METER CONVERSION

[Bigglez]

i am constructing this now, and have found (2) mistakes. c10 is not electrolytic because parts list designates as 2.2uf, 50-volt polystyrene or mylar. secondly, c10 and r25 are in opposite connection on pc component layout diagram. in other words, c10 goes directly to pin 6 of ic1 and other end of c10 goes to r25. other end of r25 goes to pin 2 of ic2. future issues of popular electronics do not publish this error, so i cannot confirm it, but i think that the hookup i just described is correct. feedback welcome.

I would speculate that C10 has to be close tolerance
to allow calibration. Common electrolytics are:
-10%,+50% or -10%, +100%, or even -10%, +150%
tolerance.

The capacitor does not have to be non-polarized
as long as the capacitor is placed correctly; the +ve
end goes to the meter diodes (in your diagram).

C10, R25 are in series. It does not matter which order
they appear in the physical layout. This unit probably
operates at audio (1kHz) and strays or correct RF
layout techniques do not come into play.

[MARK J. HABIAN]

i have just recieved a brand new blue lcd panel meter from hong kong for $8.00. only has 4 connections, 2 for power and 2 for input, great. power is supplied by 9-12 volts ac or dc, great.accuracy is +- .5%, great. there is no data sheet, how do i know what value shunt resistor to use? this is a dc ua panel meter to be used in a project that was designed to use a analog meter. i am going to install meter today in my cover. my question's answer will finish my project. please respond.

[Bigglez]

only has 4 connections, 2 for power and 2 for input
...
how do i know what value shunt resistor to use?

Check the DC resistance between all of the pins.
Most likely one side of the power is also connected
to one side of the input. If so, the power needs to be
isolated (i.e. use a 9V battery and separate on-off
switch).

THe meter measures voltage not current. It is
probably 0 - 199mV, but could be strapped internally
for 0-1.99V etc. Apply 100mV and check the display.

To read current place a resistor shunt across the
inputs.

R = V(fsd)/I(fsd)

For a 199uA full scale deflection, use 1,000 ohms.

[MARK J. HABIAN]

i have already sent circuit to forum some time ago. i cannot calibrate this thing. author says, word for word, with all wiring checked, connect a 10-volt dc voltmeter between the common and pin 6 of ic1. with the power turned on, the meter can indicate either positive or negative at this point. connect a resistance decade box between the r26 terminals on the board and start with a resistance of 100,000 ohms. increase this v alue until the dc voltmeter indicates zero. select a standard resistor nearest to the decade-box v alue, and use this for r26. the voltage at pin 6 should be less than 1 volt. first of all, what is he referring to as the common? second of all, what polarity of probes goes where? i have tried positived probe on pin 6 and negative probe on 2,4 and battery common. i get positive and negative readings with no appreciable change in numbers. calibration procedure continues after this stage is done. i am ready to give up on this project, i have already started another impedance meter. i am even trying to locate author. please respond. thankyou.

[Bigglez]

i cannot calibrate this thing.
...
first of all, what is he referring to as the common?
...
second of all, what polarity of probes goes where?
...
i have tried positived probe on pin 6 and negative probe on 2,4 and battery common. i get positive and negative readings with no appreciable change in numbers.

Common is the junction of the two 9V batteries.
What probes? Your test meter, or the project's probes?

Do you have an oscilloscope? If so is the internal 1kHz oscillator
running, stable and producing enough output?

Without that oscillator the impedance meter will be reading noise.

With the project's meter circuit disconnected (open C9) the
meter should read zero.

Have you tried feeding a test signal into the meter section
(i.e. at C9) to test the project's AC meter circuit?

If you don't have a suitable 1KHz sinewave generator handy,
try a Low voltage AC transformer fed from a variac. Even at
60Hz the meter section should read its amplitude correctly.

[Bob Scott]

Maybe you could use one of these, or strip it for the meter inside:


DC-A: 0-200µ; A-2000µ; A-20mA-200mA; Resistance: 0-200-2000-20K-200K-2000K ohm; DC-V: 0-200mV-2000mV-20V-200V-1000V; AC-V: 0-200-750V

Harbor Freight
ITEM 90899-0VGA

$2.49

[Bigglez]

Harbor Freight ITEM 90899-0VGA $2.49

Or, throw the meter away and keep those probes for another project...

[Robert Reed]

Bob
I bought two of those meters from Harbor Freight due to the low price (althogh I paid $3.50 at the time, which included batteries). I compared it against a hi end Fluke and was amazed how well it performed. I am thinking of trying an experimental gutting on one just to retrieve the display. Right now I'm thinking that this may not be possible - any one out there tried desecting these things?

[Bob Scott]

I am thinking of trying an experimental gutting on one just to retrieve the display. Right now I'm thinking that this may not be possible - any one out there tried desecting these things?

Not this particular meter, but I did dissect a Micronta autoranging DMM years ago. It uses the same original Intersil (Harris) 30 year old ICL7106 IC as all the panel meters do today, or a blob-on-a-board that does the same job, and that directly drives the display. The only other circuitry inside either the panel meters or the DMM attenuates the signal or some other circuitry to drive the blob to 200mV. The input switch selects the appropriate decimal point. Autorangers have automatic attenuators and automatically move the DP position.

They really are more simple than you might think. Just read the data sheet.
http://www.intersil.com/data/fn/fn3082.pdf

Edit: Oh, don't disturb the display glass from the rubber contacts it rests on. You may never see all the segments light up again. I ruined more than one LCD display by taking the display glass off and then and reassembling. I don't care how clean it is.

I don't live close to a Harbor Freight. I wish they'd move north across the border. The closest ones are in the Seattle area.

[MARK J. HABIAN]

i was unhappy with my pc board traces, so i covered them all with solder. i rechecked all component placement, values, etc. i tested the transistors, good. i threw together a simple 1500 hz tester for the 741 op amps and discovered that out of a total of 4 that i recently purchased from radio shack, only 1 gave me an audible signal meaning that i had a minimum of 1 bad op amp in circuit (swapping). i have to pick up another 741. i am also considering using some less noisy op amps from nte. by the way, i have been trying to calibrate this with my projects ua lcd meter disconnected from circuit. i have also read an article in your june 1998 issue by joseph j. carr stating to short the two input pins together, if output drops to zero, ic is capable of working. i have finished another impedance measuring project (not as good as this one) that i need a #49 bulb for. i am not giving up on first project, it should work, project has a pc board layout which minimizes publishing, etc. errors. again, thankyou for your time and quick response. i will get back to you with my results from new op amp. p.s., probes were my test meter for dc voltage, projects test terminals get used in next step of calibration.