DIY Laser Power Meter

TheMonk · Feb 18, 2008

I powered the board using 2x 9v batteries, and connected the sesor head, but I got 8.2v :-/ at (display out / DMM).
I should be getting ~0v right.

knimrod · Feb 18, 2008

TheMonk said:
I powered the board using 2x 9v batteries, and connected the sesor head, but I got 8.2v :-/ at (display out / DMM).
I should be getting ~0v right.

With an output of 8V, the op-amp is saturated. Are you sure you have the -9V supply hooked up? With your voltmeter, verify +9V at U1, pin 7, and -9V at U1, pin 4. Lastly, disconnect the sensor and ground the input. If the op-amp is still saturated, there's an assembly problem or a defective op-amp I think.

TheMonk · Feb 18, 2008

Is this how the wiring should be? I am still getting 8.2v. :-/

knimrod · Feb 18, 2008

knimrod said:
With an output of 8V, the op-amp is saturated. Are you sure you have the -9V supply hooked up? With your voltmeter, verify +9V at U1, pin 7, and -9V at U1, pin 4. Lastly, disconnect the sensor and ground the input. If the op-amp is still saturated, there's an assembly problem or a defective op-amp I think.

TheMonk · Feb 18, 2008

knimrod said:
[quote author=knimrod link=1200112201/240#241 date=1203363313]With an output of 8V, the op-amp is saturated. Are you sure you have the -9V supply hooked up? With your voltmeter, verify +9V at U1, pin 7, and -9V at U1, pin 4. Lastly, disconnect the sensor and ground the input. If the op-amp is still saturated, there's an assembly problem or a defective op-amp I think.

[/quote]
I got 8.2v at pin 7 and -8.2v at pin 4.
Was the batteries hooked to the board correctly?

knimrod · Feb 18, 2008

TheMonk said:
[quote author=knimrod link=1200112201/240#243 date=1203368937][quote author=knimrod link=1200112201/240#241 date=1203363313]With an output of 8V, the op-amp is saturated. Are you sure you have the -9V supply hooked up? With your voltmeter, verify +9V at U1, pin 7, and -9V at U1, pin 4. Lastly, disconnect the sensor and ground the input. If the op-amp is still saturated, there's an assembly problem or a defective op-amp I think.

[/quote]
I got 8.2v at pin 7 and -8.2v at pin 4.
Was the batteries hooked to the board correctly?[/quote]

That sounds right... Now disconnect the sensor and ground (short) the input. What's the output?

LarryQ · Feb 18, 2008

Your Gain Pot seems very far to the Right as well...

Did you follow the formulae and set the Pot before installing>>>

I have built 3 circuits..and never had one even close to that far off of center.

BTW that is the exact Trimmer I used.

It made me look closer at your components

I believe the "Green" resistors you have in there are incorrect....Especially R5.

R5 should be color coded Brn-Bla-GRN-Gld

Yours looks Yellow not green....That would be a 100K Ohm resistor where a 1 Meg is required.!!!!
Check all of those Green colored Resistors...the picture you have doesn't contrast very well...but I think that is where your problem lies

Here is a neat place to calculate color codes...http://www.dannyg.com/examples/res2/resistor.htm

Larry

TheMonk · Feb 18, 2008

knimrod said:
[quote author=TheMonk link=1200112201/240#244 date=1203373541][quote author=knimrod link=1200112201/240#243 date=1203368937][quote author=knimrod link=1200112201/240#241 date=1203363313]With an output of 8V, the op-amp is saturated. Are you sure you have the -9V supply hooked up? With your voltmeter, verify +9V at U1, pin 7, and -9V at U1, pin 4. Lastly, disconnect the sensor and ground the input. If the op-amp is still saturated, there's an assembly problem or a defective op-amp I think.

[/quote]
I got 8.2v at pin 7 and -8.2v at pin 4.
Was the batteries hooked to the board correctly?[/quote]

That sounds right... Now disconnect the sensor and ground (short) the input. What's the output?
[/quote]
The sensor head was not connected when I measured the voltage at pin7 and pin4.
How do I ground (short) the input?
BTW the board behind the sensor head do I need to remove that?

LarryQ · Feb 18, 2008

Monk..I believe your resistors are the wrong values..read my above post.

Larry

TheMonk · Feb 18, 2008

LarryQ said:
Your Gain Pot seems very far to the Right as well...

Did you follow the formulae and set the Pot before installing>>>

I have built 3 circuits..and never had one even close to that far off of center.

BTW that is the exact Trimmer I used.

Just a thought...

Larry

My vr1 = 8.3k

R2=100
R3=45.7K
R4=45.9K

100 x 1000 - (45700 + 45900) - 100 = 8300

TheMonk · Feb 18, 2008

The green resistors were salvaged from the sensor board 5 band resistors.

R1 & R2 =100 ohm Brown+Black+Black+Black+Brwon
R5 = 1 Mega ohm Bronw+Black+Black+Yellow+Brown (it measured 1mega ohm by itself, but on the DIY board it measured 0.3 and 0.5 mega ohm without batteries and I got similar reading while it was on the sensor head board.
R8 = 1k ohm Brown+Black+Black+Brown+Brown.

knimrod · Feb 18, 2008

TheMonk said:
[quote author=knimrod link=1200112201/240#245 date=1203375072][quote author=TheMonk link=1200112201/240#244 date=1203373541][quote author=knimrod link=1200112201/240#243 date=1203368937][quote author=knimrod link=1200112201/240#241 date=1203363313]With an output of 8V, the op-amp is saturated. Are you sure you have the -9V supply hooked up? With your voltmeter, verify +9V at U1, pin 7, and -9V at U1, pin 4. Lastly, disconnect the sensor and ground the input. If the op-amp is still saturated, there's an assembly problem or a defective op-amp I think.

[/quote]
I got 8.2v at pin 7 and -8.2v at pin 4.
Was the batteries hooked to the board correctly?[/quote]

That sounds right... Now disconnect the sensor and ground (short) the input. What's the output?
[/quote]
The sensor head was not connected when I measured the voltage at pin7 and pin4.
How do I ground (short) the input?
BTW the board behind the sensor head do I need to remove that?[/quote]

I think your resistors are correct.

If the sensor head is not attached and the input is floating, the amp will definitely not be happy. To short, just solder a temporary jumper across the input. The output should be 0V.

The heater control board does not need to be removed.

LarryQ · Feb 19, 2008

TheMonk said:
The green resistors were salvaged from the sensor board 5 band resistors.

R1 & R2 =100 ohm Brown+Black+Black+Black+Brwon
R5 = 1 Mega ohm Bronw+Black+Black+Yellow+Brown (it measured 1mega ohm by itself, but on the DIY board it measured 0.3 and 0.5 mega ohm without batteries and I got similar reading while it was on the sensor head board.
R8 = 1k ohm Brown+Black+Black+Brown+Brown.

AHHHHHHHHHH....Sry..I failed to notice they were 5 band resistors....

I'm just so used to 4 band..I forgot that those things even existed!

See what happens when you spend too many Lunch Hours in Radio Shack?!?!?! :-[

Sry........I just saw something that jumped out at me...

Larry

Kenom · Feb 19, 2008

so ya got another run of the boards for these comin soon?

knimrod · Feb 19, 2008

Kenom said:
so ya got another run of the boards for these comin soon?

The third group buy only lacks 3 more PCB commitments.. See the third post in this thread.

TheMonk · Feb 19, 2008

knimrod said:
[quote author=TheMonk link=1200112201/240#247 date=1203375829][quote author=knimrod link=1200112201/240#245 date=1203375072][quote author=TheMonk link=1200112201/240#244 date=1203373541][quote author=knimrod link=1200112201/240#243 date=1203368937][quote author=knimrod link=1200112201/240#241 date=1203363313]With an output of 8V, the op-amp is saturated. Are you sure you have the -9V supply hooked up? With your voltmeter, verify +9V at U1, pin 7, and -9V at U1, pin 4. Lastly, disconnect the sensor and ground the input. If the op-amp is still saturated, there's an assembly problem or a defective op-amp I think.

[/quote]
I got 8.2v at pin 7 and -8.2v at pin 4.
Was the batteries hooked to the board correctly?[/quote]

That sounds right... Now disconnect the sensor and ground (short) the input. What's the output?
[/quote]
The sensor head was not connected when I measured the voltage at pin7 and pin4.
How do I ground (short) the input?
BTW the board behind the sensor head do I need to remove that?[/quote]

I think your resistors are correct.

If the sensor head is not attached and the input is floating, the amp will definitely not be happy. To short, just solder a temporary jumper across the input. The output should be 0V.

The heater control board does not need to be removed.[/quote]
I shorted the input I got 0.56v, then I re-connected the sensor head it started at 6.5v and went up to 8.2v again.
I disconnected the sensor head and shorted the input again, it went back to 8.2v. :-/

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