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LM3914 circuit???
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irving Offline
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Post: #1
LM3914 circuit???
Hi Mike

I have a question referent the EFIE that you make; does it use the LM3914 circuit? And by the picture on the store your does no have a rotary switch, that will make it easier to adjust on the go instead of using a screw driver
Can you make one with a rotary switch that does not need a screw driver?
I really what to buy one , I am getting a magdrive 12a HHO for my 1998 460 ford eng motor home and they say that I do not need one, what can you tell me?
Irving
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11-05-2007 03:02 PM
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mike Offline
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RE: LM3914 circuit???
I know the circuit you are talking about because I built one and tested it. The EFIE circuit works on a different principal entirely than the 3914 design. The 3914 design receives an input voltage and then has 10 discreet input voltage levels that will trigger the circuit's output. Which voltage level will trigger the output is selectable by the rotary switch. The output is always 1 volt, but is only triggered when the input voltage threshold is met. So the computer always receives either 0 or 1 volt.

The trouble with this design is that the oxygen sensor doesn't behave that way. It gives voltages continuously throughout the 0 to 1 volt range. The EFIE adds a floating voltage on top of what the sensor is providing, causing a shift in the output voltage graph. But the voltages provided to the computer are continuously changing throughout the range, which is what the computer expects. The EFIE just makes those voltages a little higher, which provides the compensation for the additional oxygen in the exhaust. The potentiometer used is a 20 turn unit, meaning you can turn it 360 degrees, 20 times (or 20 X 360 degrees) to get it from it's lowest to highest setting. This ensures that the exact amount of voltage offset you need is available, not just 10 steps.

Therefore when researching these designs I chose to develop the EFIE design as opposed the to the 3914 design. If you want to buy a 3914, I'm not aware of anyone who can sell you one. I'm assuming you have already found the design on the internet, so if you want to build one, follow that. If you want to build one and don't know where to find the schematic and instructions, write me back and I'll dig it up for you.

However, I recommend you buy an EFIE from me Big Grin . I think you'll get much better results with it. Also, you really don't save much money when you finish buying everything you need to build one, and it'll save you a huge amount of your time.

Does this answer your question?

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(This post was last modified: 03-27-2008 02:46 AM by mike.)
11-05-2007 03:59 PM
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irving Offline
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RE: LM3914 circuit???
yesThank's but my only problem is, that you need a scrue driver to ajust the range, can you make one with a nob to turn the voltage up or down??I will buy it from you Smile
11-05-2007 04:16 PM
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mike Offline
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RE: LM3914 circuit???
I'm very sorry Irving, but I won't be able to make a custom unit with a knob as you suggest. The potentiometer is 20 turn, and the types of pot with knobs are usually single turn. Particularly the ones that would be small enough to fit in the EFIE's case, and not be cost prohibitive. Modifying the current pot to use a knob would also be cost prohibitive.

One thing I'd like to point out is that while you need a screwdriver to adjust the EFIE, you shouldn't need to adjust it more than 6 - 8 times before you get a good setting. I adjusted mine 4 times, and haven't touched it since. After you find the sweet spot that gets you your best mileage, you really don't need to adjust it again. Even if you like to tweak things to the ultimate degree, you might only need to adjust it a few more times. Eventually you will quit messing with it, no matter how much you like to tweak, at which point the knob is not needed anymore anyway.

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11-05-2007 05:33 PM
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qsiguy Offline
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RE: LM3914 circuit???
This is my first post but I thought I'd give my $.02. I have also worked with the 3914 circuit and I agree with Mike. The EFIE is a much better method and gives you much greater control over your output. The 3914 circuit will work but it's a little crude and not very precise.

As far as a knob for adjustments, once you get it dialed in you won't want to mess with it and worse yet you won't want someone getting in your car and saying what's this? While they reach over and spin the knob. Then you are back to square 1 with your adjustments. Furthermore, you really don't need to do much adjusting while driving. You should measure the voltage and make a small adjustment and try it again. You should be watching your voltmeter while you adjust anyway so I feel it's best to make adjustments when you can view the output and log it. Lastly, it's not safe to do much tweaking while you drive Smile

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12-04-2007 04:47 PM
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mike Offline
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Post: #6
RE: LM3914 circuit???
Hi qsiguy,

Thanks for your post. I couldn't have said it better. Big Grin

I've had a couple of requests for knobs, and I wish I could give those folks what they want, but it just isn't economicaly feasible. So, for now anyway, they'll have to make do with the screwdriver adjustment.

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(This post was last modified: 03-27-2008 02:45 AM by mike.)
12-05-2007 10:02 PM
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