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48 V ‘phantom’ powering has become the standard for professional condenser microphones. The supply (or rather bias) voltage is applied over both wires of the balanced screened cable via two 6k8 resistors (see reference [1]) – the absolute value is not critical, since a variation of ±20% is permitted, but they must be matched to an accuracy of 0.4% or better [2]. Many microphones are fitted with an output transformer, and derive their power from a center-tap on the secondary.... [read more]

In this fire alarm circuit, a Thermistor works as the heat sensor. When temperature increases, its resistance decreases, and vice versa. At normal temperature, the resistance of the Thermistor (TH1) is approximately 10 kilo-ohms, which reduces to a few ohms as the temperature increases beyond 100 C. The circuit uses readily available components and can be easily constructed on any general-purpose PCB.... [read more]

This circuit was intended to activate a relay by means of a hand clap. Further claps will turn-off the relay. An interesting and unusual feature of this project is the 3V battery operation. The circuit's sensitivity was deliberately reduced, in order to avoid unpredictable operation. Therefore, a loud hand clap will be required to allow unfailing on-off switching. Q1 acts as an audio amplifier. IC1 timer, wired as a monostable, provides a clean output signal and a reasonable time delay in order to allow proper switching of the following bistable circuit. A discrete-components circuit formed by Q2, Q3 and related parts was used for this purpose, in order to drive the Relay directly and to allow 3V supply operation.... [read more]

Many people experienced sleeping well in natural surroundings, into a tent or a wooden hut. This fact is due not only to the healthy atmosphere but also from our unconscious ability to perceive natural Earth's magnetic-fields. The circuit generates this type of Geo-magnetic-fields and lets us perceive them: in this manner our brain is surrounded by an ideal environment for a sound sleep.... [read more]

This is a single channel (on / off) universal switch that may be used with any Infra Red remote control that uses wavelengths between 850-950nm.... [read more]

This circuit was designed to detect when a call is incoming in a cellular phone (even when the calling tone of the device is switched-off) by means of a flashing LED. The device must be placed a few centimeters from the cellular phone, so its sensor coil L1 can detect the field emitted by the phone receiver during an incoming call.... [read more]