Is it permissible to use night lights that are activated by light/dark on Shabbos?

Technically they should only turn on or off when the room becomes dark or light, but opening or closing window shades and sometimes standing directly in front of the light can cause it to react.

It may be used, but you may not do anything which will definitely activate the light, such as closing the shade.

משום דבר שא״מ משא״כ פס״ר.