LDR Equivalent from an SMD (phototransistor)

Daylight is way more energy per square meter than any artificial light. Daylight is in the range of 1200 watts per square meter. Most rooms are lit to much lower intensities. Your eye is able to compensate and you feel it is almost equally bright but in reality, not even close.

Modern lights (non-incandescent) are tuned to emit in the visible. White LEDs give off very little (if any) UV and do carry into the infrared a slight bit. You should be able to look at a datasheet to get relative intensity vs wavelength. A red LED has a much sharper cutoff than a white LED and emits almost nothing in the Infrared (beyond about 800nm). The wire LEDs have an array of "phosphors" that are really a mix of fluorescent pigments - the fluorescence of some tail off into the IR with broad emissions.

A photo transistor can be better but some suffer from saturation and behave better with a dark period every so often (Even multiple times per second).

If I understood your goal, a better recommendation could be made rather than explaining how the watch works.

It's a pretty simple application, the sensor turns on/off a light and the sensor should react similarly (as best as possible) to natural and non-natural white light sources. In other words be insensitive to infrared and ultraviolet.

This could be suitable?

After a bit more searching,


Good price, right footprint.

You can also have the joy of building your own with a trick used by old Mr Forrest Mimms. An LED will generate a small current when it is receiving light of slightly lower wavelength than it naturally emits (or shorter). So, you can use a Red LED to detect the light and generate a current. Add a 100k resistor across the two leads of the diode to complete the circuit. You'll need an op amp or comparitor to trigger a switch or what ever other interface. If you're worried about UV, you can use a piece of clear polycarbonate as a filter - not much UV passes through UV stabilized PC. Look up Forrest Mimms LED light detector. Plenty of Red LED 0603 chips exist.