Safety device for socket which is connected to electrical power supply

A safety device includes a comparator circuit includes three resistors arranged in series. An IC has a positive pole coupled to the resistors and has a negative coupled to the positive pole of another IC which has a negative pole coupled to the resistors. A signal output circuit includes a transistor having a base connected to the ICs and having a collector coupled to a relay. The relay includes two common contacts for selectively contacting with two normally closed contacts and two normally open contacts so as to supply electric power to the common contacts when an electric appliance is coupled between the common contacts.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a safety device, and more particularly to 
a safety device for protecting sockets which are connected to the 
electrical power supply. 
2. Description of the Prior Art 
Typical sockets which are coupled to the electric power supply have no 
safety devices provided for preventing the shortage of the sockets. For 
example, when children plug an object into the socket, the children may be 
hurt by the electricity. 
The present invention has arisen to mitigate and/or obviate the 
afore-described disadvantages of the conventional sockets. 
SUMMARY OF THE INVENTION 
The primary objective of the present invention is to provide a safety 
device for enabling the sockets which are connected to the electric power 
supply and for preventing children from being hurt by the electric power 
supplied to the sockets. 
In accordance with one aspect of the invention, there is provided a safety 
device for a socket comprising a power supply means for converting an AC 
current to a DC current and including a first IC for stabilizing the DC 
current, the first IC including at least one output, a comparator circuit 
means including three resistors arranged in series and having a first 
point V1 and a second point V2 formed therebetween, a second IC including 
a positive pole coupled to the first point V1 and including a negative 
pole, a third IC including a negative pole coupled to the second point V2 
and including a positive pole coupled to the negative pole of the second 
IC, a third point VT being formed between the negative pole of the second 
IC and the positive pole of the third IC, the second IC and the third IC 
including an output, a first NAND gate coupled to the output of the second 
IC and the third IC and a second NAND gate coupled to the first NAND gate, 
a signal output circuit including at least one third NAND gate coupled to 
the second NAND gate, a transistor including a base connected to the third 
NAND gate and including a collector and an emitter, a relay coupled to the 
power supply means and including two first common contacts, a first 
normally closed contact coupled to the output of the power supply means, a 
second normally closed contact grounded, and two normally open contacts 
connected to the power supply means, an induction coil coupled to the a 
first of the normally open contact, a socket coupled between the two first 
common contacts, and a locking circuit including a fourth IC coupled to 
the induction coil and a fifth IC coupled to the fourth IC and having an 
output, a diode coupled to the output of the fifth IC and coupled to the 
third NAND gate. The common contacts are coupled to the normally closed 
contacts when the relay is not actuated. The common contacts are coupled 
to the normally open contacts when the relay is actuated, in order to 
supply electric power to the socket. The locking circuit maintains the 
common contacts in contact with the normally open contacts. 
Further objectives and advantages of the present invention will become 
apparent from a careful reading of a detailed description provided 
hereinbelow, with appropriate reference to accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a safety device in accordance with the present 
invention comprises a power circuit 1 including a transformer T1 coupled 
to an electric power supply for receiving AC current therefrom, and 
including a bridge rectifier D1, a capacitor filter C1 and a first 
integrated circuit U1 (abbreviated as IC hereinafter) for converting the 
electric power into a stable DC power output and for stabilizing the DC 
power output. 
A comparator circuit 2 comprises a second IC (U2) including a positive pole 
connected to one point V1 of a series of resistors R1, R2,R3, and a third 
IC (U3) including a negative pole connected to another point V2 of the 
series of the resistors R1, R2, R3. The negative pole of the second IC 
(U2) is connected to the positive pole of the third IC (U3) and has a 
point VT grounded via two resistors R4, R5. The output of the ICs (U2, U3) 
are coupled to a NAND gate G1 which includes an output connected to a 
resistor R6 which is connected to the third leg of the first IC (U1) via a 
capacitor C2 and which is connected to another NAND gate G2 of a signal 
output circuit 3. 
The signal output circuit 3 includes the NAND gate G2 having an output 
connected to the base B of a transistor Q1 via a resistor R7 and two NAND 
gates G3 and G4. The emitter E of the transistor Q1 and a capacitor C3 are 
grounded or are equivalently coupled to the second leg of the first IC 
(U1). The collector C of the transistor Q1 is coupled to a relay RY1 and 
an indicating light L1. A socket 5 is disposed in parallel to the relay 
RY1 and connected between two common contacts COM1 and COM2. The socket 5 
includes a normally closed contact NC1 connected to the third leg of the 
first IC (U1) via a diode ZD1, a normally open contact NO1 coupled to one 
pole of the AC power supply, another normally open contact NO2 coupled to 
the other pole of the AC power supply via an induction coil T2 which may 
convert the current to a voltage output signal, and another normally 
closed contact NC2 coupled to the resistor R5 at a contact VS so as to 
provide a voltage for comparing purposes. 
A locking circuit 4 includes two ICs (U4 and U5 coupled to the induction 
coil T2 for receiving the voltage output signal therefrom. Two diodes D2, 
D3 are arranged in parallel to each other and arranged in reverse 
direction for limiting the amplitude of the signal from the induction coil 
T2 to the positive pole of the IC (U4). The negative pole of the IC (U4) 
is coupled to the negative poles of both of the ICs (U3, U5). The output 
of the IC (U4) is coupled to the positive pole of the IC (U5) via a diode 
D4 and a resistor R8 which are arranged in parallel to each other. A 
capacitor C4 is coupled to the positive pole of the IC (U5) so as to 
stable the signal. The output of the IC (U5) is coupled to the input of 
the NAND gate G3 via a diode D5. 
In operation, as shown in FIG. 1, when the socket 5 is unloaded or no 
electrical appliances are connected to the socket 5, the two normally 
closed contacts NC1 and NC2 of the relay RY1 are coupled to the socket 5 
such that the socket 5 is not connected to the AC power supply. 
Accordingly, the children will not be hurt when the children plug objects 
into the socket. 
When an electrical appliance is plugged into the socket 5 and is switched 
on such that the socket 5 is loaded, a closed circuit is formed and 
includes the stable DC power output from the third leg of the first IC 
(Ul), the normally closed contact NC1, the load of the socket 5, the 
normally closed contact NC2 and the resistor R5 which is then grounded. A 
voltage VS will thus be formed and transmitted to the point VT which is 
connected to the negative pole of IC (U2) and to the positive pole of IC 
(U3). At this moment, VS equals to VT, and the resistor R4 has no voltage 
drop. The voltage of VT is compared in the comparator circuit 2. When VT 
is smaller than V1 and higher than V2; i.e., V1&gt;VT&gt;V2, both the outputs of 
the ICs (U2 and U3) will include higher voltages which are then 
transmitted to the resistor R7 of the signal output circuit 3 via the NAND 
gates G1 and G2. The resistor R6 and the capacitor C2 form a RC circuit 
for eliminating disordered signals. The high voltage from the NAND gate G2 
is transmitted to NAND gates G3, G4 via a delay circuit including the 
resistor R7 and the capacitor C3, in order to actuate the transistor Q1. 
The indicator light L1 is then energized and the relay RY1 is actuated in 
order to connect the common contacts COM1 and COM2 to the normally open 
contacts N01 and N02. At this moment, the external AC power supply is 
coupled to the socket so as to energize the electric appliance. 
The induction coil T2 may also receive a voltage signal VR from the AC 
power supply. The voltage is then transmitted to the IC (U4) via the 
diodes D2, D3 which may be provided to limit the amplitude of the voltage 
signal and to prevent the IC (U4) from being damaged. 
If the voltage V2 is far less than the voltage signal VR, the signal VR 
will be amplified by the ICs (U4, U5) and will become a high voltage which 
is then transmitted to the NAND gate G3 of the signal output circuit 3 via 
the diode D5. The signal output circuit 3 may be actuated continuously in 
order to energize the relay RY1 so as to constantly couple the common 
contacts COM1 and COM2 to the normally open contacts NO1 and NO2, such 
that the AC power supply may be constantly supplied to the electric 
appliance in order to energize the electric appliance. 
When the relay RY1 is actuated, the circuit including the diode ZD1, NC1, 
socket 5, NC2, and the resistor R5 is opened such that voltage VT is less 
than V2 which is less than V1, i.e., VT&lt;V2&lt;V1. A high voltage output will 
be generated from the ICs (U2, U3) via the NAND gate G1. A lower voltage 
may be formed in the resistor R7 when the high voltage signal is 
transmitted through the NAND gate G2. However, the locking circuit 4 has a 
high voltage output to the resistor R7 in order to actuate the signal 
output circuit 3. Therefore, the output signal from the comparator circuit 
2 may not be transmitted into the signal output circuit 3. The locking 
circuit 4 may thus amplify the current so as to continuously actuating the 
signal output circuit 3. The relay RY1 is actuated in order to constantly 
supply electric power supply to the socket 5. 
The relationships between the voltages VT, V1 and V2 are as follows: 
(1) when the socket is unloaded, V1&gt;V2&gt;VT, the NAND gate G1 has a high 
voltage output, the relay RY1 is off such that no AC power supply is 
coupled to the socket 5. 
(2) when the socket is shortaged, VT&gt;V1&gt;V2, the NAND gate G1 has a high 
voltage output such that the relay RY1 is off and such that no AC power 
supply is coupled to the socket 5. 
(3) when an electric appliance is plugged to the socket 5 and is switched 
on, V1&gt;VT&gt;V2, the NAND gate G1 has a low voltage, the relay RY1 is 
actuated. At this moment, the AC power supply is coupled to the socket in 
order to supply electric power to the socket 5. 
Accordingly, the safety device in accordance with the present invention is 
safe and has no electric power supplied thereto such that the children 
will not be hurt when the children plug any other object into the socket. 
Although this invention has been described with a certain degree of 
particularity, it is to be understood that the present disclosure has been 
made by way of example only and that numerous changes in the detailed 
construction and the combination and arrangement of parts may be resorted 
to without departing from the spirit and scope of the invention as 
hereinafter claimed.