This hack is so simple that, if there were a Book of Hacks, this one would be found at the top of the first page of Chapter 1, Volume 1. Infrared (IR) is electromagnetic radiation with a frequency lower than red light (infra means below) and higher than microwaves. Because infrared is not visible to the human eye and can pass through opaque objects, devices that utilize it can be difficult to troubleshoot. Moreover IR can be a health hazard because it won't trigger the eye's blink reflex. Many nightlights use a photosensitive circuit element to automatically switch a light source on or off when the ambient light level falls below or rises above a certain threshold. These photosensors are often still sensitive well outside the range of visible wavelengths which means they can be used to detect some types of IR.
The video below demonstrates the use of an inexpensive nightlight to detect the IR emitted from a remote control. The first part shows how to use the nightlight as a detector even if the ambient light level is high by simply putting the remote very close such that it shields the photosensor from the room's lights. Notice that the nightlight turns off when IR is detected. The second part demonstrates the nightlight's response when the room's lights are off and the distance of the IR source is varied. If you watch closely you'll see the camera detect some reflected infrared a couple times. You might be wondering "Why bother with a nightlight if a camera can detect IR?" Well, the nightlight has the advantage of being inexpensive, plus there are some cases where using the nightlight will be quicker and easier.
The next video shows the nightlight's response when it is irradiated by an IR jammer. The purple light captured by the camera is not visible to the naked eye. Notice how the nightlight doesn't flicker when lit by the IR jammer like it does when lit by the remote control. The flicker is a result of the remote control turning off it's infrared LED while it pauses before resending a code.
Placing an IR pass filter over the photosensor allows the nightlight to be used as an IR detector even when the ambient light level is above it's normal cutoff threshold. I tested two kinds of black electrical tape and one type of red electrical tape as IR filters, but both failed. I had success in using IR plastic, taken from my electronics junk box, to block out enough ambient light to turn on the nightlight while still allowing directed IR to pass and turn out the nightlight. However, the filter would not stop directed blue LED light and red laser light from turning of the nightlight.
Some green lasers from unscrupulous manufacturers emit a potentially hazardous amount of infrared. By a clever use of filters or a diffraction grating it may be possible to use a nightlight as an IR hazard indicator.
I think science educators may find photosensitive nightlights to be inexepensive yet useful tools for demonstrating concepts like transducers and detecting light beyond the range of human vision.
The Electromagnetic Spectrum
Green Laser IR Hazard Explanation and Test