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In order to obtain a good audio reproduction at different listening levels, a different tone-controls setting should be necessary to suit the well known behavior of the human ear. In fact, the human ear sensitivity varies in a non-linear manner through the entire audible frequency band, as shown by Fletcher-Munson curves.... [read more]

Apart from the obvious usage as a headphone amplifier, the circuit can be used for a range of applications where a wide bandwidth low power amplifier is needed. The circuit is based on an opamp, with its output current boosted by a pair of transistors.... [read more]

This circuit provides a visual 9 second delay using a 7 segment digital readout LED. When the switch is closed, the CD4010 up/down counter is preset to 9 and the 555 timer is disabled with the output held high.... [read more]

This circuit uses a UM3561 IC to produce four different sound effects... [read more]

This two-zone alarm has automatic exit, entry and siren cut-off timers. It was developed for the Beginner's Guide To Cmos Timers. So it has a particularly detailed circuit description. There's an optional One-Time-Only module. It will force the siren to switch off after the first ten minutes - and prevent the alarm from activating a second time.... [read more]

It beeps if the fridge door is left open for too long or hasn't closed properly, to stop food from spoiling. There are lots of other uses as well. A refrigerator or freezer door that is left open or ajar may cause the food contents to spoil. In some cases, the internal temperature of the fridge or freezer will be maintained if the refrigeration system can cope with the open door.... [read more]