AN442

Rev. 0.1

13

3.6. LED Parameters and Multiple LED Operation

Best performance is generally achieved with the highest output efficiency 850 nm LEDs. However, 940 nm LEDs

can also work. Red LEDs have reduced range but may be suitable in applications where it is desireable to have a

visible proximity indicator. White LEDs, which convert blue to white, are not recommended because of the long rise

and decay time constant of the phosphors used to broaden or whiten the wavelength.

The Si1120 can be used to drive multiple LEDs in conjunction with microcontroller GPIO pins to control multiplexing

switches to steer the TXO driver to multiple LEDs. With multiple LEDs, a wide variety of enhanced applications

become possible.

For example the proximity sensor may be placed on the front of a motorized toy with an LED on each side to allow

the toy to sense whether an obstacle is to its left or right and steer away from it or, conversely, to allow a robot doll

head to track an object within proximity range by turning its head to keep both reflections at equal amplitude.

For human interfaces, multiple appropriately located and lensed LEDs can be used to determine hand gestures

laterally, vertically, inward, and outward by measuring reflectance and applying suitable algorithms. Simple

examples include using hand motion to control the dimming on-off function of a light switch, control picture viewing

in an electronic picture frame, or page through documents on e-readers. The proximity sensor can also turn on the

picture frame when a viewer's motion is detected nearby and turn it off to save power after the viewer leaves.

3.7. Motion Detection

When used with a microcontroller, the Si1120 can be used to implement a simple motion detection system. The

simplest motion detection system requires only a single LED. However, if it is desired to cover a larger area, it may

be necessary to have multiple LEDs, each targeting a different direction. The number of LEDs used in such a

motion detection system is dependent on the angle coverage. For example, it will take approximately four to five

20° half-angle LEDs to cover 180°.

For motion systems, motion sensitivity is a function of the time between samples. Rapid sampling rates will have

less difference between adjacent samples. Typically, for human motion detection, sample periods can be anywhere

from 10 ms to 1 second between samples. If higher sampling rates are used to integrate to improve signal-to-

noise, moving averages delayed by at least 1 s are usually required. Of course, motion systems can indicate

motion towards or away from, which is sometimes useful.

When using motion detection algorithms that detect a very small percentage change in reflection, it is important

that the LED driver not saturate but stay in constant current mode in order to minimize the effects of power supply

fluctuations on LED TX current, which may cause spurious motion detection. A few percent variation in LED

current from pulse to pulse is not critical. These fluctuations can occur if the LED driver is saturated due to a high

LED drop or a series current-limiting resistor and the power supply voltage is changing due to other loads or even

the LED driver. The 400 mA LED driver is designed to be connected to an independent voltage supply up to 7 V in

order to prevent overloading of the regulated Vdd supply. Often, the LED anode supply may have a significant

voltage ripple because it is an unregulated battery or input supply with significant wire drop load ripple, etc.

Imagine that we have a simple LED circuit with only a resistor and an LED with a 3 V supply. In this case, there is

nominally a 2 V constant voltage drop across the LED with only a 1 V drop across the resistor. When there is a

100 mV change on the 3 V supply, this 100 mV change appears primarily across the resistor because the 2 V drop

across the resistor is generally constant, due to the nature of diodes. A 100 mV drop, given an initial resistor

voltage of 1 V, translates directly to a 10% drop in current. A 10% drop in current implies a 10% change in total

reflectance, and this can easily be 10 times more than the motion change threshold.

In order to prevent LED supply modulation, the Si1120 LED TXO driver has been designed to have very high

constant current impedance. The 400 mA (or 50 mA) internal current limit has less than a 1% per volt change in

current.

Consequently, a 100 mV change in LED anode supply will cause less than a 0.1% change in LED

intensity, which is much more desirable for sensitive motion detection algorithms.

Another source of spurious motion detection can arise from dc ambient modulation of the receive gain. On the

Si1120, the photodiode and analog front end may have a 1% change in gain over the extremes or dc ambient. This

might happen if, in direct sunlight, a shadow falls across the IC receiver. Obviously, if the reflectance is already very

high, a 1% change may be more than the motion detection threshold. Again, this is a reason to minimize stray

optical coupling for motion detection to keep the ambient gain modulation below the minimum detection threshold.