in order to best understand the following description it is thought that a discussion of the functional schematic of fig1 accompanied by cross references to the detailed circuitry of fig2 a and 3 b , as necessary , is a preferred approach . the backup circuitry and associated elements are housed in a circuitry panel , not shown , which is efficiently packaged to minimize the resulting size and to facilitate its location at the site to be monitored . unless otherwise indicated , all connections in the referenced schematics , and relay positions , are shown based on the assumption that the ac voltage is present and that the battery voltage , v batt , is above the selected , low voltage disconnect level as discussed below . the backup unit draws its power from the business or resident primary power line , for example , 120 vac . this is supplied on input line 10 . the 120 vac is supplied to a 24 vac step down transformer 12 . the secondary , 24 ac voltage , is supplied through a test switch 14 to a bridge rectifier circuit 16 , and to the coils of two ( 2 ), 24 vac , dpdt , form c relays , 18 and 20 . the output of bridge rectifier circuit 16 is connected through a limiting resistor to an led 22 status lamp mounted in the face of the panel and which provides a visual indication that 24 vac is present at the system . all six of the relay contacts of auxiliary relay 20 ( see fig2 ) are brought out to a terminal block 24 mounted on the panel . these would provide optional utilization for a customer when ac power is lost . for example , they can be electrically connected to a remote monitoring station ( s ) to enable continuous monitoring of the power status . when the primary voltage is lost , the 24 vac output from transformer 12 , of course , is also lost . led 22 would indicate this fact . relay 18 would be de - energized enabling the backup power supply to provide the necessary dc voltage to the alarm circuitry through “ ac loss ” spst relay switch 25 . a solar array panel 26 and a 12 vdc lead acid battery 28 are located separately from the circuitry panel . the amp - hour capacity of the battery is selected based upon an alarm system &# 39 ; s unique parameters . the nominal voltage is 12 vdc . the solar cell array panel provides charging current to the battery . the output power rating of the panel is based on the circuit and battery parameters . the positive , negative and frame ground terminals of the solar array panel are connected to the circuitry panel through terminal block 30 ( fig2 ). a surge arrestor 32 is positioned across the array panel input . the positive side of the array input is fed through a resettable fuse fig3 whose current rating , for example , four amps , is selected to accommodate the charging current determined for the battery 28 and , as well , to ensure that excessive current capable of damaging the solar array panel is not drawn by the backup unit . the array panel voltage is filtered and is supplied through a panel connection 36 to a normally closed set of contacts 38 of a low voltage disconnect latching relay , 40 ( fig3 a ). in order to provide regulated power to the system control circuitry , a zener shunt regulator 42 is employed ( fig3 b ). the shunt regulator draws on the battery power to produce a regulated output , v +. in the circuit design depicted , v + is 9 . 1 v . “ power on ” reset circuitry 44 ( fig1 and 3a ) employs three serially connected “ d ” type flip - flops to insure that all system control circuits enter initial operation under a known circuit state condition . the reset circuitry generates a “ po reset ” pulse voltage of v + at turn on at output 45 which is supplied to one input 47 of “ or ” circuitry 46 . the “ po reset ” pulse signal is also supplied to the set input 48 of battery charge control comparator 50 ( fig3 b ). the above and immediately following discussion assumes the battery voltage is above a low voltage disconnect level which will be discussed hereinafter . “ or ” circuitry 46 provides a v + gate voltage at its output 52 which is supplied to the trigger input 54 of mono - stable multi - vibrator 56 . once triggered the multi - vibrator produces a pulse - shaped voltage of v + magnitude at output 58 which in turn is supplied to the gate 60 of n - channel mosfet , 62 . the mosfet turns on , providing a ground return for one of the coils , 64 , of latching relay 40 . the schematic depicts the condition of the relay contacts for the latching relay 40 when coil 64 is energized , as just described . the nature of the latching relay is that , once energized , the coil voltage can be removed but the relay contact change remains until the other coil is pulsed . as such , in this circumstance , the array panel voltage provided at panel connection 36 is supplied to the “ panel in ” terminal 66 through closed contacts 38 . assuming the battery voltage is above its low voltage disconnect ( lvd ) level and assuming further that the battery voltage is below an upper voltage threshold for terminating the charging of the battery , all to be discussed hereinafter , then n channel mosfet 67 ( fig3 b ) will be open and the solar panel voltage and its available charging current will be supplied through steering diode 68 to the positive terminal of the battery 28 through terminal block 70 . comparator 72 ( in fact , a dual comparator in one package ) and 50 , and mosfets 67 and 88 cooperate to regulate the charging of the battery 28 by the solar panel array between a range of voltages . the range presently set is between 13 . 8 and 14 . 3 vdc . if the battery voltage is above 14 . 3 vdc the charging circuitry is shunted by mosfet 67 and battery charging is terminated . if the voltage reaches or drops below 13 . 8 vdc the charging resumes , unless the battery voltage is below the lvd voltage , for example , 11 . 3 vdc . assume the battery voltage , vbatt , is between 11 . 3 and 13 . 8 vdc . the voltage divider resistive network 76 ( fig3 b ) is set up such that the voltage at juncture 78 , which is coupled to the input of one of the comparators in dual comparator 72 , is of a value to trigger a gate voltage of v + at output 80 which in turn is coupled to clock input 82 of comparator 50 . a v + gated voltage appears at output 84 to drive the gate 86 of mosfet 88 . this turns on mosfet 88 , shorting to ground the gate 90 of mosfet 67 and cutting it off . as such , the solar panel voltage and charging current at terminal 66 can be directed through steering diode 68 to initiate ( or continue ) the charging of the battery . at this time led 91 is energized to thus give a visual indication that the charging circuit is operating . as the battery voltage rises and reaches 14 . 3 vdc , resistor divider network 92 ( fig3 b ) results in a voltage at juncture 94 , input 96 to the second comparator in dual comparator 72 , which results in a reset voltage change at output 98 which in turn is coupled to reset input 100 of comparator 50 . the voltage of output 84 ( and gate input 86 ) goes to zero . this turns off mosfet 88 which allows the v + voltage through resistor 102 to turn on mosfet 67 thereby shunting the solar array panel current to circuit ground , thereby inhibiting its battery charging ability . provided the battery voltage does not drop below the low voltage disconnect threshold ( e . g . 11 . 3 vdc ), the charging circuitry cuts out when v batt reaches 14 . 3 volts on the way up and is turned on , as v batt decreases , when it reaches 13 . 8 volts . this gentle internal charging of the battery is a significant improvement over the continuous trickle charge , prior art designs which ultimately degrade the battery life . resistor divide network 104 ( fig3 b ) is used to create this dead band between the 14 . 3 volt and 13 . 8 volt levels . the ability to readily change the charge cutout range through the manipulation of resistor values in divider networks expands the potential of this design to accommodate all types of backup batteries with various charging voltage requirements . the low voltage disconnect circuitry 106 ( fig3 b ) includes a comparator 108 . a resistor divider network 110 is placed across the battery voltage , v batt . the junction voltage at 112 is supplied to the input of the comparator . the resistor values in network 110 are selected for given comparator specifications so as to create a low voltage disconnect pulse at output 114 when v batt drops to a predetermined cut off voltage , for example 11 . 3 vdc . at this level the present circuitry will disconnect the solar panel feed to the battery charging circuitry ; disconnect the battery 28 from the backup feed path ; and provide a warning that the battery voltage has dropped below the disconnect value . when the lvd comparator circuit detects a positive going voltage level crossing the 11 . 3 volt threshold , a further pulse is fired which reestablishes the “ connect ” status . i . e ., the solar panel feed to the charging circuitry is re - established and the battery voltage is reconnected to the backup feed path . the specifics of the implementing circuitry follow . the voltage pulse at output 114 , responding to the battery voltage falling below 11 . 3 vdc , is a negative excursion pulse , from v + to zero volts . this is supplied to a d type flip - flop 116 , which produces the inverse or positive excursion pulse at its output 118 at this time . the output 118 is fed to the input 120 of a second monostable multivibrator 122 ( fig3 a ) which produces a positive gate pulse voltage at output 124 in response to the input signal . the output 124 is tied to the gate input 126 of mosfet 128 which turns on in response to the positive gate pulse voltage . a return path to ground is thus provided to the second coil 130 of latching relay 40 such that it is energized . this causes a change in state for relay contacts 38 , 132 and 134 . as a consequence , the solar panel feed is interrupted through the opening of contacts 38 . v batt is now disconnected ( due to the opening of contacts 132 ) from the “ feed ” terminal 136 through which it was provided as a back up to the alarm circuitry ( to be discussed below ). contemporaneously , through relay contacts 134 , now closed , v batt is connected to an led 137 which flashes e . g . a flashing red light , signifying that the lvd threshold has been reached and that the above results have occurred . when the v batt increases , and crosses the lvd threshold , the output voltage at output 114 changes from zero to v +. the output 114 is connected to the anode 138 of a second diode 140 of the “ or ” gate 46 ( fig3 a ). the v + voltage is seen at output 52 which is connected to input 54 of the monostable multivibrator 56 . output 58 pulses high , gating on mosfet 62 so as to provide a return path for coil 64 , energizing it and thus again changing the contact arrangement of contacts 38 , 132 and 134 . this reestablishes the solar feed to the charging circuitry and the battery feed to the alarm unit through contacts 132 . the low voltage disconnect warning is now interrupted so that led 137 no longer flashes . the backup feed path through contacts 132 and as presented at the terminal 136 , continues through a service switch 142 ( fig3 b ), which is manually activated , as desired , to break the feed path for diagnostic testing and repairs . the feed path continues through the relay contacts of relay 18 ( remember , the 24 ac is not present at this time ) to pins 144 in an internal panel connector ( not shown ). the path continues in fig2 . the battery negative lead 146 continues directly to alarm panel interface terminal 148 and auxiliary terminal 24 . the battery positive lead 150 is connected to the fixed relay contact of the spst “ ac loss ” relay 25 which is depicted in its unenergized state . the coil of this relay on one side is connected to v pos . bat through diode 151 . the remaining side is tied to the drain terminal of mosfet 152 . mosfet 152 is gated on when a positive voltage appears at terminal 2 of jumper block 154 . terminal 1 of jumper block 154 is tied to the solar battery positive voltage , as available , for example , when there is a loss of ac power to the backup unit . terminal 3 of jumper block 154 is tied to the output of flip - flop 156 which in turn is driven by an opto isolator circuit 158 . the input leads of the opto isolator circuit are connected to appropriate terminals of the alarm panel interface connector 148 . there will be , typically , a twelve volt dc voltage at these terminals when there is a loss of ac power in the alarm panel . this may or may not occur with the loss of ac power to the backup unit . assuming a loss of ac power in the alarm panel , the 12 volt dc signal , which is present and which could be of either polarity , is imposed across the back to back diodes 159 . this causes the transitor portion of the isolator to trigger on , creating a change of state , from v + to zero volts , at its output and the input to the flip - flop 156 . the output of the flip - flop , tied to terminal 3 , changes from zero to v + volts . the “ no ac ” indication from the alarm panel thus can be utilized to further qualify the “ tag on ” connection of the backup unit integral battery 28 onto the alarm system battery . this is done by jumping terminal 3 to terminal 2 of jumper block 154 , so that the energization of relay 25 occurs because the alarm panel has lost its ac power . if the installer chooses not to qualify the “ tag on ” of the backup unit , then he would jumper terminal 1 to terminal 2 of the jumper block 154 . in this situation relay 25 would be energized if ac power to the backup unit was lost , irrespective of whether or not the alarm panel lost power . once relay 25 is engaged , the backup battery voltage at lead 150 passes through its relay contacts , through steering diode 160 and through fused output leads , one to the alarm panel connector 148 and another to the auxiliary connector 24 . the latter may be optionally used by the installer to power additional lamps , sirens , etc . also , the battery voltage at lead 150 , when relay 25 is energized drives a “ backup engaged ” led , 162 , which signals that the backup battery is being used . the anode of steering diode 160 is made available via lead 164 to provide remote signaling of the engagement of the backup unit as explained below . lead 164 is connected through a resistor divider network to an input 166 of one of four fet switches packaged in 168 ( fig3 a ). the corresponding fet switch produces a gated signal at terminals 170 which is connected to the alarm panel interface connector 148 for remote monitoring of the backup unit engagement status . a second fet switch , shown functionally in fig1 as 172 , but contained in quad fet switch package 168 , receives an input signal at input 174 when the backup unit loses ac power and relay 18 is de - energized . the backup unit battery voltage now appears on lead 176 attached to the wiper contact of the relay 18 which in turn is tied through a resistor network 178 to input 174 . the output of the second fet appears on output line 180 of the quad fet package 168 and is also supplied to the alarm panel interface connector 148 . a third fet switch in the quad package 168 receives the lvd indication on line 118 ( fig3 b ) at its input 182 . the output of this fet , appearing on lines 184 is also made available to corresponding terminals on the alarm panel interface connector 148 . the various electrical component types and values identified herein and appearing on the drawings should be sufficiently familiar to and / or developable by those of ordinary skill in the circuit design art . for informational purposes , the inventors herein identify the following , select solid state components by reference number , manufacturer and manufacturer &# 39 ; s part number . further all components are available through distributors in the us and specifically , newark electronics company , having distributor offices throughout the united states ; and digikey corporation located in thief river falls , minn . ref . no . part no . manufacturer 67 irl 3705n international rectifier 62 , 88 irl l3303 international rectifier 128 , 152 158 opto isolator toshiba 50 cd4013bcm fairchild 72 , 108 ltc1442 linear tech d style flip - flops cd40106 fairchild ( e . g . 44 , 116 , 156 ) 168 dg 412 maxim 56 , 122 cd4047 fairchild while a specific preferred embodiment has been described , alternative means for implementing the various circuit functions will now be apparent . therefore , it is not intended , of course , to limit the scope of the invention to what has been described . rather the invention is to fined by the breadth of the claims which follow .