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This design was developed by request of a correspondent having made a sort of LED candle and needing to switch off the LED with a puff. This simple, easy to build gadget can be useful as a prop for Halloween and Christmas season, shows and the like. Q2 & Q3 form a self-latching pair that start operating when P1 is pushed: in this way the LED (or bulb) will illuminate steadily. When someone emits a strong puff in the vicinity of the small electret microphone.... [read more]

A single Schmitt Trigger inverter stage (1/6 of 74HC14) is used as a squarewave oscillator to produce a low frequency of about 0.5 Hertz. The 10K resistor in series with the input (pin 1) reduces the capacitor discharge current through the inverter input internal protection diodes if the circuit is suddenly disconnected from the supply. This resistor may not be needed but is a good idea to use.... [read more]

This circuit is permanently plugged into a mains socket and NI-CD batteries are trickle-charged. When a power outage occurs, the lamp automatically illuminates. Instead of illuminating a lamp, an alarm sounder can be chosen. When power supply is restored, the lamp or the alarm is switched-off. A switch provides a "latch-up" function, in order to extend lamp or alarm operation even when power is restored.... [read more]

This analyst is, a sensitive instrument, in the frequency changes and width of a acoustic signal. Thus the brightness of LED that turns on each moment of is proportional signal width, while the colour of proportionally frequency.... [read more]

This circuit emits a beep and/or illuminates a LED when someone touches the door-handle from outside.... [read more]

This circuit will emit an intermittent beep (or will flash a LED) when the water contained into a recipient has reached the desired level. It should be mounted on top of the recipient (e.g. a plastic tank) by means of two crocodile clips, acting also as probes. If a deeper sensing level is needed, the clips can be extended by means of two pieces of stiff wire (see pictures).... [read more]