Low Ohms meter

[Gene]

Seems like I am always testing conductivity around the house. I test light bulbs, fuses, circuit connections, diodes, etc. I have a several good multimeters but what I want to build is something that would be small, battery powered, with a meter or 2 digit display. The ideal device would show a poor connection (not just a nine volt battery, resistor, and a resistor) such as a resistance of 10-20 ohms - when a connection was not quite as good as it should be. It should insert very little voltage into the circuit as not to damage delicate parts. Finally, it should be so simple to use that my wife could use it. (I'm gonna be in trouble for saying that - aren't I?) Any schematics on hand?

 

[Pilot]

Gene,

How about this one - it's old but the 10mV injection sounds low enough..

https://members.tripod.com/~baec/DEC90/ohmmeter.htm

 

[Gene]

A very real possibility. This idea came to mind when I was reading the post about the erratic 7-segment display circuit. I know there are times when a circut looks right but one wire is loose or there is a "cold" solder joint, or some little problem keeps the whole thing from working. I think it would be practical and worthwhile to have a simple little checker I could use to trouble-shoot a circuit - - - but could also be used around the house without going to the shop and getting out the VOM. I found an old schematic in my stuff but it is very old and uses enough current to trouble-shoot 5 miles of wire (of course it would fry any CMOS or diode). Thanks.

 

[bogdanfirst]

look, i got something, not quite like you want, but still good.
it can only tell you if you have a resistance lower than 1R, between the testers. so it is only for conductibility. it has a buzzer. the voltage between the testers is 2mV, wich is small enough, and the current is small too.
it operates with a 9V batt.
if you are interested i can draw the schematic.
i have built it and it works fine. i even got a pcb

 

[Gene]

Please do - even if I can't use it, maybe someone else can. I would like to have a look.

 

[bogdanfirst]

ok, here it is. there are some comments: the 2K5 pot is for sound frequency, you can adjust it to your needs.
the 10K pot is for adjusting it, you need a 1R resistor to connect it to the test leads and adjust it.
the unconnected inputs of the other 3 gates go to GND.

 

[Gene]

Many thanks for the replies. I think what I want is somewhere in the middle of both circuits. I like Pilot's meter idea because this tells the relative difference between a fair connection and a great one - rather than bogdanfirst's circuit that is a go-no go switch. On the other hand, I like the input, single power source, and low parts count ideas in bogdanfirst's circuit.

The plan is to use bogdanfirst's input. Then, to drive a 339 IC (in place of the 741) using three of the four segments. Each segment should drive sucessive LEDs (like a meter) such that one LED glows at 10 ohms, 2 LEDs glow at 5 ohms, and all three LEDs glow at 1 ohm or less. Maybe drive a pezio buzzer at the 1 ohm connection and maybe an LED that shows more than 10 ohms at the input to indicate an "on" state and to show a poor connection (greater than 10 ohms).

The thing works in Electronic Workbench - so now I'll start building. I will post the schematic if there is any interest (when I figure out how).

 

[bogdanfirst]

i like your idea, i think i got something in a book that does this, but it is for ranges something like 10R, 100R and 1K, so not so good for what you need.
also, the circuit i have i need it more for connections on pcbs, because at those big ones and complicated it is sometimes hard to find a connection.
and about putting a buzzer, it would be great if you can make it sound on 3 different frequencies, because you will have to do somethig like put the testers on the circuit and turn your head to watch the leds, thats why i preffered an audio indicator, you dont have to stare at it all the time.
good luck, and if you get something nice, share it please.

 

[Gene]

Still fine tuning my circuit but, for different tones vs. continuity, here is an idea for bogdanfirst.
http://www.electronic-projects.net/Schematics/schematics.htm

 

[Phasor]

Gene, do you mean http://www.electronic-projects.net/Schematics/AudibleCircuitTracer/AudibleCircuitTracer.htm ??

 

[Gene]

That's the one. My tester has an audiable alarm but it is only active in the 0 to 1 ohm range. bogdanfirst had mentioned the possibility of using different tones for different resistances and, while this didn't interest me, I did have this reference that he might like.

 

[bogdanfirst]

the tester i used is great for testing pcb connections, the low voltage doesnt "see" diodes as a short circuit. it has a sound indicator
ok, here is what i found in a book.
it is a tester with the ranges: <5R(+buzzer); 5-100R, 100k-15M, >15M
it has the indications with 4 leds + a buzzer. but the current for testing is great enough to light a led, and it is good for testing diodes, indicating if it is direct or reversed connected.
it seems quite good. it uses a TLC074 op-amp ic.
also i got a question, since i might build it. is the only diffrance between TLC072 and 074 that one has 2 amps and the other has 4?

 

[Gene]

This sounds very much like the one I'm working on. the 074 is a quad - the 072 is a dual chip. If you've got some 72s on hand, just use two.

**broken link removed**

I'm using three of the four comparitors inside the chip. The comparison is stepped through (in my case) three levels. Although I haven't seen your circuit, it sounds like they use all four sections for four ranges.

 

[bogdanfirst]

nope, 3 are used as comparators, and one is used as an oscilator for the buzzer. i was asking because i have 072 so i will use them.

 

[Gene]

Hey bod - I am a firm believer in using what you have . . . especially in my area where there are no parts suppliers. In fact, I just finished organizing all my parts so I can find what I have. PS. Don't forget to power up that second IC (experience speaking). Sure would like to see the schematic you are using.

 

[bogdanfirst]

here is the schematic....
VLO-<5R+buzzer
LO 5-100R
HO 100K-15M
VHO >15M
if you got any other comments.......just ask

 

[Gene]

I am attaching the schematic that I decided would fit my need - many thanks to all who participated and gave ideas and circuits. The green LED (and optional buzzer) indicates a good connection (1 ohm or less), the amber LED is for a poor connection (10 ohms or less), and the red LED is for general continuity (100 ohms or less). The circuit works well on ELectronic Workbench - I will start actual construction next week unless someone has another idea or sees a problem with the schematic.

Hope the attachment thing works.....

 

[bogdanfirst]

well, i lije it. the input stage looks like the first on i have. i agree, better use a zenner diode, you get more stability when the battery discharges, but it discharges much faster. i left mine on for a day and a half and it was still functioning.

 

[Gene]

You are correct. I really liked your input design for the sensitivity and component protection. I wanted to get rid of the adjustment each time the thing is used so I added the zener and an 'off' switch. The variable resistor (I hope) will be a 15-turn PC mount. The idea is to set it once and forget it. Of course, the thing can be built without the two extra LEDs and the piezo buzzer. The multiple resistors used to adjust the comparitor leads on the second and third stage of the IC could also be replaced with PC mount variables. I am considering limiting the input current to the 2N2222 and awaiting any bad comments - then we build!

 

[bogdanfirst]

i dont think it is good to remove the buzzer, maybe thel led, but not the buzzer, because this way you will hear the short circuit and not stay with one eye on the testing leads and with the other on the led....
what are the 2 extra leds for?