A photosensitive switch including control means for turning a load on or off in response to the ambient light level. The switch is so arranged to turn the load off if the components thereof fail.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to the field of light actuated electronic 
switches, and specifically to such switches used in conjunction with 
outside loads such as street lamps or the like. 
2. Description of the Prior Art 
Light actuated switching devices employing thermal relays are known in the 
prior art. However, such devices waste energy by consuming substantial 
amounts of power during the daylight hours. Furthermore, such switches 
waste energy by being so arranged that, should the switch fail, it will 
most probably fail with the load on. During times of abundant energy, in 
the case of street lighting, in the interest of safety it was preferable 
to require the switch to fail with the light on. 
With the energy crisis, however, it is desirable to provide a switch which 
consumes considerably less power than switches heretofore, typically on 
the order of milliwatts during the daylight hours and is so arranged so 
that the unit will turn the load off if any of the components of the 
switch fail. 
SUMMARY OF THE INVENTION 
The invention provides a switch for turning a load on or off including 
control means responsive to the ambient light level. In a first 
embodiment, a silicon controlled rectifier is arranged to actuate a 
thermal relay. The SCR in turn is connected to be switched on or off by a 
voltage divider network having a photosensitive resistor in one leg 
thereof, and a fixed resistor in the other. The ratios of the 
photosensitive resistor to the fixed resistor are such that during the 
night the SCR is permitted to pass current but during the day, the SCR 
effectively blocks current. 
In a second embodiment, means are provided to vary the light sensitivity of 
the switch. 
In a third embodiment, means are provided to turn the switch off if the SCR 
shorts. 
It is therefore an object of the invention to provide a photosensitive 
switch having a reduced power consumption. 
It is another object to provide a switch that most likely fails with 
current "off".

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention provides a photosensitive switch 10 connected to an AC power 
supply 11 through line terminal L and a neutral terminal N. Switch 10 is 
connected to a load through load terminal LD such as a street lamp 12, and 
includes control means arranged to turn the load on or off in response to 
the ambient light level. 
As exemplified in FIG. 2, switch 10 includes a silicon controlled rectifier 
(SCR) 13, having an anode 14, a cathode 15 and a gate 16. The anode 14 is 
connected to the neutral line of power supply 11 at terminal N. Cathode 15 
is connected to a heater 17 in a thermal relay 18. 
Switch 10 further includes a voltage divider network 19 comprising a fixed 
resistor 20 and a photosensitive resistor 21. A diode 22 is connected in 
series with photosensitive resistor 21 in that leg of the voltage divider 
19. Voltage divider 19 is connected between terminal N and terminal L. 
Diode 22 is poled with its anode towards the photosensitive resistor and 
the cathode towards terminal L. 
Gate 16 of SCR 13 is connected to voltage divider 19 at the junction 
between fixed resistor 20 and photosensitive resistor 21. 
Thermal relay 18 includes a pair of normally-open contacts 23 and 24, 
actuable by heater 17. Contact 23 is electrically connected to terminal L 
and contact 24 is connected to the load terminal LD. 
Photosensitive resistor 21 is chosen having a characteristic such that as 
the luminous flux incident thereon increases, the resistance thereof 
decreases. Therefore, during the day, the resistance of resistor 21 is at 
its lowest whereas in the evening, its resistance increases and is at a 
maximum at night. Furthermore, the ratio of resistance of the fixed 
resistor 20 to the photosensitive resistor 21 is chosen such that the 
threshold voltage is exceeded at night, actuating the SCR gate, and is 
insufficient during the day to actuate the gate. 
The thermal relay 18 is chosen so that the contacts 23 and 24 are normally 
open when heater 17 is not actuated. Contacts 23 and 24 close when heater 
17 is thermally actuated, when current flows therethrough. 
In operation, during the half-cycle in which the voltage at terminal L is 
positive with respect to terminal N, the entire voltage drop appears 
across diode 22 and SCR 13 is prevented from actuating. No current flows 
through heater 17, and contacts 23 and 24 of relay 18 are prevented from 
closing. During the half-cycle in which terminal N is positive with 
respect to terminal L, resistors 20 and 21 act as a voltage divider. 
During the day, when the resistance of resistor 21 is low, the potential 
at gate 16 fails to reach a level sufficient to trigger the SCR. 
Therefore, during the day, the SCR is held off both when terminal L is 
positive with respect to terminal N and when terminal N is positive with 
respect to terminal L. 
However, at night when light incident on resistor 21 decreases, its 
resistance increases. When terminal N is positive with respect to terminal 
L, the gate is actuated, triggering SCR 13 and allowing current to flow 
through heater 17 and thermal relay 18. The contacts 23 and 24 are closed, 
supplying current to the load terminal. 
When terminal L is positive with respect to terminal N, the SCR is held 
off. However, the contacts 23 and 24 of thermal relay 18 are held closed 
by the residual heat in the thermal relay and the load terminal LD is held 
on. 
A second embodiment of the invention is exemplified in FIG. 3 comprising a 
switch 25 similar to switch 10, and further including a variable resistor 
26 in the leg of the voltage divider including the photosensitive resistor 
21. Variable resistor 26 varies the operating point of the voltage on gate 
16 by varying the ratio of the resistance in the two legs of the voltage 
divider. By increasing the resistance of resistor 26, the SCR is actuated 
at a threshold higher light level corresponding to a commensurately lower 
resistance required of photosensitive resistor 21. Similarly, by 
decreasing the resistance of variable resistor 26, the SCR is actuated 
only at a threshold lower light level corresponding to a commensurately 
higher resistance required of photosensitive resistor 21. 
With reference to FIG. 4, a third embodiment is exemplified providing a 
switch 27 having SCR 13, fixed resistor 20 and thermal relay 18 in the 
configuration described with respect to FIG. 2. Switch 27 further includes 
a fuse 28 connected to the junction between SCR anode 14 and resistor 20 
and to neutral terminal N. A photosensitive resistor 29 and variable 
resistor 30 are connected between the other side of resistor 20 and line 
terminal L, and a diode 31 is connected across them with anode 32 
connected towards the line terminal and cathode 33 connected towards 
resistor 20. 
Switch 27 normally operates in the manner described with respect to FIG. 2. 
Fuse 28 and diode 31 serve to turn the switch off when SCR is shorted. In 
such a situation, during the half-cycle that L is positive with respect to 
N, current will flow through diode 31, through the shorted gate of SCR 13, 
and blow the fuse 28. 
Variable resistor 30 is shown by way of illustration only, and may be 
deleted. It serves to vary the operating sensitivity of switch 27 in a 
manner similar to the operation of variable resistor 26 in switch 25, 
shown in FIG. 3. 
It may thus be seen that the objects of the invention set forth as well as 
those made apparent from the foregoing description are efficiently 
attained. While preferred embodiments of the invention have been set forth 
for purposes of disclosure, modification to the disclosed embodiments of 
the invention as well as other embodiments thereof may occur to those 
skilled in the art. Accordingly, the appended claims are intended to cover 
all embodiments of the invention and modifications to the disclosed 
embodiments which do not depart from the spirit and scope of the invention 
.