Refrigerator CW key
Does the fridge light bulb go off when the door is closed? This weighty problem has puzzled mankind ever since the modern fridge was invented. (For a profound discussion on this topic see for example this link.)
If you can't get sleep because thinking of this problem, this device might help. It is a simple light sensitive sensor using a phototransistor and an ordinary BC547 transistor as a switch.
A piece of plastic tube is added around the phototransistor so that only the fridge lamp light is detected. Connect the sensor to an audio oscillator to monitor the lamp status.
The detector becomes even more useful if it can be connected to a CW transmitter. Then you can key the TX by opening and closing the fridge door (assuming of course that the light bulb really goes on and off). Long QSOs are not recommended though because opening the door repeatedly rises the fridge temperature. Always keep some food in the fridge so that you can take a snack while having fridge-keyed CW QSOs.
Keying is carried out like this: