the ultimate objective of the invention is to facilitate inexpensive and effective means for controlling the indicator elements or active signs in car park management systems ( and is hereon referred to as the objective or the stated objective ). by the term ‘ car park management systems ’ is meant systems that are capable of informing motorists within car parking facilities which sections or areas or aisles are full and which have vacant parking places available . this would normally be achieved by way of strategically positioned signs at car park entrances or the ends of parking aisles . these signs would be active in that their display changes in relation to the availability conditions in the nominated parking areas . this invention consists of an electronic circuit with specific components of the circuit being physically located in specific positions within the car park . it uses a sensor in each car space being monitored . this invention is sufficient to fulfill the above objective in the most simple and inexpensive manor known to the inventor . other advantages over current or conventional technology are ; ease of installation , low maintenance and low cost of maintenance . the simplicity is an important factor due to the fact that , usually , numbers of parking spaces run into hundreds and thousands within typical car parking lots , necessitating that what components and labor are needed for each car space are multiplied by hundreds or thousands for a complete car park management system . this invention is capable of monitoring a group of car parking spaces and is able to determine two states of that group . the two states are ; that either the group is completely full or occupied , or , that there is one or more vacant spaces available within the group . conventional car park management systems utilising individual car space sensors , use sensors which must be directly polled periodically , each polling operation using a computer or microprocessor giving the result of only one car space at a time via a digital address bus . there must also be a micro - processor or similar attached to each sensor to code and decode data to and from the address bus , necessitating multiple components for each car space . this is not the case with this invention as it can monitor a group ( or groups ) of car spaces without these complex digital components . as mentioned this invention uses a sensor in each car space . conventional technologies that are capable of discreetly and non intrusively determining the existence or the absence of vehicles within parking spaces are ; metal detecting devices , infrared sender / receivers and sonic or radio range finders ( sonar and radar ). these devices all require multiple components for each sensor . the sensors used by this invention are light sensitive electronic components only and more specifically only those that use semiconductor technology such as phototransistors or photodiodes ( hereon referred to as light sensors ). there is at least one of these light sensors mounted on , or flush with , the floor of each car space to be monitored ; generally toward the middle or central point of the car space bounds . simply , when a vehicle is parked within the car space , it reduces the ambient light falling incident upon the light sensor which changes the internal resistance of that light sensor ( the vehicle casts its shadow over the light sensor ). this internal resistance is used to determine the occupancy state of the car space . the reason for using semiconductor light sensors is due to the way these components behave when multiples of them are connected electrically in parallel ( as against series ) configuration . the resistance between the top and bottom rails of such a parallel circuit remains reasonably stable with increasing numbers of parallel components and reduces nothing like the extent that it would with resistors connected in the same way . resistors in parallel ( r 1 , r 2 , r 3 . . . ) obey the known formula 1 / r 1 + 1 / r 2 + 1 / r 3 + . . . = 1 / r where r is the resistance between the top and bottom rails , while semiconductors do not strictly obey this law . it can be seen that with increasing numbers of parallel resistors , the resistance drops away significantly , therefore light dependent resistors are unsuitable for this invention . the behavior of the semiconductor light sensors enables a multiple of them to be connected in parallel without changing the effectiveness of the individual sensors . if all the light sensors are blocked by vehicles , they all will have relatively high internal resistance and hence the parallel array as a whole will also have relatively high resistance ( as measured between the top and bottom rails ); while if one or more light sensors are fully exposed to ambient light , the resistance of the parallel array is substantially reduced . this means that a string of these parallel light sensors on a two wire cable can be laid out within a group of car spaces ( in such fashion as previously described ) requiring only examination of the top and bottom rail resistance ( between one wire and the other ) in order to determine the occupancy of the whole group . this arrangement when combined with an electronic detection means for detecting and indicating the resistive state of the parallel array of light sensors gives a system capable of achieving the stated objective — in situations with constant or at least reasonably stable ambient light conditions . the most simple and yet effective way of achieving this electronic detection means uses a transistor and opto - coupler . ( an opto - coupler consists of a light emitting diode or led which casts its light directly onto a phototransistor or photodiode within a sealed encapsulation ). the led is connected in parallel with the transistor between the collector and emitter . the parallel array of light sensors connects to the collector and base of the transistor turning it on or off . when the transistor is on , it shorts and turns off the led hence turning off the opto - coupler output . the circuit is connected to voltage via a limiting resistor . in practice this works very well as the opto - coupler isolates the parallel array of light sensors from the switching circuitry . it may be desirable to put the opto - coupler in series with the transistor . a convention relay may also suffice as the electronic detection means being connected in parallel or in series with the transistor . in order to overcome variables such as changing ambient light conditions , the following enhancements are also included in the scope of this invention . there may be the addition of ambient light compensating circuitry for each group of car spaces monitored . in most fundamental form this ambient light compensating circuitry consists of two components ; a light sensor , combined with a transistor such that the light sensor connects to the base and emitter of the transistor in correct polarity ( this particular light sensor will hereon be referred to as an ‘ ambient light sensor ’). the two rails of the parallel array of light sensors connect to the collector and base terminals of the transistor ( in correct polarity ) controlling it by switching it on or off . the ambient light sensor in effect conducts current away from the base / emitter junction of the transistor at a rate that is directly proportional to the level of ambient light incident upon it . the transistor in this case will form part of the electronic detection means . this transistor will correspond to the transistor as described above when using an opto - coupler as part of the electronic detection means . in practice this is a very effective method for compensating for very large changes in ambient light , from faint artificial light at night , to full sunlight . the ambient light sensors should be physically located so as to receive representative or similar ambient light conditions as the parallel array of light sensors that it compensates for . the representative ambient light conditions referred to must change generally in proportion to the ambient light conditions of the light sensors . this will generally be physically close to the light sensors on or near the periphery of the car spaces mounted on the floor , walls columns or ceiling . the ambient light sensor should not be mounted where a significant reduction in its incident ambient light occurs due to the presence of any cars parked within the monitored car spaces . in order that each group of monitored car spaces may have its own ambient light sensor which more closely compensates for the particular ambient light conditions of each group , the following alternative arrangement is included in the scope of this invention . further separate parallel arrays of light sensors , each with its own ambient light sensor and transistor , may be connected in parallel ( as follows ) with the existing parallel array , ambient light sensor and transistor . the parallel connections of these sections will be such that , of the transistors , the collectors are connected together and the emitters are connected together . this arrangement may also be used in conjunction with an opto - coupler as previously described with the led in parallel to these parallel transistors . the physical location of all of these ambient light sensors should be such that each ambient light sensor receives a level of ambient light similar to the particular parallel array of light sensors that it compensates for . one or more parallel arrays of light sensors connected to each other in parallel which might have associated ambient light sensors and connect to an electronic detection means will hereon be referred to as a ‘ basic unit ’ of a car park management system . there may be one or more of these basic units controlling one indicator element ( perhaps an active sign ). this indicator element will be able to directly control other indicator elements by ; either overriding these other indicator elements directly via cable ; or by putting a short circuit ( via cable ) on any parallel array of light sensors that is part of a basic unit switching the particular indicator element . this second method will mimic a vacant car space in the particular basic unit causing its electronic detection means switch to a vacant condition from a full condition . indicator elements may also indirectly control other indicator elements via a computer or micro processor . in practice this invention may involve cutting shallow grooves in the surface of the car park and laying the cables with the described components attached into this groove . ambient light sensors may be attached adjacent to the associated transistors and , as a unit , placed within the car park between the car spaces , mounted on the floor or otherwise . two and three wire cables are generally sufficient to connect the various components back to the main part of the electronic detection means . a durable filler medium may be applied to cover the cables , fill the grooves and set in the light sensors to prevent damage . the specific arrangements documented in this specification , with regard to this invention , demonstrate the most fundamental or basic arrangements that will enable it to facilitate its stated objective , though do not limit the scope the invention which has been defined by the claims herein . it must be understood that these basic arrangements may be somewhat elaborated upon to achieve specific or general improvements , as seen fit , while still remaining within the scope of this invention . these basic arrangements , improved or otherwise , may also be used in conjunction with other contemporary electronic equipment in order to facilitate the stated objective , enhanced or otherwise , while also remaining within the scope of this invention .