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.

Precision Audio Millivoltmeter

Measures 10mV to 50Volt RMS in eight ranges
Simply connect to your Avo-meter set to 50µA range

Parts:

• R1_____909K 1/2W 1% Metal Oxide Resistor
• R2______90K9 1/2W 1% Metal Oxide Resistor
• R3_______9K09 1/2W 1% Metal Oxide Resistor
• R4_______1K01 1/2W 1% Metal Oxide Resistor
• R5_____100K 1/4W Resistor
• R6_______2M2 1/4W Resistor
• R7______82K 1/4W Resistor
• R8______12K 1/4W Resistor
• R9_______1K2 1/4W Resistor
• R10______3K3 1/4W Resistor
• R11____200R 1/2W Trimmer Cermet
• C1_____330nF 63V Polyester Capacitor
• C2,C3__100µF 25V Electrolytic Capacitor
• C4_____220µF 25V Electrolytic Capacitor
• C5______33pF 63V Polystyrene Capacitor
• C6_______2µ2 63V Electrolytic Capacitor
• D1-D4___1N4148 75V 150mA Diodes
• IC1_____CA3140 Op-amp
• IC2_____CA3130 Op-amp
• SW1_____2 poles 5 ways rotary switch
• SW2_____SPDT switch
• J1______RCA audio input socket
• J2,J3___4mm. output sockets
• B1______9V PP3 Battery
• Clip for PP3 Battery

Notes:

• Connect J2 and J3 to an Avo-meter set to 50µA range:
• Switching SW2 the four input ranges will be multiplied by 5
• Total fsd ranges are: 10mV, 50mV, 100mV, 500mV, 1V, 5V, 10V, 50V
• Set R11 to read 1V in the 1V range, with a sine wave input of 1V @ 1KHz
• Compare the reading with that of another known precision Millivoltmeter or with an oscilloscope.
• The oscilloscope reading must be a sinewave of 2.828V peak to peak amplitude
• Frequency response is flat in the 20Hz-20KHz range
• If you have difficulties in finding resistor values for R1, R2, R3 & R4, you can use the following trick:

R1 = 10M + 1M in parallel
R2 = 1M + 100K in parallel
R3 = 100K + 10K in parallel
R4 = 1K2 + 6K8 in parallel
All resistors 1/4W 1% tolerance

Sound Pressure Level Meter

Suitable for home-cinema setup
9 Volt operated portable unit

This project was requested by some correspondents wishing to setup their home-cinema set adjusting all the loudspeaker outputs to the same level when heard from the listening position.
In practice this device is a simple (though linear and precise) ac millivoltmeter, using an existing multimeter set to 50 or 100µA fsd with the probes connected to J1 and J2 to read the results.
The precision of the measure is entirely depending on the frequency response of the microphone used but, fortunately, for the main purpose of this circuit an absolutely flat response is not required. Therefore, a cheap miniature electret microphone can be used.

Use:

The amplifiers driving the loudspeakers must be fed, one at a time, with a sine wave in the 400Hz - 1KHz range, but different values can also be chosen. For this purpose you can use a simple signal generator circuit like one of those available on this site, namely: 1KHz Sine wave Generator or, better still, Spot-frequency Sine wave Generator. As an alternative, the input sine wave can be provided by a CD test track, a cassette-tape or a personal computer.

Please be careful and set the volume control very low, to avoid loudspeakers' damage. Switch-on the Sound Pressure Level Meter and increase the volume of the amplifier in order to obtain an approximate center-scale reading. Repeat the same steps with all channels.

Parts:

• R1______________10K 1/4W Resistor
• R2,R3___________22K 1/4W Resistors
• R4_____________100K 1/4W Resistor
• R5_____________100R 1/4W Resistor
• C1_______________1µF 63V Polyester or Electrolytic Capacitor
• C2_____________100µF 25V Electrolytic Capacitor
• C3_____________220µF 25V Electrolytic Capacitor
• D1-D4_________BAT46 100V 150mA Schottky-barrier Diodes
• IC1__________CA3140 Op-Amp IC
• MIC___________Miniature electret microphone (See Notes)
• J1,J2___________4mm Output sockets
• SW1____________SPST Toggle or Slider Switch
• B1______________9V PP3 Battery
• Clip for PP3 Battery

Notes:

• If external electret (battery powered) or dynamic microphone types are used, R1 must be omitted.
• Using a good quality microphone, this circuit can be very useful in setting-up parametric equalizers or tone controls in HiFi chains etc.

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.