Instrumentation (Page 3)

This a part of a complete Benchtop DMM that you can build to learn the internals of a Digital MultiMeter. This is oriented towards learning the measurement aspects and the protection of input circuits.

U2A opamp LF353 is used here as a constant current source, R10-D13-D14 is for protection in case you measure voltage in the ohms range. U1 4052 helps digitally select four different currents, the currents pass thru the unknown resistor to be measured and an voltage developed across the resistor is measured.

U2B is a buffer which passes on the voltage measured to U5A for inversion of polarity as current source is a negative current (current sink). U5B amplifies to the level required for the LM3914 display circuit. U7 555 is used as a de-bouncing for switch SW1 so as to advance counter 4029 to change the range of resistance measurement.

Build a DMM - LM3914 Analog display

Use any FET input dual opamp, TLO72 will work but LM358 will cause error as it is transistor input..

D1, D12 and R8 are to ensure that the FET can be turned off, as the opamp swings from +/-3.5V only, with some FET it needs to be tweaked. LF353 is a Wide Bandwidth Dual JFET Input Operational Amplifier.

Build a DMM – LM3914 Analog display

U1A LF353 opamp turns on a buzzer when the voltage at Test + falls very low indicating a short between Test + and Test – or a resistor less than 5 ohms. The R9-R14 divider determines the minimum voltage that can be at Test + for buzzer to just turn on.

The Opamp here is a comparator and R10 10 Meg gives a very small hysteresis so that there is no oscillation at threshold levels, that is when both inputs at same levels.The Zener is for protection and R6 to limit current.

U2A and U1B opamps form a precision rectifier, note that this is not a true RMS rectifier circuit for that see some devices from Analog Devices. When you need to rectify a 200mV AC signal you cannot use a diode like 1N4148 as the diode turns on at 700mV so this circuit is used.

AC conversion and continuity buzzer test

The AC signal measurement is best to do with a True RMS convertor. This Type is able to quantify the Heating power of a AC voltage, includes all harmonics and Noise too.

The Early Microcontroller : 80C39 is a CMOS Version of 8048 the one that preceded 8031-8051. MCS48 is the set for that, MCS51 is the current set.

Here is an example project for 80C39-8748-8749 microcontroller. This code was written by me, and it works. But the documentation is not complete or may have errors.

80C39 and MCS48 based Process Controller

Process Controller code here 80C39 Code for MCS48 (editable spreadsheet online)

The circuit for these are on this page along with other circuits. This may be difficult to put together now. But the code and hardware is near compatible to 8051 uC. Now there are may types of ADC and uC.

This was supposed to have Analog in and Analog out. Some parts of the circuit may be in the 80C51 page. This has ramp-up and ramp-down settings for the Increment and Decrement buttons. This helps setting setpoint quickly. There is also debouncing for the pushbutton.

This was designed, keeping in mind, that sometimes we have few components available and you could not carry your DMM somewhere, but you have to make some instrument for a quick need with what is available.

Analog Dial AVO Amps-Volts-Ohms meter

IC3 LM555 is to generate -5V from 9V. That reminds me of a story, In 1986 while i was studying BE electronics i got a chance to work part-time in a company that sold-serviced imported instruments, it was called trans-marketing. They were the agents for Racal-Dana, Genrad, Data I/O and so many more. Here i came across a Book called CMOS Hot Ideas of Intersil. I had a chance to observe, build and learn some things here. The ICL7107 circuit was the most fascinating, I also read the books of National Semiconductor here.

Analog Dial AVO Amps-Volts-Ohms meter

In those days small firms used to make DPM’s digital panel meters with 7107. In 1987 when i got a oppurtunity to design a DPM for a firm, I put the 555 clock in place CD4009 clock shown in intersil, to derive the -5.

This Circuit  is a DPM or digital panel meter. It has a analog bar graph display and a 3-1/2 digit digital display. ICL7107 is used in the 200mV configuration.

U4A opamp LF353 amplifies the 200mV Full scale input to the level required for the LM3914 display circuit. D13-D14 are clamping protection diodes. Adjust P1 trimpot for a reading of 1000 counts when a 100.0 mV signal is fed at Vin. Adjust R8 trimpot to get the 5th led  to just turn on at 100mV input.

Analog and Digital Voltmeter using ICL7107

A combination of digital and analog display is helpful for quick decision making. Analog indicates even from a distance the process dimension.

Human Brain understands analog better. The digital is required to note down and record values for determining a setpoint or performance of a system.

Sometimes a analog recorder with a ink-pen plot against time is a very good way of process analysis. Many systems are better studied using graphs not tables and lists of numbers. A Computer based data-logger gives greater power to this methods. You can measure and plot graphs of various types and at different points and for much longer periods.

This is a part of my Build a DMM or Digital Multi Meter

Here is a easy to read ‘Analog’ Millivoltmeter. Just like the Moving Coil Voltmeter, but does not have that resolution. This gives a easy indication of process progress or parameter magnitude from a large distance. A bargraph is easy on decision making too, compared to a digital readout.

How this Works ? – The analog input in mV – millivolts is fed to R18, RC reduces Noise and the Zener Clamps protect. The LF353 FET Opamp offers High Impedance as a Non-Inverting Amplifier, which nullifies measurement burden, Remember the Moving coil voltmeter loads the measured circuit, causing sizable errors. Then they invented the Vacuum Tube Voltmeter to solve this issue. An FET voltmeter is near ideal, they ought to have inventing this first.

The Zeners and C3 Plastic cap should not leak, even if they do it should be in Pico Amps. Get quality stuff and do a neat job putting them together. To master this leakage and other aspects, try building an Electrometer with CA3140. Another Measurement challenge is uV Microvolt measurements, you will be faced with new glitches in connectors and PCB due to thermocouple effects and contact resistance. A soldered joint near a hot resistor will set up enough thermal gradients and create many thermocouples all over the board. Try to measure 1 Microohm with a 10 Amp pulse or 1 A DC. You will learn many things. Connectors have a craze for the Precious Metal, they act funny if they do not have enough Gold on em.

Once i observed, very low voltages or circuits with nominal voltages but very low currents, cannot break a near invisible layer between the plates of a good connector. A sub-micron coat of corrosion, dust or even some organic deposit, was forming a dielectric layer which was impervious to uV and pA. A good cleaning with a volatile organic solvent solved the problem but messed up other plastics nearby.

Millivolt Meter using a LM3914 LED Dot Display

Millivolt Meter using a LM3914 LED Dot Display. – This circuit is a part of my Build a DMM or Digital Multi Meter

This circuit is a Parallel interface between 89C52 of Atmel with 7135 of Intersil.  With This circuit you can read analog data of both polarities. You can change the range scale with extra circuits, you can store data on a EEPROM or send them to PC thru RS232 or Comm port.

Using Linux in Embedded Electronic Devices

The Circuit Is Shown for One Anode Drive and one Segment Drive for Display. In similar Fashion connect rest, all 5 anodes and 7 segments and one dp decimal point. The +5 V of 2N2907 and gnd of BC547 must be directly from regulator with a big cap or even a separate supply.

AT89C52 Parallel Interface to ICL7135

Parallel interface is faster that a serial interface.It is more “Real Time” for a given processor and clock speed. The code and accompanying hardware also can speed up the data acquisition.

AT89C52 Parallel Interface to ICL7135 – Complete Page

A single maxim chip can act as a good interface between the uC and Serial Port. You could also do this with Logic chips but correct levels and isolation cannot be achieved, Hence better to use use these interface chips.

These days USB and Wireless Interfaces are being used and Tablet Computers becoming Popular. The Interfacing of the future is “Device Networking” and Wireless may be common.

Here is a current source you can build for resistance measurement. When the current is held constant, you know as per Ohm’s Law the Voltage across Resistor is proportional to Resistance value.

Precision Current Source for Resistance Measurement

The supply is +12 and -12, The total voltage across R6 + R7 is 24V. Then 24V / 120K = 0.2mA. The voltage across R6 is (10K * 0.2mA) = 2V. The same is reflected across R5 in this feedback configuration. That means Q3 is a 2V / 1K = 2mA source. If my calculations are right.

There are sources of errors in this circuit. The temperature variation of all resistor values, which is 100ppm for general calculations in 1% MFR. Let us assume you use OP07 which is close to an ideal opamp, but for this application it is not needed. The second error is Ib, the base current of Q3 which may be 0.2mA / Hfe(200) = ~ 1 uA. Then the variation of Hfe, Vcc and Vdd w.r.t. Temperature, should not be overlooked. Use LM7812 and LM7912.

So you see, design knowing that all these components are not ideal. Leakage currents, Humidity, EMI, Stray Capacitance and Inductance and much more. It is just like, even when the motor is fixed firmly on the machine, some parts Vibrate and create a Noise due to Mechanical Resonance. So Build and evaluate your design in the real environment, to learn.

Discover how resistors are color coded – Interactive Java Resistors Tutorial.