You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     This invention relates to a control circuit for initiating flow into a scrub sink and, in particular, to a circuit containing a photo sensitive component mounted on the scrub sink for contact with the torso of the user. 
     In preparation for performing a medical procedure, a doctor carefully washes his hands before placement of gloves thereon. It is necessary to complete the washing and operate the scrub sink without the use of hands so as to maintain a sterile condition. To enable the washing operation to be conducted without direct contact of the hands with the flow control mechanism or scrub sink, the combination of a light beam and photocell has been utilized. Interruption of the beam by the hands and arms of the doctor initiates flow from the spout. As long as the hands or arms continue to interrupt the beam, the flow continues. This type of device suffers from a significant disadvantage in that the doctor must continually maintain his hands in position to interrupt the beam. Since the beam width determines in part the sensitivity of the device, the use of a narrow beam is favored. Thus, the movement of the hands and/or arms during the scrubbing operation is unduly restricted else the flow of water becomes repeatedly interrupted. 
     Attempts to direct a beam of light to the torso of the user and monitor changes in reflected light have proven generally unsatisfactory. Changes in ambient conditions along with variations in the type of reflective surface cause the operation of the circuit to be unpredictable. Thus, the monitoring of changes in reflected light levels at or near scrub sinks has not proven satisfactory in these types of flow control devices. 
     Another approach has utilized an under the sink light beam and reflections sensor to detect the presence of the legs of the user. One such device is disclosed in U.S. Pat. No. 5,412,816 wherein a tubular extension is affixed under the sink for detection of the change in light level caused by the legs of the user being within a few inches of the sensor. The problems arising from changes in ambient conditions are countered by use of a short focus sensor. Thus, the doctor&#39;s movements are quite limited else the flow into the sink stops. Other under the sink approaches to providing remote actuation of flow to a scrub sink have relied on leg-actuated levers. These mechanical systems are characterized by the problems inherent in all mechanical devices subjected to repeated use. Furthermore, a doctor using this type of scrub sink actuating system is limited in movement during use since the same position is maintained to insure flow. 
     The present invention is directed to a dark-initiated flow control circuit which operates essentially independently of the sensitivity of a sensor and does not reply on reflected light levels for operation. The circuit utilizes a light-responsive resistive component mounted on the exterior of the edge adjacent the user. Contact by the torso or masking of the incident light causes the circuit to actuate a valve in the liquid flow line. The circuit permits adjustment for changes in ambient conditions to provide reliable operation. Further, the flow control circuit can be retrofitted on installed scrub sinks without requiring removal or alteration of the scrub sink or its tub. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, a scrub sink providing hands-free user control of the entering fluid includes an electrically-responsive valve located in the liquid flow line to control the flow of liquid through a spout in the tub of the sink. 
     A light-responsive component having light and dark impedance states is mounted on the exterior of the tub on the edge portion proximate to the user. The position of the cell enables the user to either rest directly against the cell or stand in close proximity thereto thereby permitting lateral movement while maintaining the cell in its dark impedance state. 
     The invention includes a control circuit containing a differential amplifier circuit. One input signal to the amplifier is derived through the light-responsive component, typically a photosensitive cell, with the other input signal being derived from the voltage across an adjustable resistor. The adjustment of the resistor enables the control circuit to operate under differing ambient conditions. When the user blocks incident light from the photosensitive cell, the differential amplifier provides an output which results in an actuating signal being supplied to the valve in the flow line. 
     The placement of the photosensitive cell and the use of the dark condition to actuate the valve enables the user to freely move the arms and hands during a scrub without experiencing interruption in flow. Further, the torso can be moved laterally without causing a cessation of flow since the photosensitive cell is considerably smaller than the torso of the user. Thus, the invention greatly enhances the mobility of the user during a scrub and facilitates the process of hands-free scrubbing. 
     Further features and advantages of the invention will become more readily apparent from the following detailed description of a specific embodiment of the invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view in section showing a scrub sink utilizing one embodiment of the present invention. 
     FIG. 2 is an electrical schematic of a preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a scrub sink is shown comprising tub  12  having a distal edge portion  15  with spout  18  extending upwardly therefrom. The proximal edge portion  16  is shown to be thicker in cross-section with a flat exterior surface. A bottom drain  14  is located in the base of the tub and coupled to the drain pipe by a conventional threaded coupler. 
     A light-responsive component  17  is attached to the exterior surface of proximal edge portion  16  with the translucent cover  19  facing outwardly toward the user. Attachment to the tub may be permanent by use of epoxy or removably adhered by another adhesive. The electrical connection to the control circuit housing  23  is made via cable  20  which is attached to the exterior of the tub by means of removable adhesive fasteners  22 . As shown, a grommet  24  is used to provide a watertight seal in housing  23 . 
     The housing  23  contains the electrically-responsive valve which is of conventional design and is normally a solenoid controlled valve. In the embodiment shown, the flow rate and temperature of the water flowing through flexible connector pipe  22  to spout  18  is preset. The mechanism and valving used are commercially available and are not part of the present invention. The housing  23  is inserted into the water supply circuit and the valve therein is normally closed. An electrical cable  25  extends between valve housing  23  and the electrical circuit housing  26  which is connected to the local power supply through a wall socket. The means of attachment of the scrub sink to the wall are not shown. However, it should be noted that the only modification to the scrub sink is the attachment of component  17  and the cable  20  to the exterior of the tub. The housing  23  is attached to the rigid vertical pipe  30  of the water supply and is normally supported thereby. Thus, the present invention is well-suited for use in connection with presently installed scrub sinks. 
     The electrical schematic diagram of the circuit is depicted in FIG. 2 with the photosensitive component  17  which is a commercially available cadmium sulfide photocell with light and dark resistance states of 100 and 500 k ohms respectively. The component  17  is coupled between the voltage regulator  41  and the second or minus input terminal of the differential amplifier  42 . Resistor  43  couples the second input terminal to ground. A potentiometer or tapped variable resistor  44  is used to apply the reference signal to the first or positive input of the differential amplifier. The resistance establishes the baseline for the operation of the amplifier and can be changed to compensate for changes in the ambient light level. 
     The output terminal of amplifier  42  is supplied to the drive transistor  46 . When the photosensitive component  17  enters the dark state, the output signal from amplifier  42  drives transistor  46  into conduction and a drive signal is supplied to relay  48 . Relay  48  is connected in series in the low voltage circuit of step down transformer  51 . The drive signal closes the normally open relay  48  to actuate the solenoid water valve  50  and permit flow through the spout into the scrub sink. A diode  49  is coupled across the relay for transient protection. 
     The low voltage side of transformer  51  is used to power the electrical control circuit. A bridge rectifier  53  is used to rectify the 24 volt stepped down voltage from the transformer. The rectified signal is supplied to a fixed positive 9 volt regulator  41 . A ceramic capacitor  57  rated at 0.1 μf 16 v. is provided at the regulator for transient protection. The regulated output signal is provided to amplifier  42  and drive transistor  46  as well as being applied across the potentiometer  44  and the combination of photocell  17  and resistor  43 . In addition, a flow indicating light-emitting-diode  56  can be utilized in the circuit. A second differential amplifier  55  is shown in FIG. 2 connected in parallel with the amplifier  42 . The output signal from amplifier  55  is supplied to diode  56  for a visual indication of the condition for flow. 
     The transformer  51  is connected to the facility power supply and may be housed individually or in combination with the electrical control circuit. The photocell  17  and the cable thereto can be removably affixed to the exterior surface of the scrub sink thereby enabling the invention to be placed in use with presently installed scrub sinks. While the embodiment shown and described is intended for use with the water flow line to a surgical scrub sink, it should be noted that the invention can be used in other cases where hands-free fluid flow control is used. It is recognized that modifications and variations may be made in the invention as described without departing from the scope of the invention as claimed.

Summary:
A control circuit for flow control of liquid into a scrub sink wherein a differential amplifier is provided with an adjustable input signal based on the ambient condition and an input signal from a photocell which is mounted on the exterior of the edge of the scrub sink. An electrically responsive valve is in the flow line. The torso of the user contacts the photocell to initiate flow into the scrub sink