End point quality control light circuit

An end point quality control light circuit for a water demineralizer. The quality control light circuit comprises a power cord, a housing for the electronics, and a probe-light assembly. The probe-light assembly includes a pair of spaced apart A.C. energized probes, and applies approximately 12 volts to the water. The probes are transformer isolated from the power input, and the final drive to an indicating lamp is transistorized. The current through the probes, directly proportional to the quality of the water, develops a voltage that is compared to a threshold voltage. The indicating lamp is illuminated to indicate high water quality, and is switched off by the final drive transistor when the water quality becomes unacceptable. An optional switching circuit, isolated from the quality control circuit, enables the actuation or control of external devices in dependence upon water quality. Various techniques for minimizing leakage currents are also disclosed.

BACKGROUND OF THE INVENTION 
The present invention relates to an end point quality control light circuit 
designed for specific use with a water purifier, or demineralizer, unit. 
Yet the inventive circuit finds application wherever the quality of an 
effluent is important. 
A demineralizer unit is disclosed in U.S. Pat. No. 3,245,537; this unit 
includes an integral end point quality control light. An improved, 
screw-in end point quality control light unit can be seen in U.S. Pat. No. 
3,334,745. Each of these patents is owned by the present assignee. In the 
latter of these known end point lights, a neon bulb, two resistors, a pair 
of probes, and power wires for bringing standard household current to the 
bulb and probes are encapsulated in a common housing. The housing is 
externally threaded to enable association with mating threads of the 
demineralizer unit's discharge spout. The probes reside in the discharge 
path of the water, and are excited by approximately 60 volts A.C.; water 
quality is indicated by the state of the neon bulb. 
This known end point quality control device is quite effective. However, 
recently enacted and expected codes, setting stringent requirements for 
electrical and electronic measuring and control instrumentation, indicate 
the possible need for redesign of such a device. As an example, the 
attention of the reader is directed to the American National Standard, 
Safety Requirements for Electrical and Electronic Measuring and 
Controlling Instrumentation (the ANSI Code), ANSI C39.5-1974. This code 
specifies safety requirements for devices such as end point quality 
control lights, and sets maximum permissible limits for electrical 
parameters such as insulation breakdown (2500v.) and leakage current 
(0.5ma). In this latter regard, a code enacted by the city of Los Angeles 
limits leakage current to 0.005ma for medical devices. 
It is toward the development of an end point quality control light circuit 
capable of meeting the most stringent codes, that the present invention is 
directed. 
SUMMARY OF THE INVENTION 
The present invention relates to an end point quality control light circuit 
that is powered by standard household current, that includes a pair of 
probes isolated from the source of power, and that applies only on the 
order of 12 volts to the water being monitored. The quality of the water 
is indicated by a high luminosity incandescent lamp switched by means of a 
sharp turn-off integrated circuit. The electronic circuit associating with 
the probe-light unit is totally enclosed, is transformer-isolated from the 
power source, and has extremely low leakage current and a high insulation 
breakdown point. An optional feature of the inventive device is a totally 
isolated switching circuit which follows the operation of the indicator 
light and which provides switching capabilities for external loads up to 
on the order of 4 amperes. 
Accordingly, it is the main object of the present invention to provide an 
end point quality control light for a demineralizer unit which is capable 
of meeting the most stringent electrical codes. 
A more specific object of the present invention is to provide an end point 
quality control light whose major components are isolated from the source 
of electrical power. 
Yet another object of the present invention is to provide an end point 
quality control light which applies a low voltage to the liquid being 
monitored. 
A further object of the present invention is to provide an end point 
quality control light circuit having exceedingly low leakage currents and 
an exceedingly high insulation breakdown point. 
Still another object of the present invention is to provide an end point 
quality control light having an indicating lamp of extremely high 
luminosity. 
A further object of the present invention is to provide an end point 
quality control light which responds quickly and accurately to the quality 
of the liquid being monitored. 
Another object of the present invention is to provide an end point quality 
control light having switching capabilities for external loads. 
Yet a further object of the present invention is to provide an improved 
circuit for monitoring the quality of an effluent. 
Still another object of the present invention is to provide a circuit, for 
monitoring the quality of an effluent, and for controlling external 
devices in dependence upon the quality of the effluent being monitored. 
The foregoing and other objects of the present invention, as well as many 
of the attendant advantages thereof, will become more readily apparent 
when reference is made to the following description, taken in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
With reference first to FIG. 1, the configuration of the inventive end 
point quality control light circuit will be described, in the environment 
of a water demineralizer unit. The inventive circuit is shown generally at 
10, and receives standard household 110-120v A.C. energization through a 
transformer 12, the primary winding 14 of which associates with a grounded 
male plug 16. Transformer 12 is of the type which has a high insulation 
breakdown point; one such transformer is manufactured by Signal 
Transformer Co., Inc. of Brooklyn, New York under Size 5, 12VA, Part No. 
241-5-24. The secondary winding of transformer 12 is denoted 18, and 
develops on the order of 25 volts from the input voltage across primary 
winding 14. In series with the secondary winding 18 of transformer 12, is 
a probe 20 and a dropping resistor 22, probe 20 taking the form of two 
conductive probe elements 24 and 26, adapted to reside within the water 
whose quality is being monitored. 
An operational amplifier 28 is a major component of the inventive circuit 
10, and receives the necessary operating bias from power lines 30 and 32. 
Line 30 is connected directly to one side of the secondary winding 18, 
while line 32 is connected to the opposite side of winding 18, through a 
voltage supply and rectifying diode 34, and a voltage dropping resistor 
36. 
The current flow between probe elements 24 and 26 is converted to a voltage 
and is sensed at the non-inverting input 38 of operational amplifier 28 
through a rectifying diode 40 and an input resistor 42. This voltage is 
compared with a fixed reference voltage appearing at the inverting input 
44 of operational amplifier 28, the magnitude of the fixed reference 
voltage being determined by a voltage divider network including a 
potentiometer 46, a voltage divider resistor 48 and an input resistor 50. 
Input resistor 50 and a positive feedback resistor 52 determine the gain 
of the operational amplifier 28. For example, if feedback resistor 52 is 
ten times the value of resistor 50, then the gain of amplifier 28 is 10. A 
filtering capacitor 54 is connected across the power supply lines of the 
operational amplifier 28, and serves to smooth the output of the supply 
voltage rectifier 34. The amplifier 28 is made to see a more constant 
impedance by the operation of an impedance matching capacitor 56 connected 
across the input leads 38 and 44 of the operational amplifier 28. 
An incandescent quality light 58 (24v rating) and an NPN transistor 60 
complete a series circuit with the secondary winding 18 of transformer 12. 
And as can be seen, current is permitted to flow through quality light 58 
when transistor 60 is in its conductive state, but is interrupted when 
transistor 60 is non-conductive. In this regard, the base 62 of transistor 
60 is biased by a voltage divider network including resistors 64 and 66. 
The collector 68 of transistor 60 is connected directly to the quality 
light 58, while the emitter 70 feeds to one side of the secondary winding 
18 of transformer 12. A bypass capacitor 72 is connected between ground 
and the secondary winding 18 of transformer 12. The function of capacitor 
72 is to bypass any leakage current across transformer 12 to ground, and 
it is this capacitor which enhances the leakage current properties of the 
inventive circuit. Capacitor 72 is, in particular, a polarized 
electrolytic of the tantalum wet anode variety such as is sold by 
Cornell-Dubilier as Type TX65B; the value of capacitor is between 10 and 
160 mfd, and is more preferably between 25 and 47 mfd. 
With continuing reference to FIG. 1, the operation of the inventive quality 
control light circuit will be described. Conductive elements 24 and 26 of 
probe 20, A.C. energized by secondary winding 18, are submerged in the 
water being monitored. When the water is pure, it exhibits a high 
impedance, and very little current flows between elements 24 and 26. 
Accordingly, a high voltage appears at the non-inverting input 38 of 
operational amplifier 28. The fixed reference voltage that is impressed 
upon the inverting input 44 is of a value less than this high voltage on 
the non-inverting input 38, and hence operational amplifier 28 issues an 
output which biases transistor 60 into its conductive state. When 
transistor 60 conducts, current flows through the quality light 58, and 
the light is illuminated. The illuminated condition of light 58 indicates 
high quality water. 
As the quality of the water deteriorates, the conductivity of the water 
increases. Accordingly, more and more current passes across the probe 
elements 24 and 26. The voltage on conductive element 24 therefore moves 
toward that potential of conductive element 26, and hence the voltage on 
the non-inverting input 38 of operational amplifier 28 decreases. 
Potentiometer 46, used to set the value of the reference voltage at input 
44, is preadjusted to a value so that the reference voltage equals the 
voltage on input 38 just when the quality of the water becomes 
unacceptable. As soon as the water quality becomes unacceptable, 
therefore, the voltage on non-inverting input 38 drops to a value less 
than the fixed reference voltage on inverting input 44. At this instant, 
operational amplifier 28 switches over, and the bias to transistor 60 is 
removed. Accordingly, transistor 60 reverts to its non-conductive state, 
and blocks the flow of current to the quality light 58. Quality light 58 
therefore turns off, indicating an unacceptable quality of water. 
With reference now to FIG. 2, the optional remote switching feature will be 
described. This switching option is shown generally at 80, following the 
end point quality control circuit 10 already described when reference was 
made to FIG. 1. 
The switching circuit 80 derives power from standard household 110-120v 
A.C., is completely isolated from circuit 10, and is actuated as a result 
of current flow through a light emitting diode 88 in series with the 
quality light 58 of circuit 10. Power is delivered to switching circuit 80 
from primary winding 14, through the means of power supply lines 82 and 
84, and a ground conductor 86 originates from the ground pin of male plug 
16. With the exception that the light emitting diode 88 is series 
connected to the quality light 58, the end point quality control circuit 
10 shown in FIG. 2 is identical to that shown in FIG. 1. Accordingly, 
common reference numbers are utilized. 
Associating with light emitting diode 88 is a phototransistor 90. Voltage 
dividing resistors 92 and 94 are connected, respectively, to the collector 
and emitter sides of phototransistor 90, and a bypass resistor 96 is 
shunted across the collector and emitter. 
A zero voltage switch 98 is the central element of the remote switching 
circuit 80. When the input terminal 100 of zero voltage switch 98 
experiences a voltage in excess of a predetermined threshold, then output 
pulses are issued at output line 102. This threshold is internally set by 
a comparator circuit, the wiring for which is indicated at shorted 
terminals 106. Phototransistor 90 is D.C. biased from a D.C. output lead 
104 of the zero voltage switch. 
The zero voltage switch 98 derives its power from a lead 108 connected to 
power supply line 82 through a voltage dropping resistor 110, and a lead 
112 connected to power supply line 84. The zero voltage switch 98 has a 
return line 114 associating with the internal voltage divider determining 
the threshold, and a feedback line 116 which biases the output stage of 
the switch so that output current capability is increased. In addition, a 
filtering capacitor 118 is utilized to smooth the D.C. output appearing at 
lead 104. 
The output line 102 of the zero voltage switch 98 is connected to the gate 
120 of a triac 122, the annodes of which are in series with the power 
lines 82 and 84 and load receptacles shown at 124 and 126. Therefore, when 
triac 122 is conductive, the load receptacles 124 and 126 are energized. A 
capacitor 128 associates with the supply line to zero voltage switch 98, 
and serves to internally widen the output pulses of the switch so that the 
triac 122 has sufficient time to reach holding current. 
The operation of the remote switching option will now be described. It is 
sometimes desirable to operate external devices, such as lights, motors, 
alarms, and the like, during the time when water quality is acceptable, 
and to automatically deactuate such external devices when the water 
quality drops. The remote switching option 80 provides such capabilities 
when external devices are plugged into the load receptacles 124 and 126. 
The switching option may also be utilized to control down-stream equipment 
such as water pumps, or could actuate a double-pole relay to switch from 
one water treatment leg to another. And the quality light 58 can be used 
with or without the switching option 80 in the circuit. 
It will be recalled that when the water quality is acceptable, current 
flows through the circuit including the quality light 58 and the 
transistor 60. With current flowing, the light emitting diode 88 is 
energized and issues a light signal that is received by the electrically 
isolated phototransistor 90. Phototransistor 90 then becomes conductive, 
shorting the bypass resistor 96 out of the circuit. This results in the 
emitter of phototransistor 90 going high (to on the order of 4.3v). Upon 
the emitter of phototransistor 90 going high and exceeding the threshold 
voltage of the zero voltage switch 98 (on the order of 3.1v), the switch 
98 begins to issue a pulsating signal at its output 102. A pulse is 
developed each time the A.C. supply voltage crosses zero. The signal at 
output 102 gates the triac 122 conductive, and hence closes the series 
circuit connection of load receptacles 124 and 126. Accordingly, the 
external devices connected to the load receptacles 124 and 126 are 
energized. 
When the quality of the water falls below the acceptable threshold, on the 
other hand, operational amplifier 28 switches over, transistor 60 becomes 
non-conductive, and hence light emitting diode 88 is turned off. 
Phototransistor 90 therefore also becomes non-conductive, and shunt 
resistor 96 returns to the circuit. At this occurrence, the voltage on the 
emitter of the photoresistor 90 drops, bringing with it, the input to the 
zero voltage switch 98 appearing at lead 100. The voltage on lead 100 will 
then drop below the internal threshold value of the zero voltage switch 
98, and hence the switch will turn off. Accordingly, triac 122 becomes 
non-conductive, opening the series circuit including the load receptacles 
124 and 126, and deactuating or switching whatever external devices are 
connected to the receptacles. 
FIG. 3 illustrates the overall configuration of the inventive device. As 
can be seen, the device includes three basic components. The first is a 
power cord shown generally at 130, and including the male plug 16 already 
described. A three-conductor cord 132 extends from plug 16 and enters the 
second basic component, a housing shown generally at 134. Cord 132 is 
strain relieved. Housing 134 supports the two load receptacles 124 and 
126, and encases all of the electronic circuitry. 
The third basic component is a probe-light assembly shown generally at 136. 
A four-conductor cord 138, also strain relieved, emerges from the housing 
134 and terminates in a probe casing 140 which encapsulates the quality 
light 58, and from which extends the conductive elements 24 and 26 of the 
probe 20. Casing 140 is of a transparent plastic material. The base of 
casing 140 is threaded, as shown at 142, to be received in the discharge 
spout of the demineralizer unit. The outline of a demineralizer unit is 
shown in phantom, but can be seen in detail by referring to U.S. Pat. No. 
3,334,745 mentioned above. A plug from a piece of auxiliary equipment 144 
associates with receptable 126. 
With reference now to FIG. 4, a first alternate circuit for minimizing 
leakage currents will be described. As in the circuits of FIGS. 1 and 2, a 
probe 20 in the form of two conductive probe elements 24 and 26 is adapted 
to reside within the liquid being monitored. In FIG. 4, probe elements 24 
and 26 are part of a resistive bridge circuit, the remaining elements of 
which are resistors 146, 148 and 150, and the inherent and varying 
resistance between probe elements 24 and 26, shown in phantom at 152. The 
bridge circuit is excited by a 24v A.C. source at terminals 154 and 156. 
The second stage of the circuit illustrated in FIG. 4 is an isolation stage 
158 comprising a pair of amplifiers 160 and 162 connected in unity, or 
buffer, configuration. Input 164 of amplifier 160 senses a threshold 
voltage set by resistors 148 and 150, and the 24 volt excitation source. 
The input 166 of amplifier 162, on the other hand, senses a variable 
voltage proportional to the conductivity of the liquid being monitored and 
dependent upon the inherent resistance (152) of the liquid. Amplifiers 
160, 162, and 182 receive +12 volts power supply voltage at terminal 168 
and -12 volts power supply voltage at terminal 170. The second input of 
amplifier 160 is part of a feedback circuit including line 172 and 
resistor 174. Amplifier 162 has a similar feedback path comprising line 
176 and resistor 178. A resistor 180 couples the respective outputs of 
amplifiers 160 and 162. 
A comparator 182 receives input from the respective isolation amplifiers 
160 and 162. The output of amplifier 160 is fed to a first input 184 of 
comparator 182 through a diode 186 and a resistor 188. The output of 
amplifier 162 is delivered to the other input 190 of comparator 182 
through a diode 192 and a resistor 194. A resistor 196 provides feedback 
for comparator 182, while resistor 198 connects input 190 to ground. 
The output of comparator 182 is fed to the gate of a transistor 200, the 
collector of which is biased by +12 volts, and the emitter of which 
associates with -12 volt bias and the coil of a control relay 202. Relay 
202 controls the operation of contacts 204 which opens and closes a series 
circuit including incandescent quality light 58. As illustrated in FIG. 4, 
relay 202 optionally controls contacts 206 for operating auxiliary 
equipment 144. 
The operation of the circuit illustrated in FIG. 4 is as follows. When the 
quality of the liquid being monitored is acceptable, comparator 182 issues 
an output which actuates relay 202 and closes the respective circuits of 
quality light 58 and the auxiliary equipment 144. When the quality becomes 
unacceptable, on the other hand, the conductivity of the liquid increases, 
representing a decrease in the value of resistor 152. Accordingly, the 
voltage appearing at input 166 of amplifier 162 would drop below the 
threshold voltage impressed at the input 164 of amplifier 160. The output 
of amplifier 160 appearing at input 184 of comparator 182 therefore goes 
above the output of amplifier 162, in turn, impressed at terminal 190 of 
comparator 182. Comparator 182 therefore turns off, releasing relay 202, 
opening contacts 204, and hence turning quality light 58 off. At the same 
time, relay actuated contacts 206 would open, and the operation of the 
auxiliary equipment indicated at 144 would switch over. 
The circuit illustrated in FIG. 4 serves well to minimize leakage currents, 
without the use of the transformer 12 illustrated in FIGS. 1 and 2. 
Amplifiers 160 and 162 are in buffer configuration, exhibit exceedingly 
high input impedance, and are virtually insensitive to changes in current. 
Because of this configuration, the input terminals of amplifiers 160 and 
162 experience almost identical inputs relative to ground, and hence 
leakage currents are rejected. 
Turning now to FIG. 5, there is illustrated still another embodiment of an 
input circuit for minimizing leakage current. Here, power is derived 
through a male plug 208, adapted to associate with standard household 
current. The plug 208 feeds the primary winding 209 of an isolation 
transformer 210, preferably a one-to-one type transformer, and including 
three internal shields designated 212, 214 and 216. As is common, internal 
shield 212 is connected to the ground lead of male plug 208, and forms a 
common chassis ground. The secondary winding 218 of transformer 210 feeds 
power to a circuit such as that shown in FIG. 1 immediately to the right 
of input transformer 12. It may of course be necessary to step down the 
voltage across the secondary winding 218 of transformer 210, or to utilize 
transformer 210 as a low voltage transformer by employing a step-down 
stage immediately after the input from household current. 
The third internal shield 214 of transformer 210 is connected to one side 
of the secondary winding 218 through a variable trimming capacitor 220. It 
is this trimming capacitor which results in low leakage current operation 
of the input circuit illustrated in FIG. 5. Capacitor 220 serves as cancel 
the inter-electrode capacitance between the windings of the transformer 
210, and is adjusted to minimize leakage current. This FIG. 5 arrangement 
can replace transformer 12 of FIGS. 1 and 2 or the high input impedance 
circuit illustrated in FIG. 4. 
Above, specific embodiments of the present invention have been described. 
It should be appreciated, however, that these embodiments were described 
for purposes of illustration only, without any intention of limiting the 
scope of the present invention. Rather, it is the intention that the 
present invention be limited not by the above but only as is defined in 
the appended claims.