ok, I need help

Thread Starter

jimbarstow

Joined Nov 8, 2011
24
Freescale makes pressure sensors with a 5v supply that will simplify using with an Arduino.
This looks like it is exactly what I need and it appears to eliminate virtually the entire circuit I'm having trouble with.

On the downside, I'm trying to learn how to do this. Going with this component, and not figuring out what I'm doing wrong, seems like I'm giving up. I'd really like to figure out what is wrong with the circuit, make it work, then replace it with this self contained component. If I don't do that, I'm just going to make the same mistake some tome later.
 

praondevou

Joined Jul 9, 2011
2,942
Which exact pressure sensor model did you use?

On page 8 of this datasheet there seem to be 2 different pinouts for a very similar package. Since the sensor seems to have some circuitry inside the power supply probably needs to be at the correct pins.

Btw, if C2 is the only bypass capacitor available for the Opamp, I think it is very far away from it. The ground goes all around the board, right? Not that it matters for this circuit though.

It still looks to me like the regulator's flat side is toward the camera, and if so, the regulator is in backwards.
I agree.
 

tracecom

Joined Apr 16, 2010
3,944
It still looks to me like the regulator's flat side is toward the camera, and if so, the regulator is in backwards.
I agree.
Which would explain why the OP measured 8 volts on the "output" as he reported in post 10; he was measuring the battery. According to the spec sheet, that regulator has a typical dropout voltage of 1.7V. It really needs a minimum of 10 volts in; 12 would be better.

 

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Thread Starter

jimbarstow

Joined Nov 8, 2011
24
It still looks to me like the regulator's flat side is toward the camera, and if so, the regulator is in backwards.


You're right. I looked at the datasheet when I first put the regulator on the board but didn't go back to verify when checking everything else.

Unfortunately, it doesn't change the behavior: 3.8V (or so) constant output unchanged with increasing pressure.
 

tracecom

Joined Apr 16, 2010
3,944
In 957, the end of R1 that is supposed to connect to R2 and C3 doesn't seem to be connected; it looks to be off by one row. But in 958, it seems to have been moved. How can that be?

What is the voltage measured between pins 1 and 2 of U1?

What is the voltage measured between pins 2 and 3 of U1?

What is the voltage measured between pins 4 and 8 of U2?

Where are the red and blue wires going off the top of 957?
 
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thatoneguy

Joined Feb 19, 2009
6,359
You have both the input and output of the voltage regulator going to the red rails on the protoboard, and are using both for the circuit, this could be a problem.

In the first attachment, the flat side looks to be facing toward the battery pack. Is that correct?
 

Thread Starter

jimbarstow

Joined Nov 8, 2011
24
In 957, the end of R1 that is supposed to connect to R2 and C3 doesn't seem to be connected; it looks to be off by one row. But in 958, it seems to have been moved. How can that be?

What is the voltage measured between pins 1 and 2 of U1?

What is the voltage measured between pins 2 and 3 of U1?

What is the voltage measured between pins 4 and 8 of U2?

Where are the red and blue wires going off the top of 957?
I'll check the voltages.

Nothing changed between the pictures. I think it is an artifact.
 

Thread Starter

jimbarstow

Joined Nov 8, 2011
24
You have both the input and output of the voltage regulator going to the red rails on the protoboard, and are using both for the circuit, this could be a problem.

In the first attachment, the flat side looks to be facing toward the battery pack. Is that correct?
The red rail on one side is used for the battery input. The voltage regulator is the only thing that connects to this side.

The red rail on the other side is used for the output from the regulator.
 

Thread Starter

jimbarstow

Joined Nov 8, 2011
24
In 957, the end of R1 that is supposed to connect to R2 and C3 doesn't seem to be connected; it looks to be off by one row. But in 958, it seems to have been moved. How can that be?

What is the voltage measured between pins 1 and 2 of U1?

What is the voltage measured between pins 2 and 3 of U1?

What is the voltage measured between pins 4 and 8 of U2?

Where are the red and blue wires going off the top of 957?
The red/blue wires are the for measuring the output. The red is connected to pin 7 and the blue to ground.
 

Thread Starter

jimbarstow

Joined Nov 8, 2011
24
In 957, the end of R1 that is supposed to connect to R2 and C3 doesn't seem to be connected; it looks to be off by one row. But in 958, it seems to have been moved. How can that be?

What is the voltage measured between pins 1 and 2 of U1?

What is the voltage measured between pins 2 and 3 of U1?

What is the voltage measured between pins 4 and 8 of U2?

Where are the red and blue wires going off the top of 957?
The op amps are labeled U2A and U2B and are both in a single
8 pin chip. I just measured the voltages between the specified
pins on that chip.


between pins 1 and 2
0 mv
between pins 2 and 3
1.33 v
between pins 4 and 8
7.2v
 

tracecom

Joined Apr 16, 2010
3,944
The op amps are labeled U2A and U2B and are both in a single
8 pin chip. I just measured the voltages between the specified
pins on that chip.


between pins 1 and 2
0 mv
between pins 2 and 3
1.33 v
between pins 4 and 8
7.2v
Except for the R1 connection problem I noted previously, I don't see any wiring errors, however the flywires make it difficult to follow from the photo's, and I could be wrong.

I understand that both op-amps are in the same package: U2. My questions about pins 1, 2, and 3 were about U1, the regulator. I am trying to determine the input and output voltages because the entire circuit's function is based on providing precise voltages to the two comparator op-amps.

However, the 7.2 V between pins 4 and 8 of U2 is clear evidence that U1 is not functioning properly. It is either damaged or does not have sufficient input voltage. In addition, U2 may have been damaged.

U1 must have a minimum input of 10 V (12 is better) in order to regulate. The quote about a 6.8 V input that you included in post 10 is wrong; in fact, it is contradicted by a table in the same document, a copy of which is attached. Although the circuit would function with a steady 7.2 V supply, it would not be accurate, and without at least a 10 V input, U1 will not regulate.

It is my opinion that until U1 is functioning properly providing a steady 8 V output, and U2 has been replaced, further troubleshooting is pointless.

Good luck.
 

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Thread Starter

jimbarstow

Joined Nov 8, 2011
24
Ok, I'll try that. I have a 12V supply I might be able to use or I'll use multiple batteries.

I really do appreciate all the help.

thanks
 
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