Abstract: Disclosed are filters which are particularly useful in sampling devices for detecting radioactive particulate contaminants in a gaseous medium, such as air. The filters comprise a filter membrane sandwiched between two pieces of plastic material having holes exposing the filter membrane to the gaseous medium. The filters can readily be handled by grasping them by their plastic portions and there is little risk of the filter medium being damaged. An air tight seal may easily be made with the plastic surrounding the filter medium. The filter is easily constructed by placing a piece of filter material to cover a hole in a plastic base layer and then covering them both with a plastic cover layer having a hole to match the hole in the base layer. The cover layer is preferably formed of self-adhesive layer.

Abstract: Modular electronic trunk selectors are known which have a number of incoming trunk modules and a common control module. Incoming trunks are connected to outgoing trunks by the common control module. The invention disclosed is a trunk selector of this type but it includes automatic testing and fault location. Periodic tests are randomly made of incoming trunks to see if correct signals are present on certain leads between the incoming trunk module and the common control module. Checks are also made for continuity and open circuit conditions on the switching paths at appropriate times. Tests are also made when regular calls are being set up. Detection of a fault initiates a "loop-back" in the common control module to see if the fault is in the incoming trunk module or the common control module and an appropriate alarm is activated.

Abstract: Disclosed is an electronic hybrid circuit which enables telephone conferences to be set up. The known systems have various limitations. For example, in one type of system, more than one person cannot be heard simultaneously. Known systems also tend to be fairly limited in the number of telephones which may be connected up in a single conference. Otherwise, signal degradation takes place due to worsening signal to noise ratio and impedance matching problems. The present invention provides an electronic hybrid circuit which enables a large number of telephones to be connected in a conference set up and which has a number of advantages, including the fact that more than one person may talk at one time. The disclosed hybrid circuit comprises a local input and output and a system input and output. The local input is connected to an analog switch and to a comparator which provides a control voltage to turn on the analog switch if a signal on the local input exceeds a predetermined minimum.

Abstract: Disclosed is a switching arrangement operable by a tone of a predetermined frequency. An automatic gain control amplifier has an input connectable to a source of signals and an output connected to an input of a bandpass filter which rejects all frequencies except those of a narrow band centered on the predetermined frequency. The bandpass filter is connected to an input of a phase locked loop which produces a change in voltage from a first level to a second level at its output only in response to a narrow band of frequencies centered on the predetermined frequency. The output of the phase locked loop is connected to a timer which only produces a pulse at its output upon receipt of the second level of voltage for a predetermined time. A pulse from the timer is fed to a flip-flop which changes state and operates a relay.

Abstract: Various types of static DC to DC converters are known. Some of these have output rectifiers including phase controlled SCRs. The present invention eliminates the need for controlled output SCRs by pulse width modulation of switching elements in a control circuit controlling the converter circuitry itself. This results in the elimination of some switching devices, higher reliability and higher-efficiency, A unijunction oscillator drives a one-shot multivibrator and a flip flop. The unijunction oscillator, one-shot multivibrator and flip flop feed two AND gates which are alternately enabled to allow pulses of current to flow from a DC source through opposite halves of a primary winding of a transformer. The output of the secondary winding is rectified and filtered to provide the converter output. Voltage regulation is accomplished by controlling the one-shot multivibrator. In one embodiment, paralleling of a number of converters is enabled by providing for frequency and pulse width synchronization.