## misterhouse-users

 Re: [mh] 1-wire Pressure Gauge questions From: David Satterfield - 2007-08-28 14:20:22 Attachments: Message as HTML ```Hi Jim, Thanks for the reply (again), I am getting closer, but still have a few more questions: I did some more research after my second post requesting information (and before your response) and realized that you were trying to make a constant current source with the 4th op-amp section. What is a bit strange is that the current you get is .7V/4.7K = .15ma. From all the googling I've done, it apppears to me that most of these sensors want more like 1.5ma bias current. That had me very confused since with a 4.7K resistor, one would need a 7V reference (higher than the supply voltage) to get 1.5ma. Perhaps the lower resistor should be 470? If so, I assume the two resistors above it would also change value to 470 accomodate the higher current (right now they offset the bridge by (.15ma * 9400) or 1.4V + .7V provided by the lower resistor, for a total of 2.1V shift Also, I was wondering how you actually attached the sensor to the gauge. It appears from your photo that you accomplished this while leaving the pressure gauge functional (which is cool). Can you describe how you did that? Did you need to drill a hole into the brass on the gauge and braze a fitting on or something, or did you glue it somehow? I've never brazed brass before, so hints on your construction method would be appreciated. Thanks in advance, David Satterfield Jim Serack wrote: >David, >I have updated the Wiki to add the missing information that your questions >hilighted. Good luck and please let me know of your success. >Jim --------------------------------- Pinpoint customers who are looking for what you sell. ```
 Re: [mh] 1-wire Pressure Gauge questions From: David Satterfield - 2007-08-30 03:59:54 Attachments: Message as HTML ```Jim wrote: >I updated the Wiki with the mechanical assembly information. Thanks! >Looking at the high resolution images of the board the resistors are marked >472 - which is 47 followed by 2 zeros - or 4700 which is 4.7K. I can't really >remember doing anything other than measuring the voltage across the sensor >under operation in the original circuit and matching that in my circuit - it could >well be that the designer of the battery operated circuit wanted to minimize the >current drawn to get longer battery life. The impact is really the amount of gain >that you need in the analog section, and noise rejection. I also thought lower >current = longer life ... I expect that the strain guages are very linear with >current - unless you get too much and cause heating. Sorry about that, I think you are correct in using a 4.7K resistor. I got the same tire gauge you recommended in the wiki yesterday, and sure enough I measured around .1ma, not 1.5ma. So a .15ma bias current sounds like a good place to be. >You are right about the second and third resistors - they are just there to center >the signal into the working range of the 100X amplifiers. One minor nit: I think the 100x amplifiers are actually 201x amplifiers... I think the equation for the first stage is: (1 + 2R6/R5) = 201 Is this correct? >I also noticed in the photo that Vdd gets to the LM324 via a red jumper on the >surface - I remember now that was so I didn't have to do double sided circuit >board and vias. Perhaps the jumper is there to repair a manufacturing defect? The artwork does connect VDD to the LM324 pin 4...I think you managed to get all the connections made with no need for a jumper or vias. >It's in the wiki - but the mechanical connection for the pressure is a brass hose >barb 1/8 inch cut from an adaptor and soldered into a hole drilled at an angle >into the block of the existing guage. That is beyond cool :) I suspected you did something like that. Thanks again for all the help. You really did an awesome job not only creating this project but in documenting it for folks like me! I really appreciate it. I'm going out tomorrow to buy PC board etching supplies :) Thanks again, David Satterfield >Jim --------------------------------- Choose the right car based on your needs. Check out Yahoo! Autos new Car Finder tool.```
 Re: [mh] 1-wire Pressure Gauge questions From: Jim Serack - 2007-08-30 13:02:09 Attachments: Message as HTML ```Yes - thanks for the corrections - there is nothing like someone who is trying to understand to proofread :) -----Original Message----- From: misterhouse-users-bounces@... [mailto:misterhouse-users-bounces@...] On Behalf Of = David Satterfield Sent: August 30, 2007 12:00 AM To: misterhouse-users@... Subject: Re: [mh] 1-wire Pressure Gauge questions Jim wrote: >I updated the Wiki with the mechanical assembly information. Thanks! =20 >Looking at the high resolution images of the board the resistors are = marked >472 - which is 47 followed by 2 zeros - or 4700 which is 4.7K. I can't really >remember doing anything other than measuring the voltage across = the sensor >under operation in the original circuit and matching that in my circuit - it could >well be that the designer of the battery operated circuit wanted to minimize the >current drawn to get longer battery = life. The impact is really the amount of gain >that you need in the analog section, and noise rejection. I also thought lower >current =3D longer = life ... I expect that the strain guages are very linear with >current - = unless you get too much and cause heating. Sorry about that, I think you are correct in using a 4.7K resistor. I = got the same tire gauge you recommended in the wiki yesterday, and sure = enough I measured around .1ma, not 1.5ma. So a .15ma bias current sounds like a = good place to be. =20 >You are right about the second and third resistors - they are just = there to center >the signal into the working range of the 100X amplifiers. One minor nit: I think the 100x amplifiers are actually 201x = amplifiers... I think the equation for the first stage is: (1 + 2R6/R5) =3D 201 Is this correct? =20 >I also noticed in the photo that Vdd gets to the LM324 via a red jumper = on the >surface - I remember now that was so I didn't have to do double = sided circuit >board and vias. Perhaps the jumper is there to repair a manufacturing defect? The = artwork does connect VDD to the LM324 pin 4...I think you managed to get all the connections made with no need for a jumper or vias. =20 >It's in the wiki - but the mechanical connection for the pressure is a brass hose >barb 1/8 inch cut from an adaptor and soldered into a hole drilled at an angle >into the block of the existing guage. That is beyond cool :) I suspected you did something like that.=20 Thanks again for all the help. You really did an awesome job not only creating this project but in documenting it for folks like me! I really appreciate it. I'm going out tomorrow to buy PC board etching supplies = :) Thanks again, David Satterfield =20 >Jim _____ =20 Choose the right car based on your needs. Check out Yahoo! Autos new Car Finder tool.=20 ```
 Re: [mh] 1-wire Pressure Gauge questions From: Jim Serack - 2007-08-30 02:09:52 Attachments: Message as HTML ```David, =20 I updated the Wiki with the mechanical assembly information.=20 =20 Looking at the high resolution images of the board the resistors are = marked 472 - which is 47 followed by 2 zeros - or 4700 which is 4.7K. I can't really remember doing anything other than measuring the voltage across = the sensor under operation in the original circuit and matching that in my circuit - it could well be that the designer of the battery operated = circuit wanted to minimize the current drawn to get longer battery life. The = impact is really the amount of gain that you need in the analog section, and = noise rejection. I also thought lower current =3D longer life ... I expect = that the strain guages are very linear with current - unless you get too much and cause heating. =20 You are right about the second and third resistors - they are just there = to center the signal into the working range of the 100X amplifiers. =20 I also noticed in the photo that Vdd gets to the LM324 via a red jumper = on the surface - I remember now that was so I didn't have to do double = sided circuit board and vias. =20 It's in the wiki - but the mechanical connection for the pressure is a = brass hose barb 1/8 inch cut from an adaptor and soldered into a hole drilled = at an angle into the block of the existing guage. =20 Jim -----Original Message----- From: misterhouse-users-bounces@... [mailto:misterhouse-users-bounces@...] On Behalf Of = David Satterfield Sent: August 28, 2007 10:20 AM To: misterhouse-users@... Subject: Re: [mh] 1-wire Pressure Gauge questions Hi Jim, Thanks for the reply (again), I am getting closer, but still have a few = more questions: I did some more research after my second post requesting information = (and before your response) and realized that you were trying to make a = constant current source with the 4th op-amp section. What is a bit strange is = that the current you get is .7V/4.7K =3D .15ma. From all the googling I've = done, it apppears to me that most of these sensors want more like 1.5ma bias = current. That had me very confused since with a 4.7K resistor, one would need a = 7V reference (higher than the supply voltage) to get 1.5ma. Perhaps the = lower resistor should be 470? If so, I assume the two resistors above it would also change value to 470 accomodate the higher current (right now they offset the bridge by (.15ma * 9400) or 1.4V + .7V provided by the lower resistor, for a total of 2.1V shift Also, I was wondering how you actually attached the sensor to the gauge. = It appears from your photo that you accomplished this while leaving the pressure gauge functional (which is cool). Can you describe how you did that? Did you need to drill a hole into the brass on the gauge and braze = a fitting on or something, or did you glue it somehow? I've never brazed = brass before, so hints on your construction method would be appreciated. Thanks in advance, David Satterfield Jim Serack wrote: >David, >I have updated the Wiki to add the missing information that your = questions >hilighted. Good luck and please let me know of your success. >Jim=20 _____ =20 Pinpoint customers who are looking for what you sell.=20 ```

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