Sensing devices are used in many appliances around the home. Similar circuits to these two appear in many products to warn us of the need to take some action. Make a list of products around the home that have 'sensing' devices as a part of them. Start by thinking about getting up and 'step-by-step' through all the items you use before the end of the day. After making that list extend the exercise by thinking about 'sensing' devices in the rest of the world... Car-parks, lifts, etc.
The water detector must switch off the LED alarm light when water is added to the soil. If a thyristor was used in this circuit the LED would not go off once the soil had dried out for the first time.
The steady hand game needs an 'electronic switch' that stays locked on when the contact between the handset and the test track is made. The THYRISTOR is the perfect component for this task as it latches on when a low voltage is sensed at it 'gate' connection. If a transistor was used here the buzzer would sound only as the contact was made and would then switch off as the contact was broken.
The circuit would have to be reset by switching off. A TRANSISTOR is the perfect electronic switch' to use in this circuit. When a low voltage is detected at the 'base' of the transistor the warning circuit is switched on ~ and off when the voltage at the base disappears.