TSL235R

LIGHT-TO-FREQUENCY CONVERTER

TAOS038E −SEPTEMBER 2007

5

The LUMENOLOGY r Company

r

r

Copyright E 2007, TAOS Inc.

www.taosinc.com

APPLICATION INFORMATION

Power-supply considerations

Power-supply lines must be decoupled by a 0.01-μF to 0.1-μF capacitor with short leads placed close to the

TSL235R (Figure 6).

Output interface

The output of the device is designed to drive a standard TTL or CMOS logic input over short distances. If lines

greater than 12 inches are used on the output, a buffer or line driver is recommended.

Measuring the frequency

The choice of interface and measurement technique depends on the desired resolution and data-acquisition

rate. For maximum data-acquisition rate, period-measurement techniques are used.

Period measurement requires the use of a fast reference clock with available resolution directly related to

reference-clock rate. The technique is employed to measure rapidly varying light levels or to make a fast

measurement of a constant light source.

Maximum resolution and accuracy may be obtained using frequency-measurement, pulse-accumulation,

or integration techniques. Frequency measurements provide the added benefit of averaging out random- or

high-frequency variations (jitter) resulting from noise in the light signal. Resolution is limited mainly by available

counter registers and allowable measurement time. Frequency measurement is well suited for slowly varying

or constant light levels and for reading average light levels over short periods of time. Integration, the

accumulation of pulses over a very long period of time, can be used to measure exposure — the amount of light

present in an area over a given time period.

TSL235R

Timer / Port

MCU

0.1 μF

2

3

1

Figure 6. Typical TSL235R Interface to a Microcontroller