MULTI-FUNCTION LED

Publishing Venue

Motorola

Related People

Abstract

A single LED is shown here configurable for any of several dual purpose applications. Traditionally, an LED or an LED array is used only as an optical transmission device. All of the following applications are a result of the ability of the LED to perform bidirectionally. It can convert photons to electrical current as well as its intended mode of light pro- duction. The test circuitry used for light detection is straightforward as depicted in figure 1. The LED drives an OP AMP in a current to voltage mode. Then an OP AMP is used as a voltage gain stage. Table #1 shows the circuit output voltage obtained while the LED was exposed to three different ambient lighting conditions.

m MOTOROLA Technical Developments March 1990

MULTI-FUNCTION LED

by Stefan Peana, David Winfree and Daniel Peana

Introduction:

   A single LED is shown here configurable for any of several dual purpose applications. Traditionally, an LED or an LED array is used only as an optical transmission device. All of the following applications are a result of the ability of the LED to perform bidirectionally. It can convert photons to electrical current as well as its intended mode of light pro- duction. The test circuitry used for light detection is straightforward as depicted in figure 1. The LED drives an OP AMP in a current to voltage mode. Then an OP AMP is used as a voltage gain stage. Table #1 shows the circuit output voltage obtained while the LED was exposed to three different ambient lighting conditions.

Proposed Applications:

Use of the LED as alert light and reset switch.

When a message is received by the pager, the alert light is activated along with other alert modes such as tone or vibrator. The LED is flashing intermittently for a predetermined period of time until the user resets the pager. To reset the pager, a separate switch must be activated. The LED can be used as a switch if the LED is used as a sensor during the nonflashing state (see figure 2). When the LED is pulsed the ambient light sens- ing take place during the off duty cycle. The first sensing value is used as the reference and compared with the value obtained when the LED is covered by the finger. Use of the LED as a switch is not restricted only to the reset switch.

Use of the LED as alert light and ambient light sensor.

This approach is similar to the one described above in which the sensed ambient light value is used to deter- mine the required level of illumination for the LCD backlighting. Once the alert light is off the LED becomes a sensor. The ambient light level is continuously measured and the illumination level is adjusted accordingly.

Use of the LED as backlight...