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This device beeps intermittently for about two seconds when a person in a range of around 10 meters emits a whistle. The first two inverters contained in IC1 are used as audio amplifiers. IC1A amplifies consistently the signal picked-up by the small electret-microphone and IC1B acts as a band-pass filter, its frequency being centered at about 1.8KHz. The filter is required in order to select a specific frequency, the whistle's one, stopping other frequencies that would cause undesired beeper operation. IC1C is wired as a Schmitt trigger, squaring the incoming audio signal. IC1D is a 2 second-delay monostable driving the astable formed by IC1E & IC1F. This oscillator generates a 3 to 5Hz square wave feeding Q1 and BZ1, thus providing intermittent beeper operation.... [read more]

It many times happens, we want erase or decrease in a musical song, the voice of singer or some other sound that usually is written in same level and in the two channels. If we remove between them two signals of channels, we can take in the output a mono signal without the voice.... [read more]

This is a selection of small self-contained alarm circuits. They have a very low standby current; and are suitable for battery operation. Some are triggered by normally-open and normally-closed switches. Some react to changes in light or temperature. The result is a variety of output times and patterns.... [read more]

The PR4403 is an enhanced cousin of the PR4402 40mA LED driver. It has an extra input called LS which can be taken low to turn the LED on. This makes it very easy to build an automatic LED lamp using a rechargeable battery and a solar module. The LS input is connected directly to the solar cell, which allows the module to be used as a light sensor at the same time as it charges the battery via a diode. When darkness falls so does the voltage across the solar module: when it is below a threshold value the PR4403 switches on. During the day the battery is charged and, with the LED of, the driver only draws 100µA.... [read more]

This timer was designed for people wanting to get tanned but at the same time wishing to avoid an excessive exposure to sunlight. A Rotary Switch sets the timer according to six classified Photo-types (see table). A Photo resistor extends the preset time value according to sunlight brightness (see table). When preset time ends, the beeper emits an intermittent signal and, to stop it, a complete switch-off of the circuit via SW2 is necessary.... [read more]

This circuit provides a delayed visual indication when a door bell switch is pressed. In addition, a DPDT switch can be moved from within the house which will light a lamp in the door bell switch. The lamp can illuminate the words "Please Wait" for anyone with walking difficulties.... [read more]