Discusgeo
01-14-2004, 07:54 PM
I translated it and printed it all out. I have a friend who builds Ham Radios and things of that nature including his own circut boards. I will have him look at it this week. I will see if I can post the translation here.
You will need to go back and look at the pictures.
Also if you go down about 3/4 of the page you can download the document and you will also need photo editing software to view the pictures.
Ph meter translated I hope.
First approximation.
Since we are going to utilize a probes of pH standard, we will begin for describing its operation to electric level. In theory, a probes of pH provides to its exit an inversely proportional tension al pH of the liquid, having like zero the pH neutro. That is to say, for pH 7, the tension at the outset is 0V; when the pH is greater that 7, the tension at the outset is negative and when the pH is more less than 7 positive, being the changes of, approximately, 60mV by degree of pH. For example, for pH 5, the tension at the outset would be of some 120mV and for pH 8.5 would be of approximately -90mV. I have said that this is the theory, because the reality is that, depending on various factors (quality and aging of it probes, temperature, etc.), is able that do not give exactly 0V for pH 7 ó that the increments of tension be not of 60mV by degree of pH. For all it, the circuit will have to be calibrated periodically so that its measure be reliable.
With these data, we will present now of schematic form what will do our circuit. We have the tension that arrives al circuit is:
In the first place, the tension of entrance is amplified carrying it to a convenient level. This amplificador multiplies the tension of entrance for 1.7 approximately, so that to its exit we will have:
Subsequently we have a circuit that reduces this tension of another tension of reference of 0.7V, with what to its exit we will have:
and once adjusted the amplificador of entrance and the reference of 0.7V will have:
That is to say, 100mV by degree of pH. Only it remains to add a voltímetro in a volt tenths scale to obtain a direct reading of the pH.
Besides there is a series of filters step-under with a frequency of cut of approximately 10hz to eliminate the noise and the interferences and some comparadores regarding two references of tension ajustables to generate the alarms.
The circuit.
In the figure 1 the plan of the circuit is shown. It is convenient to print it to continue this explanation. As it can be seen, the references of some components do not continue a logical order. This owes to the different modifications that has suffered the circuit to its final version.
The exit of it probes goes connected al amplificador U3 through a filter step under of 10hz formed by R1, C1 and the own internal resistance of it probes. These components and the trails that join with them U3 and al connector of it probes CON1, constitute an authentic antenna, for which the length of the trails to should be reduced al most minimum. The amplificador operational U3, together with R2, R3 and POT2 constitutes an amplificador not investor of profit ajustable between 1.5 and 2.5 approximately. This model has elected himself of amplificador operational (LF356) and this configuración due to its highest impedance of entrance, greater that a Tera-ohmio (1012 Ohmios). It is keep in mind that the impedance at the outset of them you probe can reach various Giga-ohmios. For you probe with at the outset more high impedance, is possible that have that to elevate more still the profit. In that case can be substituted POT2 by an ajustable multivuelta of 20K ó 50K. The condensers C2 and C3 are a protection against noise due to car-oscillation recommended by the manufacturer of the LF356 (National Semiconductor). In my design, all this block has armored of interferences by means of a metallic little box united to mass.
The divisor of tension formed by R8, R9 and POT1, constitutes a reference of tension ajustable among 400mV and 950mV. Since the current that circulates for him is minimum, the temperature of the resistances will depend almost exclusively of the temperature environment, and al to have equal coefficients of temperature, their relation, and therefore the reference of tension, they will be maintained stable.
This tension of reference is carried through the follower of tension U1D to the positive entrance of the restador constituted by U1A, R4, R5, R6 and R7. The purpose of the follower of tension U1D is independizar the reference of tension of the remainder of the circuit. To the negative entrance of the restador applies the exit of the amplificador of entrance U3. To maintain the precision of this restador is convenient that R4 to R7 be resistances of the 1% of precision. In the circuit themselves there are position of 22K, but can be of any value among 10K and 50K, provided that being all equals. The linearity of the circuit depends enormously of the equality of these resistances, so would not be useless to buy, for example, 10 resistances, and to measure them with a polímetro digital to elect four it more seemed possible.
Figure 1
The following step, formed by U1B, U1C, R23, R24, C4 and C5, is a filter step-under of two phases, with a frequency of cut of some 10hz, to eliminate the noise accumulated in the previous steps. In normal conditions of operation, the exit of U1C goes connected through the jumper J1 al voltímetro. On this point we have a tension of 0V to 999mV, that coincides with the rank of measures of the voltímetro, and corresponds to levels of pH between 0 and 9.99. Further on we will deepen in the voltímetro, but we say that a reading will give ourselves between 0.00 and 9.99 pertaining to tensions among 0V and 999mV. This exit also is utilized for the circuits of alarm of pH too much high and pH too much under.
The level of shot of the alarm of pH too much high remains set by means of the reference of tension constituted by R10 and POT3, while that of the alarm of pH too much under himself fixed by means of the reference of tension formed by R11 and POT4. Both can be adjusted approximately in the rank of 0V to 999mV. U2D is configured like comparador of tension; their positive entrance is connected to the exit of U1C whose tension corresponds al pH measured, through the follower of tension U2C, while their negative exit goes connected to the reference of tension pertaining to the alarm of pH too much high. When the pH measured be greater that the set by means of POT3, the exit of U2D exchanges to its equivalent, positive state to some 4V; in contrary case, exchanges to its negative state, that is to say approximately –4V. D3, R12 and R14 introduce a histéresis (differentiates among the tension of commutation of the state under al high and the tension of commutation of the high state al under) of approximately 0.12 degrees of pH. If we call VIN to the tension in the positive entrance of U2D, VOUT to the tension in the exit of U2D and VD3 to the fall of tension in the diodo D3, the value of the tension of histéresis can be calculated as:
and since at the moment of the commutation the tension in the two entrances of U2D is practically equal, we would have:
being VALR The tension set for the alarm of pH too much high. For example, if we have set the alarm so that shoot to pH 8, VALR will be 0.8V and the histéresis will be:
That equals to 0.12 degrees of pH. That is to say, that the alarm will shoot itself when the pH rise from 8.00, but itself will not be defused until descend of 7.88. The level can be modified of histéresis changing the value of R12 ó R14 in accordance with the previous formulae and even can be reduced to 0 eliminating R14 and D3, but if intends to utilize the exit of alarm to activate a corrector device of the pH, always there will be that to leave something of histéresis so that said device do not receive very short impulses, but walk during more long periodsTo correct the level of pH.
When the exit of U2D is positive, that is to say when the pH is greater that the set for the alarm of pH too much high, T1 enters conduction and circulates current through D5, R22 and D7, so that the LED is lit. Of the same way, if to the exit of alarm is connected the LED of an optoacoplador, this will conduct through R20 and will activate the external device. With these values, the cable that goes to said external device can measure to 5 meters without need of apantallado. For more long lengths would be able to substitute R20 by a resistance of 150 ohmios. When an external device be connected, will be done always through an optoacoplador, keeping in mind that the common one of the connector corresponds al ánodo of the LED internal of said optoacoplador. In the figure 2 the form can be seen of conexionado.
Figure 2
The circuit of shot of the alarm of pH too much under is similar, unless the entrances have been invested al comparador and a percentage has been set of histéresis somewhat smaller, keeping in mind that the tension of shot set for this alarm will be more less than it set for the other. Concretely, if we set the alarm of pH too much under in 5, we will have a histéresis of 0.11 degrees of pH. As before, it can change the value of R13 ó R15 to obtain the histéresis desired.
They remain a pair of details by commenting. In the first place to cause to note that, although only there is a LED warning of alarm (D7), the exits to control external devices are independent. On the other hand, to facilitate the adjustment of the alarms, the exits of their corresponding references of tension go connected al jumper J1, so that putting J1 in the position 3-4, the voltímetro will reflect the pH set for the alarm of pH too much high, and putting J1 in the position 5-6, the level set for the alarm of pH under. Finally, the condensers of filtered of the diet CU1 + and CU1- they should be it more nearby possible of U1, CU2 + and CU2- it more nearby possible of U2, and CU3 + and CU3- it more nearby possible of U3.
We will pass now to speak of the source of diet, whose plan appears in the figure 3. It is a matter of a symmetrical source of + 5V and –5V of little power to feed the amplificadores operational and the conversor analog-digital of the voltímetro and a second source of + 5V and more power to feed the circuits of more consumption, that is to say the display and the LEDs. The reason to implement this second independent source of the other, is to avoid that the disruptions caused by the lit and dull of the displays affect al circuit of measure. By the same motive the line of mass of this source has designed himself aside from the other, although in its origin be common.
Figure 3
The symmetrical source does not have greater mystery. The semiciclo positive of the tension provided by the alimentador external (that can be of 9V ó 12V but necessarily alternate) is extracted by means of D102. It is filtered by means of C105 and C106 and feeds al regulating U102. The exit of this returns to filter by means of C107 and C108. The semiciclo negative is obtained by means of D103 and is filtered by C109 and C110 before arriving al regulating U103. The regular exit is again filtered by C111 and C112. The regulating U102 and U103 work very under its maximum power, for which will not be necessary to add them disipadores of heat.
The source of power is something more complex. It obtains the semiciclo positive of the alternate tension by means of D101 and filters it by means of C101 and C102. As regulator has been utilized a L200 instead of the typical one 7805 because in the tests was seen that this last one introduced large disruptions in the mass that did very unstable the circuit of measure, due to that in the peaks of current caused by the commutation among the displays of the voltímetro, the protection shot himself against sobreintensidad of the 7805. The L200 is something more expensive and requires additional components, but the result is worthwhile. R101 fixed the protection against sobreintensidad of U101 in approximately 1.5A while the network formed by R102, POT101 and R103 adjusts the tension at the outset. The exit of U101 is filtered by C103 and C104. Before putting the jumper J101 there will be that to connect a voltímetro digital in the rank of 20V among the test-points TP101 (positive) and TP102 (negative), and to adjust POT101 to obtain a reading of 5V. Later himself cortocircuita J101 and returns to do the regulation, adjusting it with load. The regulating one U101 dissipates enough power, for which there will be that to endow it of a disipador of adequate heat.
Figure 4
The last block is the voltímetro digital, that is shown in the figure 4. Its design is enormously simple due to the utilization of a couple of circuits integrated specialized, and corresponds almost exactly to a note of application of the manufacturer. The CA3162 (U201) is a conversor analog-digital complete, with exit BCD of 3 digits. Its entrance tensions rank goes since –99mV to + 999mV, with a precision of 1mV. The potentiometers multivuelta POT201 and POT202 regulate respectively the offset and the profit of its amplificador of entrance. Further on we will explain how to adjust them. The circuit integrated CA3161, U202, is a conversor BCD to 7 segments a little special, since includes in its logic the possibility to generate some special characters, besides the digits 0 to 9. In combination with the CA3162, is capable to show in the display the legend “-nn” for negative tensions to –99mV, “---“ For tensions more less than –99mV (out of rank by defect), and “EEE” for greater tensions that + 999mV (out of rank by excess). The point decimal among the first one and the second digits is maintained always on by means of R202. Only it remains to comment the importance of the condensers of filtered of the diet. Each one it should be placed it more nearby possible of the device al that go destined: C202 next to U202, C-DP next to the transistors T201 to T203, and C203 next to U201.
Practical execution.
The final design has been summarized in a printed plate of circuit of simple face of 14.9cm by 6.7cm. These measures, as well as the position of the connectors and the drills for the anchorage have come you imposed by the size of the box in which was going to to be located. Naturally, it can adapt to other greater of cash balance models with only utilizing a greater plate, or to redo the plate with other measures. The design here presented, and that can be seen in the figure 5, is only one of the possible although that yes, has been tested with results more than good.
Figure 5
Although I do not want to frighten nobody, I have to notify that this is not an assembly for principiantes, al less with the layout of trails and the chosen distribution of components. The density of components is high, the trails are relatively fine and the pads are quite small, for which is obliged to utilize a taladradora miniatura (of the ones that are used for modelismo) and brocas of 0.7mm for the majority of components. Nevertheless, it should not present greater complications for whom has done already some plates before. In my page is found a tutorial on printed production of circuits that can be useful. Also it is found in format Word 2000 compressed to descend it to him complete. Of any form and to confront its construction is convenient to be descended the fichero PH-TO PUT . ZIP that contains the following files:
PHMETER.DOC
This same document in format Word 2000.
FOTOLITO.TIF
Layout of the trails to create a fotolito.
SILKSCR.GIF
Distribution of components in the plate.
READY.TXT
Ready of components and budget in format text.
ARMOR.TIF
Flat it mentioned of the armor.
MECANI.TIF
Flat it mentioned for the mechanized of the box.
PHMETER.GIF
Plan of the main circuit.
SOURCE.GIF
Plan of the source of diet.
VOLT.GIF
Plan of the voltímetro digital.
It notes: some you program do not recover the ficheros TIF with the original size, for which I recommend that be utilized Adobe Photoshop or simply Kodak Imaging (that comes among the accessories of Windows 98), that yes do it correctly. For the fotolito, the better results are obtained with printers of injection, rising al most maximum the volume of ink in the configuración and utilizing special transparencies for this type of printer.
Once insolada and attacked the plate, there will be that to cut the surplus parts if there are them and to do the holes for the obsession of the plate to the box. Subsequently the drills for the components are done “rare”. I refer to the connectors, the switch, the disipador of U101 (if carries obsessions) and the armor of the amplificador of entrance, whose drills leave of the normal thing by its size or its form. To carry out holes extended (for example for the connector of diet) I recommend to do various very together drills and then to unite them using the broca to way of fresadora with care for not leaving it. Then the drills for the regulators are done, jumpers, test-points, C101, D101, D102 and D103 with a broca of 0.9mm. If a long base of legs is utilized to elevate the display, also there will be that to utilize for him the broca of 0.9mm. Finally they are done the remainder of holes with a broca of 0.7mm. Only it remains to remove the rebabas that have remained in the copper with a piece of sandpaper superfina and to clean the serrín that have remained to have the plate prepared.
Once it manufactured the printed plate of circuit and with all the done drills, the 9 bridges will be welded putting special care so that the two that go situated under U201 do not enlarge too much. Subsequently the bases are mounted for all the circuits integrated, the jumpers and the test-points. Later they are adjusted and weld the connectors, the switch, the disipador of U101 and the base for the display. This it is the best moment to do the mechanized of the box, by if there was that to do small corrections in the position of the connectors or the switch. It is convenient to do various orifices in the nearby zone al disipador of U101 to guarantee a correct cooling. Later resistances are mounted, condensers, diodos, transistors and regulating (care with the orientation of electrolytic condensers and diodos). The regulating one U101 should be screwed to its disipador (does not do lack rock aisladora) before welding it, so that adjust perfectly. The last components in being soldiers are the potentiometers, putting care of not heating them too much.
The following step is a little delicate, therefore we have to puncture U3 in its base (verifying its correct orientation) and later to weld the armor of the amplificador of entrance. To avoid that U3 be damaged in this process (the LF356 is very sensitive to the electricity static) is convenient to connect the mass of the circuit to it takes of land of an electrical socket by means of a cable and then to weld the four ducklings of the armor in its place. Only it remains to puncture the 3 displays and the remainder of circuits integrated putting attention to its orientation. Finally, the result should be something seemed to the figure 6.
Figure 6
If the box in which is going to locate the circuit is metallic, there will be that to be assured that remain connected to the mass of the circuit so that act like blidaje. I it do not recommend, by the problems of corrosión that involves to utilize a metallic box near salty water. In its place I utilized a box of plastic although to change I had to add another plate that acted like armor under the main plate as is shown in the images 7 to 10. To assure the electric connection among the mass of the circuit and this plate, I welded it a terminal that coincides with the tornillo of obsession situated together al connector BNC. Setting it al bottom of the box, simply I hit it with pegamento of contact with the face of copper downward. The possible solutions can be various and depend on the box utilized, but should be assured the correct armor against interferences of, al less, the zone of the circuit that contains the amplificadores operational U1, U2, U3 and the conversor analog-digital U201.
Figure 7
Figure 8
Figure 9
Figure 10
Before mounting the circuit in the box, there will be that to finish the mechanized of the box. One must prepare some form to be able setting it. In my case intended hanging it in the wall, so I did a pair of orifices in form of pear in the rear one of the box to be able then holding it with a pair of tornillos as can be seen in the figure 11. This it had been predicted before drawing the circuit, so that in the zones of the plate that will coincide on these tornillos himself not trails have designed, to avoid that these be damaged for the graze of the tornillos. To do it a hole of 8mm can be practiced and another of 4mm and then to cut the surplus with a segueta of marquetry. Finally, in the cover or frontal of the box there will be that to trim the window that permit to see the display and to practice a drill of 5mm for the LED of alarm. I covered the window for the display with a plaquita of metacrilato of color smoked (trimmed of the case of a tape of cassette). The final aspect of the hood can be seen in the figure 12. The last step is to mount the plate in the box and setting it, as is seen in the figure 13. The meter of pH functioning can see in the figure 14.
Figure 11
Figure 12
Figure 13
Figure 14
Adjust and set in motion.
Figure 15
If the adjustment of any circuit is important, in this case is fundamental, since is a matter of a precision measure instrument, and an incorrect adjustment would throw for land its accuracy. I recommend to utilize for the adjustment of the potentiometers multivuelta a screwdriver of ajustador. Is not very expensive and is almost indispensable among the tools of any fan to the Electronics. Likewise, before connecting the diet al circuit, is very recommendable to verify again that the circuits integrated are correctly placed, given the tendency of these devices to achicharrarse when it do not be.
In the first place we will adjust the power diet source, helping us of the figure 15. For it there it will be that to put a voltímetro digital in the continuous current scale of 20V and to connect its pinzas to the test-points TP101 (positive) and TP102 (negative). The bridge is removed J101 and is connected the alimentador. Once it activated the switch, the voltímetro should give a reading from among 4V and 6V. Now one must act on the potentiometer POT101 until the reading be 5.00V. To finish this adjustment itself cortocircuita J101 and is verified that the tension measured by the voltímetro continue being 5.00V, retouching POT101 if is precise to achieve it. Since we will not return to touch this potentiometer is recommendable to seal it applying a nails shellac drop.
Figure 16
Subsequently we will adjust the voltímetro. The zone that interests appears us in the figure 16. This adjustment is of great importance, and of its correct execution will depend in great measure the linearity and the precision of the assembly. We will put the jumper J1 in the position 7-8 and we will adjust POT201 until obtaining in the display a reading “0.00”. We put a voltímetro digital in the continuous current scale of 2V and we connect its pinzas to the test-points TP2 (positive) and TP1 (negative). We put the jumper J1 in the position 3-4 and we adjust POT3 (we are going to use as tension for the adjustment one of the references of the circuits of alarm) until obtaining in the polímetro a reading of 0.700V (700mV). Now we adjust POT202 until the display of the meter of pH show a reading of “7.00”. We return to put the jumper J1 in the position 7-8 and we retouch POT201 so that the display mark “0.00”. Finally, we return to put the jumper J1 in the position 3-4 and we retouch POT202 until the display indicate “7.00”. Only it remains to seal POT201 and POT202 with shellac of nails and to put the jumper J1 in the position 1-2, that is that of normal operation.
Calibración.
Figure 17
The correct one calibración of the meter of pH is essential, and should be carried out periodically (the characteristics of them you probe vary with the time due to the filth accumulated and to its own wear) and when be substituted or it be cleaned probes. Carrying out will be necessary two solutions buffer of pH 7 and pH 4 (is not valid to utilize buffers of pH 10, since the scale of measure goes from 0 to 9.99). Before each calibración is convenient to clean probes it.
Fijémonos in the figure 17. With the jumper J1 in its normal position (1-2), we will begin by introducing probes it in the solution of pH 7. We will leave to pass some seconds until the measure indicated in the display be stable and we will act on the potentiometer POT1 until obtaining an indication in the display of 7.00 (if the solution buffer indicates some correction in function of the temperature, there will be that to apply said correction). Subsequently, we wash probes it and we introduce it in the solution of pH 4. We wait for that the measure be stable and we adjust the potentiometer POT2 until obtaining a reading in the display of 4.00 (or the value indicated for the temperature of the solution buffer).
In the first one calibración or al to change of probes, there will be that to repeat this several process times, until the measure be correct for both solutions. In the calibraciones periodic, this condition will be achieved with a pair of repetitions.
Adjustment of the alarms.
To adjust the alarm of pH high, we will put the jumper J1 in the position 3-4. In this position the display visualizes the value of pH al that this alarm will shoot himself. Acting on the potentiometer POT3 will set the exact level of shot. It is keep in mind that once shot the alarm of pH high, this will continue assets until the pH descend 0.12 degrees approximately, under the level set.
To adjust the alarm of pH under, we will put the jumper J1 in the position 5-6. In this position the display visualizes the value of pH al that this alarm will shoot himself. Acting on the potentiometer POT4 will set the exact level of shot. Once shot the alarm of pH under, this will continue assets until the pH rise 0.12 degrees approximately.
To avoid that the alarms be always shot, there will be that to set them in values separated al less 0.15 degrees. If himself not indication of some of the alarms is needed, should be adjusted to a level “unattainable”. For example, to annul the alarm of pH under, can be adjusted to 0.00 degrees of pH.
Once adjusted the alarms, is important not to be forgot to return to put the jumper J1 in its normal position of operation, that is to say 1-2.
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