Abstract: The invention provides an electronic horn for an alarm circuits of the type in which the d.c. power supply for the horn will be of a particular polarity to indicate an alarm condition. The circuit provides a piezo sounder connected across the terminals of the power supply, an inductor connected in parallel with the piezo sounder to form a tank circuit therewith and current modifying means connected in series with the tank circuit to modify the current through the tank circuit in response to the output of an R/S flip-flop which is periodically set by a clock signal at a frequency corresponding to the frequency desired for the fundamental audible tones of the sounder. The duration of the clock output pulse which makes the modifying means conductive must be such that the capacitance of the sounder is charged by the supply before the output of the clock permits a change of state in the flip-flop to make the modifying means nonconductive.

Abstract: A strobe light circuit is provided for flashing a strobe flash unit at a desired frequency. An inductor is repetitively connected and disconnected across a d.c. power line by a switch means so that energy is stored in the inductor during the period when the circuit is complete. The flash unit and a capacitor are connected in parallel so that the capacitor can discharge its stored energy to the flash unit when the voltage across the capacitor exceeds the threshold firing voltage of the flash unit. The parallel combination of the flash unit and the capacitor is in turn connected in series with a diode, and the resulting series circuit is connected across the inductor with the diode being connected in polarity such that current will not flow from the power line through the flash unit or the capacitor.

Abstract: In an electronic horn for alarm systems of the type in which an alarm condition is indicated by a reversal of polarity in the d.c. power supply, an improvement provides a pair of terminals for connection to said d.c.

Abstract: Doorbell mechanism that employs only one solenoid to achieve both single tone and double tone operation. The solenoiod has a split coil with the full coil being employed for double tone operation and a portion of the coil being employed for single tone operation.

Abstract: Apparatus for indicating the presence of power on the third rail of a transit system having two running rails and a third rail includes a first connector for connecting to the third rail and a second connector for connecting to one of the running rails and a circuit for indicating the presence of a voltage between the first connector and the second connector and thereby indicating a voltage between the one running rail and the third rail, the means for indicating including visual and audible signaling apparatus. In some forms of the invention the apparatus may include an annunciator and a strobe light. Other forms of the invention include third and fourth connectors which are magnetically held to the rails.

Abstract: A multimode electronic circuit for driving bells or other electromagnetic devices. An important feature of this design is that it allows one circuit to function in a variety of modes: either in the vibrating mode or single stroke mode, both being operative with either an AC or DC source.