED by itself (no battery or other circuitry) to a multimeter in voltage measurement mode. Put the LEDagainst a light source, such as a desk lamp.

Color Sensor from a Reversed LED and Op Amp
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See the voltage Now, hide the LED in a dark place. See a decrease in voltage Photodiodes are used in robots and devices as light sensors. Photodiodes have a spectrum wavelength to which they are most sensitive, usually infrared. But, not surprisingly, a reversed LED is most sensitive to the same color of visible light as it normally emits. For example, if a circuit uses a reversed green LED, the most current will flow from exposure to green light. Unfortunately, even under the best conditions, photodiodes (and reversed LEDs) don`t provide a lot of current flow. The output of the photodiode needs to be amplified for the light-detection signal to be useful in most circuits. A photodiode amplified by a built-in transistor is called a phototransistor. You can connect a standalone photodiode to the input of a standalone transistor. But, it isn`t easy to control the gain of a single-transistor amplifier, and there are issues with signal noise and the amount of input current required. Instead, a better method for amplifying low-power signals in a high-quality repeatable way is an op amp chip (operational amplifier). Putting this all together - a color sensor can be made from a reversed LED and an op amp chip. In fact, TAOS did just that with their TSLR257 (red), TSLG257 (green), and TSLB257 (blue) sensors. LED1: Normally an LED has the diode arrow pointed down toward ground because conventional current flows that way. But, this...

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