Hi,

I've had a go at doing a circuit (albeit badly).

Attached is the circuit...the main component is the pressure sensor which outputs between 85mV and 145mV. I connected this to an amplifier to increase the voltage to between 0V and 5V (think I am missing a few connections). The output then goes to a LED controller chip which then lights the LED's (bar mode) from left to right depending on the range of pressure.

I also placed a A/D convertor on there as well as I want to sample the voltage and then send it to a computer, eventually, as well.

Any advice recommendations welcome! Its been sooo long since I wired something up I feel embarassed posting the diagram!

Cheers,

Craig

Attached Images

Tank Level.gif (17.0 KB, 35 views)

 

you need to add some resistors in line with the led's to limit the current

 

You missed a couple of things.
Your schematic is small and fuzzy so the details cannot be seen.

The 5V zener diode and the 9V battery will smoke and burn without a current-limiting resistor between them.

The Vss pin 4 of the 741 opamp is supposed to be connected to 0V as part of its power supply. Your opamp doesn't work without power.

The opamp would have a voltage gain of about 200,000 without negative feedback. Yours doesn't have negative feedback.

The opamp is supposed to have its input biased at half the supply voltage. I don't know if the sensor biases it. Its bias voltage might be too low.

The lousy old 741 opamp is 40 years old this year and does not work with a supply as low as 5V. It is spec'd with a 30V supply.

The LM3914 does not have its pin 2 and pin 4 connected to 0V so it doesn't work.

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Uncle $crooge

 

No.
Look at the datasheet for the LM3914. The outputs have adjustable regulated current.
In this circuit the LED current would be only 10mA if the 1.21k resistor is connected properly (it is connected to pin 6 by mistake).
Pin 6 and pin 7 are connected wrong.

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Uncle $crooge

 

Cool, thanks heaps for the comments.

I will have a go at a version 2 (after a bit of research as to what all this stuff means).

Will post a newer version later.

Cheers!

 

I miss the smiley ..
the schematic is small so I made wrong assumption ... 0300am .. not good time for reviews

 

Sorry that first image was so small. I've tried to make it clearer.

Attached is the revised circuit diagram with these modifications:
1. I changed the op amp to a MCP6001 1Mhz low power op amp (only 5 pins)
2. I took the output from the pressure sensor and passed it through a resistor to lower the 85mV down to 0V and the 145mV down to 60mV
3. I added a gain to the op amp of 81 so 60mV becomes 4.8V
4. added a 1K resistor before the zener diode

Also note that LM3914 has pins 2 and 4 connected to GND and I checked the schematics again for the chip and it showed pin 6 and 7 wired as is in the diagram (the example they had).

Thanks again for the help.

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Tank Level v2.gif (19.6 KB, 15 views)

 

The voltage gain of your new opamp is only 1.
Look at the datasheet for the LM3914 to see how pin 8 is connected between two resistors in series from pin 7 (and pin 6) to ground.
The 1k resistor feeding the zener diode is fine to feed low current to the opamp and sensor but it certainly cannot supply high current to the LEDs. They should be powered directly from the battery.

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Uncle $crooge

 

Thanks for the review.

When I scaled the diagram the op-amp got wired wrong, it should be 1 + 80k/1k = 81 gain.

I've reset the wiring of the LED's from the battery - thanks.

Finally also saw what you meant about the LM3914. I don't know how you can pick things like that up!

Just for completeness here is the final diagram (I hope).

Cheers,

Craig

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Tank Level v3.gif (19.5 KB, 12 views)

 

I couldn't find a datasheet for the strain guage.

The circuit looks good now but it needs about 100uF across the battery and 10uF across the zener diode to keep the voltages from bouncing around especially when the battery is getting weak.

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Uncle $crooge

 

Thanks.

Here are the doc's for the pressure gauge.

http://docs-asia.electrocomponents.c...6b80026163.pdf
http://docs-asia.electrocomponents.c...6b8009849c.pdf
http://docs-asia.electrocomponents.c...6b8034f1be.pdf

Buying it from RS-Components.

So when you say I need a capacitor across battery and zener diode that is to store the voltage and avoid spikes right? Also I place it in parallel with one terminal connected to ground.

Thanks again.

Craig

 

All electronic circuits need a supply bypass capacitor, especially when powered from a battery.

The pressure sensor needs a 10V supply and might not work when the supply is as low as only 5.1V. The opamp must have the same supply voltage as the sensor because the output of the sensor averages at half of its supply voltage.
Your circuit does not need the 42.5k resistor. I think the sensor is supposed to feed a fully-differential pair of opamps so that both inputs are at a very high resistance.
Doesn't the sensor have applications notes?

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Uncle $crooge

 

I've been hunting around trying to get application notes and finally found some on a related type of sensor...

http://sensing.honeywell.com/index.c...ment/1/re_id/0

I've applied the dual op-amps like in the schematic.

Thanks,

Craig

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Now your sensor is getting a supply voltage of only 3.15V. it wants 10V.
Use the other sensor if you want to use its circuit.

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Uncle $crooge

 

Yeah I see what you mean. I might have to wait until the pressure sensor arrives and then do some tests on it with an op-amp. I am getting really confused as to what voltages I need and what gain etc.

Once I have a refresh of the mind I will give the circuit another go....back to the drawing board eh!

Cheers,

Craig