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wuemura

Simple O2 meter with LM3915

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Hello Guys!

I'm doing a O2 meter for my car because pay about $150 bucks for a simple meter will not do for me, too much. So, i want to build a meter with 2 LM3915 for better resolution but i have a few questions:

1 - the car voltage is too unstable, there is too much oscilation, mechanical relays turning on/off, sparks, so i will use a 7805 to regulate the 12v. But there is one more thing, how to protect the 7805 from over voltage at the input from the car electric system?

2- In the National LM3915 datasheet there is a way to make the display to flash if the "warning" led lit up (Bar Display with alarm flasher), how to make this work with 2 LM3915?

3- The signal from the lambda sensor came with some noise, so grounding the signal with a 1M ohm resistor with a 1n capacitor could filter that with out killing the signal? Suggestions?

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The car O2 meters in cars that I have seen use the linear LM3914 IC and not the logarithmic LM3915 IC. Two LM3915 ICs have an input voltage range of 1000:1. Two LM3914 ICs have an input voltage range of 20:1.
The output voltage of an O2 sensor oscillates between the positive supply and ground so will need heavy filtering to show a steady meter reading.

Flashing the display is done by having the selected output reduce the IC's programmed reference voltage and can be done with two ICs and a couple of diodes.

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Hello Capitain!
Or should i call you General ;D

Thanks for the schematic, there is a lot os these on the net. I want to try building one from scratch but if everything goes wrong i might use yours as reference! Audioguru knows me better when i pull out the stunt with 2 LM386 in a circuit that i called Super Headbanger.

I enjoy learning new stuff ;D

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The car O2 meters in cars that I have seen use the linear LM3914 IC and not the logarithmic LM3915 IC. Two LM3915 ICs have an input voltage range of 1000:1. Two LM3914 ICs have an input voltage range of 20:1.
The output voltage of an O2 sensor oscillates between the positive supply and ground so will need heavy filtering to show a steady meter reading.

Flashing the display is done by having the selected output reduce the IC's programmed reference voltage and can be done with two ICs and a couple of diodes.


Hello audioguru, when you say "need heavy filtering" what do you have in mind?

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Yes, i got it!
I have a few LM3914 here ;)

I'm working on the schematic right now, the worst part is the "heavy filtering" part because the signal is only 1v max, there is the alternator part that can blow the regulator off (some friends os mine did that).

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Hi Mr. Super Headbanger. Wilson, isn't it?
The O2 sensor in a car normally produces a fluctuating voltage. One meter circuit on the web uses an opamp buffer driving a 2.7M resistor, a 0.1uF capacitor to ground then another opamp beffer.
Do the same thing with a 270k resistor from the O2 sensor to pin 5 of the LM3914, then a 1uF capacitor to round at pin 5.

A 7805 regulator has a max input voltage of 35V, which is not exceeded if the battery is never disconnected with the engine running. Good protection would be a 10 ohm resistor from the battery, then a 30V zener diode in parallel with a 10uF capacitor both to ground, at the input to the regulator.

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Hello Audioguru, sorry, not Wilson but Wellington ;) Long time no see Audioguru, how are you?!
You don't have that schematic around, did you?

I was thinking the same about the input protection for the 7805, true is, i was thinking in using a 7806 instead to compensate the output protection also with a 5.1 zener, just in case the 7806 fails or who knows, stop working. I wil do the suggestions, i will keep monitoring the foruns for your reply :)

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Using LDR will be like "plug and play" so you don't need to mess up with the car cables. But the circuit is not ready yet, something must be done with the second LM3914, after that i still have to make the circuit blink or buzz when it reaches a certain level.

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Does any one know or have any idea how o make the "Bar Display with alarm flasher" work with this configuation?? Because the design is too big already, i did have an idea to add to this circuit, a few extra 4148 diodes and a 555 so when the especific led turn on it will trigger the 555 to flash something or to buzz someting.

Thanks.

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Hello guys, after a few months (problem to find LM3914 around here) i've build the thing, but is not working :(
Attached to this message is the full schematic and board design and the Eagle project files.

Problems

1- When i turn the device on, the 10th led lit and stay fixed, when i turned off the 20th led flashes. Removind F2 the display start to move from 10th to 20th led with full bright.

2- D1 and D6 heat up alot, i am doing a test with a 9v battery. IC3/IC4 output is only 3.4v (?), input is 5.9v (battery has 9.2v charge) looks like a short some where, but disconecting D1 and D6 IC3/IC4 operates fine outputing 5v.

3- D8 should drop the voltage to 2v and D9 should drop 0,7v giving me aprox 1.3 ref to LM3914, but there is a 4.3v final output, no good :-X

4- At point ENT-2 (signal input) there is a 1.5v, there is nothing connected there, where the voltage is coming from? (IC1/IC2 ??)

5- The LED brightness is not working also, that part of the circuit is doing what is supposed to do, it variates according to the light in the ambient but the display do not react to it.... :-\

Did i miss something???

PS: There is a point in the board that is not connected (IC4 GND), i connected that with a wire outsid the board.
PS2: Board was build using Silk Screen.

Thank you.

O2-project.pdf

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Hi Wellington. ;D


1- When i turn the device on, the 10th led lit and stay fixed, when i turned off the 20th led flashes. Removind F2 the display start to move from 10th to 20th led with full bright.

Might be caused by other problems.
The schematic has the definitions for pins 6 and 8 of the LM3914's backwards.

2- D1 and D6 heat up alot, i am doing a test with a 9v battery. IC3/IC4 output is only 3.4v (?), input is 5.9v (battery has 9.2v charge) looks like a short some where, but disconecting D1 and D6 IC3/IC4 operates fine outputing 5v.

D1 and D6 are correct on the schematic. 30V zeners don't conduct with an input of only 9V so they must be the wrong voltage.

3- D8 should drop the voltage to 2v and D9 should drop 0,7v giving me aprox 1.3 ref to LM3914, but there is a 4.3v final output, no good :-X

I have never seen a zener with such a low voltage as 2.0V. Pin 7 of the LM3914 is a voltage output, not an input. Pin 7 is like the output of a built-in LM317 adjustable voltage regulator with a regulated output voltage at pin 7 from 1.25V up to the limit created by the resistor joining it to its adjustment pin 8.

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R5 in this DIY circuit is 10k. Up to 20 LEDs will have their current flowing through it so its value is way too high. Maybe it should be 10 ohms.
The circuit needs supply bypass caps as per the datasheet.
The value of R3 should be the same as R1. Maybe the values for R3 and R4 are swapped.

I understand that the output voltage of an O2 sensor continuously oscillates high and low. If it doesn't then the car's ECU thinks about tampering or a failed O2 sensor. Therefore a filter is needed at the input of the LM3914 to average the swinging voltage.

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Hi Welington,
I am sorry to hear that the brightness adjustment transistors or the LDR in your circuit don't work.
Here is what you have:
1) the VR terminal of each LM3914 is 1.25V.
2) The LED currents are roughly 10 times the current from VR.
3) The current from VR when the LDR is dark (if it is a very high resistance) is just the 10k resistor so its current is (1.25V/10k= 0.125mA and the LED currents are 1.25mA.
4) The current from VR when the LDR is bright (if it is a very low resistance) is (1.25V-0.6V (transistor Vbe))/ 680= 0.96mA, plus  the 0.125mA current in the 10k resistor above for a total of 1.085mA so the LED currents are 10.85mA.

I don't think your LDR goes from a very high resistance to a very low resistance.
Disconnect the LDR from the circuit to see if the 10k resistors make the LEDs dim enough. Then short the LDR to ground to see if the 680 ohm resistors make the LEDs bright enough. Your vision's response to brightness is logarithmic so 9 times the current in LEDs doesn't make much difference in brightness.

post-1706-14279143018742_thumb.png

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