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wuemura

Simple O2 meter with LM3915

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Hello audioguru!

The circuit work in the reverse way, when is dark the led must bright more, the day light must make the led dim off ;D

Changing the LDR with a 10k resitor make the led bright a bit, about 25% or 30%, shorting the connection make the led bright at 100%, when i say that it bright more i mean the full display. At your schematic i've noticed that you removed VR2 and VR3, VR3 is used to set the low level input and VR3 the high level.

When you turn the device on the first led must turn on,  if not, you ajust VR3.
Injecting 1v DC at the input terminal the 20th led should lit and stop there, if not VR2 make the magic happens.

So, looks like the circuit is working but not the way it should, so working with the 680r resistor or the transistor could make things working, right?

post-20472-14279143018871_thumb.png

post-20472-1427914301946_thumb.png

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After I attached my simplified copy of your circuit I realized that I forgot to include VR2 and VR3 which are loading the VR of the LM3914 they are connected to which increases its minimum brightness.
Measure the resistance of the LDR when it is lighted and when it is dark to see how it is a very low and a very high resistance to control the transistors.

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Hi Wellington,
Wow, your photodetector has a very wide range of resistance. Too wide for an LDR so I think it is a photo-transistor. No wonder its response is backwards.

Swap its position with VR1 and the resistor connected to VR1 (R9?) but its ends must be swapped to keep the correct polarity.

LDRs are used in automatic mains night-lights and in garden solar lights.

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Hi Audioguru, well i've done the changes but.... some how works. At normal day light the led bright is strong, but just a little shadow make the LED to shut down. :(
Putting all back again, if i change the R9 to 100K the effect become a little more effective, but not that much. Increasing the resistor to 220k and so on, make no changes at all.

If i change the transistor to BC327 or BD140?

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Hi audioguru, i've added resistors up to 1M and make not change at all, just a little shadow makes the led to turn off. And by the way, is the first photo-transistor that looks very much like a LDR to me.

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I wish all your PMs are in this thread so I can see them.
I think you said that the brightness control worked backwards so I thought the LDR is a phototransistor. Now I see that I was wrong and they would both work about the same.

The LDR is a low resistance when it is lighted, so if it is at the bases of the transistors to ground then the transistors will turn on and make the LEDs bright.
Without light on the LDR then it is a high resistance and the transistors turn off but the 10k resistors from pin VR of each LM3915 keeps some current in the LEDs.
If the transistors turn off too much then add a resistor in parallel to the LDR to turn on the transistors a little.

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Today is a great day, there is no way to put in to words, to develop something and see it working is a great feeling. Since i was a kid and started with electronic at 6 years, we had a very difficult time, my family was very poor so i've never got the oportunity to go to college or learn english at a decent school, all that i know today was result of my own fight to get it and without or few resource.

That's why i would like to thank this space and audioguru for his patience with me, some of you might think that this is too simple, but for a guy like me that has no graduation at electronics, is a great thing.

Audioguru, i've done your changes but did not work the way it should be, so let it be man, the main part it's working.

So here is the photo of the main board (i took that with my cellphone camera)
0708061234wr9.jpg

Here is the circuit when you turn the ignition on
0708061235iv3.jpg

And here is the baby working
0708061236ox5.jpg

This circuit has no precision, but compared with professional equipament, the circuit will respond to very small variations from 0,1mv to more or less 1.4v max. This computers/equipament that mesures this oxigen sensor you see the changes as jumps. Later i will record a video with the circuit installed and working.

Thank you audioguru.

PS: The full schematic and board design is attached with this message, use it, make modifications, sell it i dont care ;D

HallMeter.pdf

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I have been following this post for a little while.  Good to hear you got everything working.  I have a few questions about this setup.

What o2 sensor are you using?  Is it a wideband or narrowband??

in the last pdf you attached on pages 4 and 8 are those schematics printable for transfer??

what size are the boards you are using??

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Hi gearhead,

They are printable, only adjust the size if necessary to fit your printer!
I am sure you know the 2.54 mm (or one tenth of an inch) distance between the IC pins, use it as a reference!

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Hello, i really dont know what type of O2 sensor my car uses, i know that is 4 wire type, looks like a cigar and has a hole in the middle.
About printing, here is the tip, load the PDF in to Photoshop.

In resolution set to 600 pixel/inch, grayscale mode, 16bit and remove (uncheck) anti-aliased option.

Duplicate the layer, so it will became more dark, press CTRL+E to join this parts, press CTRL+A and CTRL+C, create a new layer for printing, press CTRL+N to create a new layer, select A4 paper type, resolution set to 600 pixel/inch, 16bit and transparent background and paste the design (CTRL+V).

Duplicate the layer if you want one or more bord designs in on sheet, print the design, Photoshop will complain about the size, just ignore.

You are done, the design will be printed at the correct size.

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A 4-wire O2 sensor has 2 wires for its electric heater, so it warms up fast. If the heater burns out or becomes disconnected (by tinkering?) then the car's computer flags an error.


Yes, and the engine will not start.

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if you are using the sensor that is already on the car it will most likely be a narrow band sensor.  They are accurate enough for the car's computer but not that accurate.  A better option would be a wideband sensor.  These sensors can read a bigger range of air/fuel ratios.  Your car should still start if the O2 sensor's heater wires are messed up as the sensor will heat up with the car exhaust.

No the car's computer does not know the difference between a real O2 or a resistor in most cases.  Some cars this does not apply to.

If you want a more accurate Air/Fuel readout you should go with a wideband O2.  These sensors have 5 wires most times.  I am currently trying to figure out how to make my own wideband O2 readout similar to what you did.  The only problem right now is cost.  The schematic i found uses a sensor that is about $200.  I have found units that you can buy that use a less expensive sensor but the unit retails for about $200.  So not much trade off.  I will try and post some schematic of the wideband O2 tonight.  If you want you can try and find them on the net.  search for DIY WBO2.  there will be a website WWW DIY-WB COM.  this is the site i got the schematic from, unfortunetly it is no longer up.  so you have to go to the internet archive page, www.archive.org and enter the website above and you can view the stuff on it.  THere is also an australian site that sells wideband o2 kits.  there website is wbo2.com.  They have some schematics on there, but i can't really understand them (too much of a newb right now)  maybe some of you guys can make sense of them and explain them to me.

I might have gotton to car technical if i did let me know i can explain further.  Hopefully with more time and help from everyone here, we can get a schematic for a wideband O2 that uses the less expensive sensor.

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Hi all,
May I use this circuit for Fuel indicator with LM3914? The resistance of rheostat on fuel tank as follows: 10 ohm when filled and 100 ohm when empty. Pls help me to design the input fo this circuit.
Thank you very much.

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Hi all,
May I use this circuit for Fuel indicator with LM3914? The resistance of rheostat on fuel tank as follows: 10 ohm when filled and 100 ohm when empty. Pls help me to design the input fo this circuit.

Use a voltage regulator and add a fixed resistor to the rheostat so that the voltage to the input of the LM3914 changes as the rheostat resistance changes.

Do you know what is the maximum allowed voltage to the rheostat? You do not want it to make a spark that ignites the fuel.

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