RELATED APPLICATIONS  
       [0001]     This application is a Continuation Application of U.S. application Ser. No. 11/396,296, filed Mar. 31, 2006, which is a Continuation Application of U.S. application Ser. No. 11/243,471, filed Oct. 4, 2005, which is a Continuation Application of U.S. application Ser. No. 11/077,960, filed Mar. 11, 2005, which is a Continuation Application of U.S. application Ser. No. 10/890,817, filed Jul. 14, 2004, which is a Continuation Application of U.S. application Ser. No. 10/799,445, filed Mar. 12, 2004, which is a Continuation Application of U.S. application Ser. No. 10/642,113, filed Aug. 15, 2003, which is a Continuation Application of U.S. application Ser. No. 10/352,374, filed Jan. 27, 2003, now abandoned, which is a Continuation Application of U.S. application Ser. No. 10/211,935 filed Aug. 1, 2002, which is a Continuation Application of U.S. application Ser. No. 10/040,259, filed Jan. 2, 2002, which is a Continuation Application of U.S. application Ser. No. 09/709,081, filed Nov. 8, 2000, which is a Continuation Application of U.S. application Ser. No. 08/996,567, filed Dec. 23, 1997, now U.S. Pat. No. 6,741,164, which is a Divisional Application of U.S. application Ser. No. 08/682,140, filed Jul. 17, 1996, now U.S. Pat. No. 5,886,620, which is a Continuation Application of U.S. application Ser. No. 08/591,902, filed on Jan. 25, 1996, now U.S. Pat. No. 5,559,492, which is a File Wrapper Continuation of U.S. application Ser. No. 08/126,791, filed on Sep. 24, 1993. The entire teachings of the above applications are incorporated herein by reference.  
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Typical building fire alarm systems include a number of fire detectors positioned through a building. Signals from those detectors are monitored by a system controller which, upon sensing an alarm condition, sounds audible alarms throughout the building. Flashing light strobes may also be positioned throughout the building to provide a visual alarm indication, with a number of audible alarms and strobes typically being connected between common power lines in a network. A first polarity DC voltage may be applied across those power lines in a supervisory mode of operation. In the supervisory mode, rectifiers at the alarm inputs are reverse biased so that the alarms are not energized, but current flows through the power lines so that the condition of those lines can be monitored. With an alarm condition, the polarity of the voltage applied across the power lines is reversed to energize all alarms on the network.  
         [0003]     Typical strobes are xenon flash tubes which discharge very high voltages in the range of about 250 volts. Those high voltages are reached from a nominal 24 volt DC supply by charging a capacitor in increments with a rapid sequence of current pulses to the capacitor through a diode from an oscillator circuit. When the voltage from the capacitor reaches the level required by the flash tube, a very high voltage trigger pulse of between 4,000 and 10,000 volts is applied through a step-up transformer to a trigger coil about the flash tube. The trigger pulse causes the gas in the tube to ionize, drawing energy from the capacitor through the flash tube to create the light output.  
         [0004]     Under the American Disability Act, and as specified in Underwriters Laboratories Standard UL 1971, the strobes must provide greater light intensity in order that the strobes can alone serve as a sufficient alarm indication to hearing impaired persons. Unfortunately, the strobes at the higher intensity levels have been reported to trigger epileptic seizures in some people.  
       SUMMARY OF THE INVENTION  
       [0005]     In typical strobe systems, each strobe fires as the required firing voltage on the capacitor is reached. Since the strobes are free-running and tolerances dictate that the time constants of various strobes are not identical, the strobes appear to flash at random relative to each other. It is believed that a high apparent flash rate that results from the randomness of the high intensity strobes causes the epileptic seizures.  
         [0006]     In accordance with the present invention, all strobes on a network are synchronized such that they all fire together at a predetermined safe frequency to avoid causing epileptic seizures. Additional timing lines for synchronizing the strobes are not required because the synchronizing signals are applied through the existing common power lines.  
         [0007]     Accordingly, in a building alarm system having a plurality of warning strobes powered through common power lines, each strobe includes a flash lamp and a capacitor to be discharged through the flash lamp. A charging circuit powered by the common power lines applies a series of current pulses to the capacitor to charge the capacitor. The firing circuit responds to a change in voltage across the power lines to discharge the capacitor through the flash lamp.  
         [0008]     In order to avoid overcharging of the capacitor as a strobe waits for the firing signal, each strobe further includes a voltage sensor for disabling the charging circuit when the capacitor reaches a firing voltage level.  
         [0009]     In a preferred system, a network operates in a supervisory mode in which current flows from a system controller through the power lines to assure the integrity of the network during nonalarm conditions. Further, during an alarm condition, the system controller may code the synchronizing signals so that the timing of the flashing strobes indicates the location in the building at which the alarm condition was triggered. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.  
         [0011]      FIG. 1  illustrates an alarm system embodying the present invention.  
         [0012]      FIG. 2  is a detailed electrical schematic of a strobe in the system of  FIG. 1 .  
         [0013]      FIG. 3  is a timing diagram illustrating the synchronization signals on the power lines. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     A description of preferred embodiments of the invention follows.  
         [0015]     A system embodying the present invention is illustrated in  FIG. 1 . As in a conventional alarm system, the system includes one or more detector networks  12  having individual fire detectors D which are monitored by a system controller  14 . When an alarm condition is sensed, the system controller signals the alarm through at least one network  16  of alarm indicators. The alarm indicators may include any variety of audible alarms A and light strobe alarms S. As shown, all of the alarms are coupled across a pair of power lines  18  and  20 , and the lines  18  and  20  are terminated at a resistance R L .  
         [0016]     Each of the alarms A and S includes a rectifier at its input which enables it to be energized with only one supply polarity as indicated. When there is no alarm condition, the network  16  may be monitored by applying a reverse polarity DC voltage across the network. Specifically, line  20  would be positive relative to line  18 . Due to the rectifiers within the alarm devices, no alarm would be sounded, but current would still flow through the resistor R L . Any fault in the lines  18  and  20  would prevent that current flow and would be recognized as a fault by the system controller. With an alarm condition, the system controller would apply power across lines  18  and  20  with a positive polarity to cause all alarms to provide their respective audible and visual indications.  
         [0017]     A preferred circuit of a light strobe S is presented in  FIG. 2 . Line  18  is coupled through the diode rectifier D 3  so that the strobe only responds to a positive polarity voltage across the lines  18  and  20  as discussed above. Diode D 3  is followed by a noise spike suppression metal oxide varistor RV 1  and a current regulator of transistors Q 4  and Q 5 . During normal current flow, Q 5  is biased on through resistors R 7  and R 13 . The current flow thus maintains a charge Vcc across capacitor C 7 . However, during an in-rush situation such as during start-up, the several alarm circuits may draw too much current and overload the power supply. In situations of high current, the higher voltage across resistor R 7  turns transistor Q 4  on, which in turn turns Q 5  off.  
         [0018]     Zener diode D 4  and transistor Q 3  are part of a flash tube trigger circuit to be discussed further below. At normal values of Vcc, nominally 24 volts, zener diode D 4  is turned on through resistors R 11  and R 12 . The resultant voltage across R 14  turns Q 3  on to pull the node below resistor R 10  to ground. With that node grounded, the silicon controlled rectifier Q 2  to the right of the circuit remains off.  
         [0019]     The overall function of the circuit is to charge a capacitor C 5  to a level of about 250 volts and periodically discharge that voltage through a flash tube DS 1  as a strobe of light. The flash tube is triggered by applying a high voltage in the range of 4,000 to 10,000 volts through a trigger coil connected to line  22 . That very high voltage is obtained from the 250 volts across C 5  through a transformer T 1 . Specifically, when SCR Q 2  is gated on, the node below resistor R 3  rapidly changes from 250 volts to 0 volts. That quick change in voltage passes a voltage spike through the differentiating capacitor C 6  which is transformed to a 4,000 to 10,000 volt pulse on line  22 .  
         [0020]     Capacitor C 5  is charged in incremental steps with a rapid series of current pulses applied through diode D 1 . To generate those current pulses, a UC3843A pulse width modulator is used in an oscillator circuit. The oscillating output of the pulse width modulator is applied through resistor R 4  to switch Q 1 . Zener diode D 2  serves to limit the voltage output of the pulse width modulator. When Q 1  turns on, current is drawn through the inductor L 1 . The output of the modulator goes low when a predetermined voltage is sensed across resistor R 5  through resistor R 1  and capacitor C 1 . When Q 1  is then switched off, the collapsing field from inductor L 1  drives a large transient current through diode D 1  to incrementally charge C 5 .  
         [0021]     The pulse width modulator is powered through resistor R 6  and capacitor C 4 . The frequency of oscillations of the modulator U 1  are controlled by resistor R 2  and capacitors C 2  and C 3 .  
         [0022]     The voltage across capacitor C 5  is sensed by voltage divider resistors R 8  and R 9 . When that voltage reaches a predetermined level such as 250 volts, the pulse width modulator U 1  is disabled through its EA input. This prevents overcharging of capacitor C 5  while the strobe circuit waits for a synchronizing pulse at its input.  
         [0023]      FIG. 3  illustrates the signal across lines  18  and  20  during an alarm condition. Normally, the voltage is high so that the charging circuit charges the capacitor C 5  to 250 volts and then holds that voltage. Periodically, however, the voltage across the power lines goes low as illustrated. For example, the voltage might drop to zero for ten milliseconds every 2.4 seconds. That voltage drop is not perceived in the audible alarms, but is sufficient to trigger the strobes. As the voltage goes low, zener diode D 4  stops conducting and transistor Q 3  turns off. There remains, however, sufficient voltage on capacitor C 7  to raise the voltage between Q 3  and R 10  to a level sufficient to gate the SCR Q 2  on. With SCR Q 2  on, the trigger pulse is applied to line  22  so that capacitor C 5  is discharged through the flash lamp. Subsequently, when the power supply voltage is returned to its normal level, the charging circuit including modulator U 1  recharges capacitor C 5  to the 250 volt level.  
         [0024]     Prior strobes have been free running, an equivalent to capacitor C 5  being discharged as it reached the 250 volt level. Thus, timing of the strobe flash was dictated solely by the charging time constant of the particular circuit, and strobes flashed at different intervals. The circuit disclosed enables the synchronization of the entire network of strobes, and does so without the need for a separate synchronization line. Synchronization is obtained by triggering all strobes of a network with a pulse in the power supply. The circuit is able to respond to the synchronization signal in the power lines without loss of the ability to supervise the network over those same two power lines during the supervisory mode of operation. Thus, the two lines provide supervisory current to monitor for faults, power to the audible and visual alarms during an alarm condition, and synchronization of the strobes.  
         [0025]     Circuitry is no more complicated than would be a free running strobe. In fact, the circuit of  FIG. 2  can be readily converted to a free running strobe by removing the resistor R 12  and applying a gating voltage above R 11  from a COMP output of the modulator U 1 . The COMP output goes high with sensing of the desired voltage level at input EA.  
         [0026]     In the past, audible alarms have been coded in their audible outputs to indicate, for example, the source of the alarm condition. For example, an alarm output of two beeps followed by three beeps followed by seven beeps could indicate that the alarm condition was triggered at room  237 . By synchronizing all strobes in accordance with the present invention, encoding of the strobe alarm signal can also be obtained. The system controller need only time the synchronization pulses accordingly. When the network includes audible alarms, the fall in voltage which ends an audible beep triggers the flash.  
         [0027]     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
In a building fire alarm system, the light strobes of a network of strobes are synchronized to flash simultaneously. Each strobe has a charging circuit to charge a capacitor which discharges through a flash tube. Once a capacitor is charged, the charging circuit is disabled. A synchronization pulse is applied through common power lines to trigger discharge of each strobe capacitor through the flash tube followed by recharging of the capacitor.