Port-Powered Temperature Meter

This is a four-channel temperature measurmet adapter that works without external power supply. It will suitable for measureing temperature and logging its data with a PC. The circuit diagram is very simple and no adjustment is required, everybody will able to build it with ease :-)


Specs. Micro-controller ATtiny15L (Atmel)
Number of channels Four channels
Measurement Range -40°C to +105°C (0.1°C/step)
or raw A-D value
Measurement Error ±0.5°C (at room temperature)
Sensor 103AT (Semitec)
Power Supply Supplyed from COM port (typ. 5mA)
Cost Approx. 1200JPY (All parts)

HARDWARE

Micro-controller

I chose an Atmel ATtiny15L for this project. It is the only device that has a built-in 10bit A-D converter in the 8 pin AVRs. The A-D converter has a bandgap reference and differencial amplifire as its front-end. The AVR core is clocked by only internal RC oscillator (calibrated to 1.6MHz), any other clock souce cannot be used. Also 25.6MHz clock source that 16x multiplied from core clock is available for timer/counter. This means that a fast PWM output can be generated. Therefore the ATtiny15L has good analog I/O capabiltity.

In this project, the A-D converter is used as four channels, single-ended, no gain and VREF from Vcc configuration. However RSTDISBL fuse must be programmed in order to use pin #1 as one of the analog inputs, an AVR programmer that can program in HVS mode is required.

Power Supply

The devices that works on the COM port without external power supply, such as serial mouse, are powered from the COM port. When an application program opens COM port, ER and RS signals will go high. The high level voltage is from 6V to 12V at most PCs, and it can supply 5mA at least. This is sufficient for low power micro-controllers.

Sensors

Four 103AT precision thermisters are used as temperature sensor. Its variation is very small, its temperature - resistance error at room temperature is ±0.3°C. The error of series resisters should be within ±0.5% to enable calibration-free design.

FIRMWARE

The program only respond the values of each channel to the PC by trigger command. The temperature - resistance curve of the thermister is not linear so that the raw A-D value is linearlized and converted to temperature value in software process. When replace the thermister with any oters, the linearlization table in the source code must be re-built. The raw A-D value can also be read, it will be used as voltage meter.

The trigger command is one "T" or "R" character, returened results are the temperarute for "T" command, raw A-D value for "R" command. Each value is separated by a comma and terminated by a .

Free Firmware

ICL7107 Digital LED Voltmeter

This circuit is a digital voltmeter with LED display. It's ideal to use for measuring the output voltage of your DC power supply. It includes a 3.5-digit LED display with a negative voltage indicator. It measures DC voltages from 0 to 199.9V with a resolution of 0.1V. The voltmeter is based on single ICL7107 chip and may be fitted on a small 3cm x 7cm printed circuit board. The circuit should be supplied with a 5V voltage supply and consumes only around 25mA.



The use of 7805 5V voltage regulator is highly recommended to prevent the damage of ICL7107, 555 ICs and to extend the operating voltages.


Parts list of The Digital LED Voltmeter:

• R1 = 8K2 R1 = 8K2
• R2 = 47K / 470K R2 = 47k / 470K
• R3 = 100K R3 = 100K
• R4 = 2K R4 = 2K
• R5, R6 = 47K R5, R6 = 47k
• R7 = 0R / 4K7 R7 = 0R / 4K7
• R8 = 560R R8 = 560R
• C1,C5, C6, C8, C9 = 100n C1, C5, C6, C8, C9 = 100n
• C2 = 470n / 47n C2 = 470n / 47n
• C3 = 220n C3 = 220n
• C4 = 100p C4 = 100p
• C7 = 10-22u C7 = 10-22U
• D1, D2 = 1N4148 D1, D2 = 1N4148
• IC1 = ICL7107 IC1 = ICL7107
• IC2 = NE555 IC2 = NE555
• OPTO = CA 10 pin FTA = CA 10 pin

The digital LED voltmeter can also be configured to measure different voltage ranges and display higher voltage resolution.

Bench Power Supply Unit

Here is a rather novel Power Supply Unit (PSU) for the workbench which can deliver 0-15 volts and up to 0-3 amperes. I have not shown any mains fuses as this I will leave to your own devices. You should as a minimum have a fuse in the primary and secondary of T1.


The circuit is a little cramped, but this is because I will also be posting the circuit to QRP@WW on the packet radio system. There are two resistors in the circuit marked "*see text" and these set the maximum output current limiter to a suitable value. Omit the 0R2 resistor and the PSU will deliver 100mA maximum. In this event the output TR8 can be almost any medium power transistor such as the 2N3053, BFY51 etc. It must be mounted on a heatsink no matter what output you need.

The 6R8 resistor may be an ordinary 1/4 watt resistor and is used on all versions of the PSU. The additional 0R2 (0.2 ohms) sets the maximum current limiting to about 3 amperes. If you only want a 0-1 ampere PSU then use 0.68 ohms. T1 is a simple transformer suitably rated for the output power you want and having a 15 volt secondary. This should give over 21v of DC when rectified (measured at D1 cathode - ground). The anode of D7 should also have -21 volts with respect to ground.

If you have problems getting a 15 volt transformer then you can use a 12-volt transformer and add a few extra turns to the secondary. Most transformers have enough room for you to thread another 20 turns of wire through the former to put in series with the secondary winding. By sure to use the same thickness of wire as used for the factory made secondary.

D1, D2, D3, D4, D5, D6, D7 and D8 are all 1N4001 or better for up to 500mA but 1N5401 should be used if you want up to 3 amperes. D9 and D10 may be almost any small silicon diode such as 1N4148 etc. The 1K0 potentiometer in parallel with D9 is the current set pot. The 47K pot is the output voltage pot. With the values shown you should be able to get a range of 0v to a little over 15v.

HOW IT WORKS

TR4 and TR5 compare the ground (0v) with the wiper of the 47K pot which should also be 0v. If the voltages are not equal then TR5 controls the output darlington (TR6 and TR7) until the voltages become equal. If the 47K pot is set to maximum (top) then the output voltage of the PSU must be zero in order to get a balance. If the output voltage pot is set to the bottom then the voltage balance will only occur if there is +15 volts on the top of the 47K pot; the other end of the 15K being -5v.

In the event TR7 draws too much current then the voltage drop accross the 6R8 resistor will turn on TR1 which in turn turns on TR2. TR2 sinks the 0v reference to -20 volts. The TIP41 is incapable of delivering a negative output voltage so the output remains at 0v. The base of TR1 is "lifted" by up to 0.7v due to the current flowing through D10. This overcomes the TR1 base-emitter voltage drop needed to turn it on. You can adjust the 22K resistor a little so that the current control goes all the way down to 0mA and no further. 5K6 is about the absolute minimum resistance you should use.

The power supply itself is very simple and is nothing more than a full-wave bridge rectifier. The AC from the transformer is also capacitively coupled to yet another rectifier to provide the negative voltage required to allow the PSU to be controlled down to 0 volts. TR3 regulates this to stabilise the negative reference voltage.

POSSIBLE PROBLEMS

In the event of self-oscillation of the PSU then under no circumstances put any capacitance accross any signal in the control path. This will just make the problem worse. The cure for this would be to add capacitance between the base/emitter of the controlling transistors TR4 and TR5.

Fluid Level Sensor # 2

This Fluid-Level Sensor circuit uses an AC-sensing signal to eliminate electrolytic corrosion on the probes. The AC signal is rectified and used to drive Transistor T1 that drives the relay. The relay is a 12-V type of your choice.


Transistor T1 can also be a TUP. Check out the TUP/TUN document for a large selection of European transistors and what this system is all about. Diodes D2 and D3 are both small signal diodes (1N4148). Diode D1 (1N4001) eliminates transients and possible sparking over the relay coil. Do not use a signal diode for this but a rectifier diode like the 1N4001 or other types of the 1N400x series. Resistor R2 controls the sensitivity. Also your choice. Select one between 10 and 22 Mega-ohm, or use a trim-pot.

The MC14093B is a CMOS quad 2-input NAND Schmitt trigger. The supply voltage can be between 3.0 and 18Vdc. It is pin-for-pin compatible with the CD4093. The capacitors are standard ceramic types but try others if you have them available. (download schematic)

Parts List:

• R1 = 470K N1,N2 = MC14093B
• R2 = 15M* T1 = 2N3906 (these will work also: PN200, 2N4413)
• C1-C4 = 2N2 (2.2nF) (NTE159, ECG159, BC557, BC157, TUP)
• D1 = 1N4001 Ry = Relay (12V or matching supply voltage)
• D2,3 = 1N4148 Sensor = Stainless Steel probes, brass, chrome, etc.

Please note:

Unused inputs MUST be tied to an appropriate voltage level, either ground or +12V. In this case, tie input pins 8, 9, 12, and 13 to either ground or +12v. Unused outputs (10 & 11) MUST be left open. You can use them as spares when needed.
In regards to the sensor, use your imagination. Stainless steel would be preferred but try other materials too. Depending on what type of fluid you use it for you naturally would choose your type of sensor which would resist corrosion for that particular fluid. I often use chrome bicycle spokes with very good success. The 'Sensor' works via the capacitive method.

The "RESET" switch in the circuit is optional. The relay can be replaced with anything you like; buzzer, lamps, other relays, etc.

Below are a couple valuable comments from Dave Burton of Burton Systems Software:

Thanks, Tony, for publishing your Fluid-Level Sensor design. I'm using it to detect sewer line plugs (water backing up toward the access port), and hot water heater / clothes washer / AC condensate pump overflows/leaks (water on the basement floor). It works very well.

Also, it says "the 'Sensor' works via the capacitive method." But I don't think that is correct. It would be more accurate to say that, for detecting fluids that are perfect insulators, the circuit CAN be made to work by detecting an increase in capacitance when the fluid replaces air in an air gap in the sensor.

But for the more common case of fluids that are not perfect insulators (like water on my basement floor), the circuit works by detecting resistive conduction through the fluid. It is lowered resistance that is detected, not increased capacitance.

To detect insulating fluids via the capacitive method would require good sized plates separated by an air gap, and careful adjustment of the sensitivity via R2 to distinguish between the possibly small change in capacitance due to the presence of the fluid. The difference might be small because there is only a fairly small differences between the dielectric constants of air and some common fluids. E.g., air has a dielectric constant of 1, and typical oils have dielectric constants of 2 to 5. Note, too, that desire to get a measurably large amount of capacitance leads us to desire that the gap between the plates be small (because the capacitance is inversely proportional to the distance between the plates), but the gap cannot be too small, lest capillary action hold fluid between the plates even after the fluid level has dropped below our sensor.

But to detect dirty water or tap water you can use almost anything: even a pair of bare wire ends several am apart works just fine.

Also, one handy feature not mentioned in the article is that several resistive "sensors" can be hooked up together (in parallel) to detect fluid at any of several different locations.

VO Fuel Controller

A small circuit to avoid cross contamination of diesel and VO in dual tank vehicles. This is a schematic for a vegetable oil fuel controller, the function is to enforce that VO goes back to the VO tank and diesel goes back to the diesel tank.

Notable Parts:

1. K1 is the return fuel line relay
2. K2 is the send fuel line relay
3. S1 is the primary switch
4. S2 is the bypass switch
5. S3 is the purge button
6. S4 is the On/Off switch
7. LED1 is the return line indicator
8. LED2 is the send line indicator (download schematic)

Usage

When the vehicle is warm enough so that it can run on vegetable oil turn on S1. The send line will immediately switch to VO and the return line will stay on diesel for a user specified amount of time. To determine correct timing switch your engine to VO and time how long it takes for the diesel to be purge from the system. Now you set the time in the circuit by changing R1 to the correct value based on 1.1 * R1 * C2. To make it easier, I set C2 at 1000µF, so if you want about 45 seconds use the closest value below 45Kohms (45,000 ohms). In the circuit as set up above R1 is 39K ohms giving a timing of just under 45 seconds (1.1 * 39 = 42.9).

When you are a few minutes from home turn off S1 and press S3. By turning off S1 you will switch the send line back to diesel and by pressing S3 you will keep the return line on VO for a user specified amount of time. To set timing use the same value resistor for R4 as you did for R1.

If you stop for a short period of time and the engine is still warm enough to run on VO when you restart it then either switch on S2 for a minute or so or press the purge button. In either case you will bypass the on-delay timer and keep the VO going to the VO tank.

Caveats

Do not expect exact timing from this circuit because capacitors are not perfect and voltage leakage will increase the time to some extent. When I timed the above circuit I found that it varied approximately 2 - 5 seconds (though I used a stopwatch and might have hit the start early or late, so YMMV). The timing can also be affected by length of time of discharge of the capacitors. If you turn off the circuit and turn it on again pretty quickly the timing can be much shorter than expected. I do not consider this an issue because the time it takes for the vehicle to cool down should be well longer than the time it takes for the capacitors to discharge. If this does become a problem use a lower value capacitor and a higher value resistor, for instance you can use a 220uF capacitor and a 180K ohm resistor to get approximately the same amount of time but the timing errors I initially stated may become more noticeable.

Modifications

If you want the circuit to be more automated so you can just switch it on when you turn on the vehicle and it will wait until temperatures are high enough before switching from diesel to VO, just add a thermostat into the circuit directly before S1. Use a NO (normally open) thermostat set to close it's contacts when the desired temperature is reached.

Parts List

1. (1) 7805 voltage regulator
2. (2) 1N4148 diodes
3. (2) SPST switch
4. (1) DPST switch
5. (1) N/O momentary push button switch
6. (2) LM555 timer
7. (2) 1000uF polarized capacitors
8. (1) 0.01uF non-polarized capacitor
9. (3) 0.1uF non-polarized capacitors
10. (2) LEDs
11. (2) 500 ohm resistors
12. (2) 100K ohm resistors
13. (2) resistors chosen for timing value (R1 and R4)
14. (2) solid state relays capable of handling the current your solenoid valves draw

All capacitors should be rated at least 25 volts, anything higher is fine.
Resistors should be rated for 1/4 watt.
7805 is a generic voltage regulator, if it says 78L05AZ or something it's still fine. visit page

Scriptable Thermometer and Vending Machine

A unified thermometric controller that can be programmed with simple scripts, integrating the "classic" thermometer/controller pair.


You can build a variety of simple machines with the same hardware and a different script : a charting thermometer, a vending machine that dials your number when empty, a leavening cell

• thermometer with 1°F (0.5C) resolution
• four switch inputs, four relay outputs
• only an handful of cheap parts
• graphic LCD display
• script interpreter built-in, can run ASCII programs (basic-like)
• 6KB non volatile script memory
• full screen editor built-in
• modular C source code (need a graphic LCD driver?)
• Two applications provided:

CHARTING THERMOMETER


Load this script to make Scriptherm draw temperature graphs

VAR x,max,min
max = THERM
min = THERM
PRINT ' PROCESS TEMPERATURE'

WHILE 1
PRINT COM,THERM,CR
BOX x+1, 0, x+1, 63, 0
BOX x, 0, x, THERM-32
PRINT @ 7,0,'NOW',THERM, ' MIN',min,' MAX',max,' '
IF THERM>max THEN max=THERM ENDIF
IF THERM
IF IN THEN
max=THERM
min=THERM
IN=0
ENDIF
IF TIME >= 1 THEN
x=(x+1)%128
TIME=0
ENDIF
WEND


SMS PHONE-MAIL ENPOWERED VENDING MACHINE



Load this script to control a refrigerated vending machine that sends you a SMS phone mail message when empty.

VAR sel,sold
THRE=38  HYS=-2

PRINT
@ 1,0,'1)    COCA COLA',
@ 3,0,'2)    DIET COKE',
@ 5,0,'3)    ICE TEA'
BOX 0,0, 127,9

WHILE 1
IF TIME>30 THEN select(0) ENDIF
IF IN&32   THEN select(1) ENDIF
IF IN&64   THEN select(2) ENDIF
IF IN&128  THEN select(3) ENDIF
IF IN&8    THEN dispense() ENDIF
IF (IN&7) != sold THEN soldout() ENDIF
WEND

FUNCTION select(n)
IF n THEN
 PRINT @7,0,'     INSERT MONEY  '
 IF (sold & (1<<(n-1))) THEN     n=0     PRINT @7,0,'  CHANGE SELECTION!'   ENDIF ELSE   PRINT @7,0,'  SELECT YOUR DRINK' ENDIF  IF n != sel THEN   IF sel != 0 THEN     BOX 30,13+(sel-1)*16,100,12+sel*16, 0   ENDIF   IF n != 0 THEN     BOX 30,13+(n-1)*16,100,12+n*16   ENDIF   sel=n ENDIF IN=IN&15 TIME=0 FEND  FUNCTION dispense() IF sel THEN   IF sel==1 THEN OUT = OUT | 1 ENDIF   IF sel==2 THEN OUT = OUT | 2 ENDIF   IF sel==3 THEN OUT = OUT | 4 ENDIF   TIME=0   WHILE TIME<1 out =" 0" in="IN&(255-8)" sold ="="" sold =" IN&7" cpms="ME" cmss="1',CR">

Fluid Level Detector

The LM903 uses the thermal-resistive probe technique to measure the level of nonflammable fluids. A low fluid level is indicated by a warning lamp operating in continuous or flashing mode. All supervisory requirements to control the thermal - resistive probe, including short and open circuit probe detection, are incorporated within the device. The circuit has possible applications in the detection of hydraulic fluid, oil level, etc., and may be used with partially conducting fluids.


Circuit Operation

A measurement is initiated when the supply is applied, provided the control input pin 7 is low. Once a measurement is commenced, pin 7 is latched low and the ramp capacitor on pin 12 begins to charge. After 25 ms when switch-on transients have subsided, a constant current is applied to the thermo-resistive probe. The value of probe current, which is supplied by an external PNP transistor, is set by an external resistor across an internally generated 21V reference. The lamp current is applied at the start of probe current.

35 ms after switch-on, the voltage across the probe is sampled and held on external capacitor C1 (leakage current at pin 1 less than 1 nA). After a further 1.5 seconds the difference between the present probe voltage and the initial probe voltage is measured, multiplied by 3 and compared with a reference voltage of 850 mV (externally adjustable via pin 16). If the amplified voltage difference is less than the reference voltage the lamp is switched off, otherwise the lamp commences flashing at 1 Hz to 2 Hz. 10 ms later the measurement latch operates to store the result and after a further 8 ms the probe current is switched off.

A second measurement can only be initiated by interrupting the supply. An external CR can be arranged on pin 7 to prevent a second measurement attempt for 1 minute. The measurement condition stored in the latch will control the lamp.

PROBES

The circuit effectively measures the thermal resistance of the probe. This varies depending on the surrounding medium (Figure 1). It is necessary to be able to heat the probe with the current applied and, for there to be sufficient change in resistance with the temperature change, to provide the voltage to be measured.


Probes require resistance wire with a high resistivity and temperature coefficient. Nickel cobalt alloy resistance wires are available with resistivity of 50 mXcm and temperature coefficient of 3300 ppm which can be made into suitable probes. Wires used in probes for use in liquids must be designed to drain freely to avoid clogging. A possible arrangement is shown in Figure 2.

The probe voltage has to be greater than 0.7V to prevent short circuit probe detection less than 5V to avoid open circuit detection. With a 200 mA probe current this gives a probe resistance range of 4X to 25X. This low value makes it possible to use the probe in partially conducting fluids.

Using resistance wire of 50 mXcm resistivity, 8 cm of 0.08 mm (40 AWG) give approximately 8X at 25ßC. Such a probe will give about 500 mV change between first and second measurements in air, and 100 mV change with oil, hydraulic fluid, etc., in the application circuit. With an alarm threshold of 280 mV (typ) lack of fluid can readily be detected. As the probe current, measurement reference and measurement period are all externally adjustable, there is freedom to use different probes and fluids.

Another possibility is the use of high temperature coefficient resistors made for special applications and positive temperature coefficient thermistors. The encapsulation must have a sufficiently low thermal resistance so as not to mask the change due to the different surrounding mediums, and the thermal time constant must be quick enough to enable the temperature change to take place between the two measurements. The ramp timing could be adjusted to assist this. Probes in liquids must be able to drain freely.

I2C Temperature Sensor

The electronic circuit is Temperature Sensor with I2C program. It is a circuit that plugs into the link port of a TI-85, TI-83, or TI-92 calculator and displays the temperature on the screen. Currently, there is only software for the TI-85, but I plan to write some for the TI-92 also. The sensor circuit draws power from the link port, so there is no need for any external batteries. The overall size of the unit will depend on the size you make it. Mine is about .75" by .5".


Parts

• Two small switching diodes. I think that just about any kind will work.
• I used 1N914 ones.
• One small electrolytic capacitor. I tried 2.2uF, 10uF, and 100uF, and all of them worked.
• An LM75CIM-5 integrated circuit (IC) made by National Semiconductor. For a more detailed description, you can read the data sheets on it onNational's WEB page at www.national.com and also check theirdistributors. It is a surface mount chip, so it is pretty hard to workwith.
• A 2.5mm stereo plug and cord. You can buy one, but I just cut a calculator link in half.
• A kit that allows you to etch your own boards. You can buy these at Radio Shack for about $15 and they can be used more than once. You canalso use another method, but I found this to be the cheapest for workingwith surface mount devices.
• Construction Materials. Just general things like a soldering iron,solder, etc.

Directions

1. Come up with some way of using the surface mount IC. I etched my own PCB for it.
2. Solder all the parts for the circuit on the board or however you chose to make the circuit. Be sure to follow the schematic for this step. And make sure the red and white wires are connected correctly accordingto the schematic. The only part you don't have to follow is how youconnect the A0-A2 pins. You can find out what pins these are in the data sheets available on National's WEB page www.national.com read the next couple sections of these plans. If you connect them differently, you will have to change the chip ID which is explained below.

Software

The software I wrote to control the I2C Temperature Sensor is very simple to understand. Simply run the ZShell program and it will display the temperature on the screen in both Celcius and Farenheit. If an error message appears on the screen, this section will help you.

The program continuously updates the temperature about twice every second. To exit the program, simply press [EXIT]. There is also a feature that can be activated by pressing [F5]. This will allow you to change the chip ID that is set by the A0 to A2 pins on the IC. If this is set wrong, it will display an error on the screen. After pressing [F5], you can change the ID number by pressing [UP] and [DOWN]. Then, simply press [ENTER] and it will bring you back into the program. This also allows you to have up to 8 chips in the same circuit if you change around the circuit a little bit and give each one a separate chip ID set by the A0 to A2 pins. If changing the chip ID does not get rid of the error, make sure the plug is plugged into the calculator all of the way. If the error is still there, check over your construction of the circuit. If you have an error and change something external, either plugging in the cord or fixing something on the circuit, be sure to restart the program or alter the chip ID to restart the program.

A Tiny and Accurate pH-meter

This electronic circuit is a tiny pH-meter. It is very tiny: 11cm2 including the PSU circuit! The schematic is shown below. It is basically a simple gain/offset circuit with a high impedance input (several giga-Ohm) and frankly the explanation could stop here: anyone with an background in electronics can understand this. But I started to write a webpage about this, so let's try to do it right and describe the schematic.


Juste because it's damn small does not mean that you have to settle down for second best when it comes to performance. The repeatability is around 0.01 pH and the accuracy, while depending on how well you will calibrate it, is around 0.02 pH. The main characteristics are:

• very small footprint (11cm2)
• very lightweight
• pluggable module for easy replacement
• requires only an external transformer and a display unit to work
• slope/offset settings
• repeatability 0.01 pH
• accuracy 0.02 pH
• low power
• low-cost single-sided PCB
• total unit price (including case and display unit): less than 100 euros.

The circuit input is pin 15 of K1. The probe signal enters IC1 via an RC circuit designed to allow only relatively slow signal variations (and avoid getting parasite HF signals). IC1 is a CMOS op-amp and thus has a very high impedance. The gain of IC1 is adjusted with the potentiometer R14. C2 is there for the amplifier stability. The R5/R11 circuit is the adjustment of the amplifier offset which is necessary for a high-precision application like this (see calibration below).

Once the signal has been amplified it enters an offset circuit built around IC2. IC2 is a more classic TL081 op-amp commonly found in audio devices, among others. The offset is defined by two potentiometers R12 and R13. The first one is on the PCB and the second one on the front panel. This improvement on the original design (single pot) allows the range swept by R13 to be symmetric, albeit smaller than without R12. It can be skipped if you wish (those small SMD trimmers can be damn expensive...). The circuit is designed to provide an average offset of 2V.

After the offset circuit the signal passes through a voltage divider before reaching the display unit. The divider roughly changes the signal range to something that is acceptable for the display. The real setting will be done on the display itself which contains a multiturn trimmer to precisely adjust its input gain.

The voltages for the signal evolve in the circuit as follows:

• Before IC1: -0.414/+0.414V (this might depend on the electrode used and its age, hence the gain/offset control)
• After IC1: -2/+2V
• After IC2: 0-4V
• After the voltage divider: 0-140mV (roughly)
• After the on-display trimmer: 0-140mV
• On the display: 0.00 - 14.00 pH (the display measures mV but the decimal point is placed accordingly to show a 0-14pH range)

As you can see the electrode voltage is symmetric and must undergo a linear transformation to fit the 0-14 pH range. This is all very classic stuff... Note that even if the supply rails are at +/-5V the circuit can cope with a 0-4V signal because the output swing is almost equal to the rails (no 0.7V drop, more around 0.3V IIRC).

A little remark concerning the integrated power supply circuit: it is a very small circuit that supplies a maximum of 50mA. Be careful of you want to add a power LED or something like that as it might be too much for the circuit. Check the total power used by the circuit before adding extras.

PCB


The PCB is very small and you are advised to build it with through-hole mounting components if you're not familiar with SMDs. That means start the PCB design from scratch. I personally think that it looks much cooler with a small footprint... No other special remarks concerning the PCB, except that the PCBs that were manufactured were slightly different (see the photos below). This is actually also true for the schematic. No functional difference, but I changed from Protel to Eagle for designing the circuit so I had to reenter the schematic and PCB manually. Hence some differences in layout but this is not a big deal.


Component list
This is the list of components used in this circuit. I only mention the display and probe at this time as the other components are generic. Maybe more info will follow in the future.

A little link to the display unit used in this project. I chose this one because it has a nice 'pH' unit that can be activated on the display.

Another link to the probe used with this circuit (IIRC). Most probes should work but I only tested the circuit with this one.
Construction

Random remarks: start with the smallest components, go slow, don't forget to set all the solder bridges correctly on the display unit (what you want is a 0-200mV range, an appropriately set dot and 'pH' shown as the unit). Check your cables,... before powering up.

The cabling diagram of K1 is:

1. AC 1
2. GND
3. +5v OUT (to display)
4. SIGNAL OUT (to display)
5. R13 / 2
6. R14 / 1
7. -5V OUT
8. R14 / 3
9. AC 2
10. GND (from transformer)
11. GND (to display)
12. GND (to BNC input)
13. R13 / 1
14. R14 / 2
15. * 15: INPUT

ESR Meter

The ESR Meter is basically an AC Ohmmeter with special scales and protective circuitry. It provides a continuous reading of series resistance in electrolytic capacitors. It operates at 100 kHz to keep the capacitive reactance factor near zero. The remaining series resistance is due to the electrolyte between the capacitor plates and indicates the state of dryness. Capacitor termination problems also show up plainly due to the continuous ohmic reading.

The ESR meter uses 8 operational ainplifiers. An op-amp is an idealized basic amplifier with two inputs. The non-inverting input (+) has an in-phase relationship with the op-amp output, and the inverting input (-) an out-of-phase relationship. Op-amps are usually used with negative feedback and reach a stable operating condition when their two inputs are equal in voltage.

Op-amps IA & 1B form a regenerative 100 kHz oscillatnr circuit. Capacitor C1 is the basic tiining capacitor and RI is selected to set frequency. Diodes D2 & D3 clip the bottom and top of the output waveform so that the output level and frequency are resistant to battery voltage changes.

The oscillator output of op-amp 1B drives 10-ohm source resistor R8F. The test-capacitor, thru the test leads, couples this 100 kllz signal to 10-ohm load resistor R9F. The amount of voltage developed here is indicative of the capacitors ESR value. (The 10-ohm resistors determine the basic iieter scaling.)

Capacitor C3 blocks any DC voltage present on the test-capacitor. Diodes D4 & D5 protect the ESR Meter from any initial charging current to C3. Resistor R7 discharges C3 after test.

A DC operating bias of 0.55 V is established by diode D1 for the oscillator stage and for all subsequent stages, which are DCcoupled and operated class A. DC bias from D1 and ESR signal from R9F are combined at the input of op-amp 1D. Both voltages are amplified by 1D, 1C, & 2A. Each of these three stages has an amplification factor of about 2.8 due to the ratio of output-voltage to feedback~voltage at the (-) input, which is determined -by feedback resistors R13F & R14F, etc.

Op-amp 2D is configured as a peak-to-peak detector. when the in-corning AC signal goes more positive than the normal bias level of about 0.77 Volt, the output of 2D also goes positive. But it must go positive enough to overcome the voltage drop across diode D6 before a fully equalizing positive voltage can be fed back to the -(-) input thru R20 to stabilize the op-amp.

-Capacitor C4 is charged to the peak value of the AC signal and accurately represents the peak of the incoming AC signal. The voltage drop across the diode becomes almost inconsequential due to the feedback process, and the circuit works down to a few mV.

A similar action occurs during the negative peak, using D7 & C5.

Resistor R21 provides a constant minimum amount of negative feed--back around op-amp 2D. The negative feedback increases the op-amp bandwidth which, most importantly, keeps the amplifier input-to-output phase-shift low enough for proper circuit operation.

The two outputs from the peak-to-peak detector are connected to two high-input-impedance unity-gain DC amplifiers, which drive the 1 mA meter movement differentially.