Sensing flowmeter / protecting analog input

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1

Hi,

I’m looking to build a system to sense ‘steps’ of a hall-effect flowmeter.

My first throught is to use a voltage divider and rectifier circuit to get down to the 0-1.8v I need.

The problem I have is that every install may potentially have a different input voltage.

I’m wondering if there’s a simpler way to do this. I really only need to clip the voltage between 0-1.8v, it doesn’t matter if it spends 90% of it’s time clipped and throws away the negative half of the signal completely as I’m only interested in a count.

The current draw will be in the millivolt-range used by the adc input, so it shouldn’t need anything too heavy.

My first thought is a simple voltage regulator set to 1.8v, but I don’t know enough about them to know whether it’s a viable option or which to choose.

Any suggestions?

2

Hmm

Maybe a 1.8v zener ?

Or 1N4007’s in series = 1.4V

“No one could make a greater mistake than he who did nothing because he could do only a little.”

“All that is necessary for the triumph of evil is that good men do nothing” Edmond Burke (1729 - 1797)

http://www.packtpub.com/building-a-home-security-system-with-beaglebone/book

3

Hmm

Maybe a 1.8v zener ?

4

John,

Digital input, hadn’t thought of that but it makes sense, but I’m even less sure of myself in that arena…

I have to admit first my electronics knowledge is very rusty…

Maybe I can use a voltage divider after the diode/s to halve that 1.8v+ down to a safe 1.0v-ish.

My first thought here is to use a zener diode as in here http://www.electronics-tutorials.ws/diode/diode42.gif) with a voltage divider after.

Is the reverse voltage low enough to be safe with the BBB?

If not, what if I set up something like this, but make D_Z1 a normal diode so that the reverse current is completely stopped?

Again, not interested at all in the quality of the signal as long as I can determine the number of cycles.

Sorry for the electronics 101 questions, appreciate all your time.

Cheers,
Paul.

5

John,

Digital input, hadn’t thought of that but it makes sense, but I’m even less sure of myself in that arena…

I have to admit first my electronics knowledge is very rusty…

Maybe I can use a voltage divider after the diode/s to halve that 1.8v+ down to a safe 1.0v-ish.

My first thought here is to use a zener diode as in here http://www.electronics-tutorials.ws/diode/diode42.gif) with a voltage divider after.

Is the reverse voltage low enough to be safe with the BBB?

If not, what if I set up something like this, but make D_Z1 a normal diode so that the reverse current is completely stopped?

Again, not interested at all in the quality of the signal as long as I can determine the number of cycles.

Sorry for the electronics 101 questions, appreciate all your time.

Yeah, the reverse voltage will exceed the limits set on the ADC so a zener will not work for you. Here is a document that explains the principles that will help you.

http://www.ti.com.cn/cn/lit/an/sboa097a/sboa097a.pdf

Or search Google for "adc signal conditioning circuit” for more references.

Regards,
John

6

John,

Digital input, hadn’t thought of that but it makes sense, but I’m even less sure of myself in that arena…

I have to admit first my electronics knowledge is very rusty…

Maybe I can use a voltage divider after the diode/s to halve that 1.8v+ down to a safe 1.0v-ish.

My first thought here is to use a zener diode as in here http://www.electronics-tutorials.ws/diode/diode42.gif) with a voltage divider after.

Is the reverse voltage low enough to be safe with the BBB?

If not, what if I set up something like this, but make D_Z1 a normal diode so that the reverse current is completely stopped?

Again, not interested at all in the quality of the signal as long as I can determine the number of cycles.

Sorry for the electronics 101 questions, appreciate all your time.

BTW, what is the flowmeter part number. Perhaps there is a much simpler way to interface this to the BBB.

Regards,
John

7

John,

Cheers, it’s a Gicar 9.0.95.05g – sits in a cafe-style coffee machine, though this’ll only be the first install and the others may be different (but presumably similar) parts.

8

John,

Cheers, it’s a Gicar 9.0.95.05g – sits in a cafe-style coffee machine, though this’ll only be the first install and the others may be different (but presumably similar) parts.

OK, I couldn’t find a data sheet, but I think this is a three pin device, GND, POWER (4.5 - 20V) and PULSE. If this is correct, then all you need is a resistor divider as you suggested and a schmitt trigger to clean up the signal and then feed this directly into one of the GPIO pins. The schmitt trigger should be powered from VDD_3V3B and enabled with SYS_RESETn.

Regards,
John

9

Yes, that sounds exactly like the device.

Awesome, didn’t even know such a beast existed. Will do my reading tonight.

Thanks again,
Paul.

10

Yes, that sounds exactly like the device.

Awesome, didn’t even know such a beast existed. Will do my reading tonight.

http://www.ti.com/lit/an/scea046/scea046.pdf

Page 4 explains the concept.

Regards,
John

11

I’ve had a bit of a dig on element14, this seems to fit the bill (http://au.element14.com/texas-instruments/sn74lvc1g14dbvr/ic-schmitt-trigger-inverter-smd/dp/1470878) but it’s not suitable for a breadboard prototype. You don’t know where I should be looking for a suitable prototyping product by any chance?

Thanks,
Paul.

12

I’ve had a bit of a dig on element14, this seems to fit the bill (http://au.element14.com/texas-instruments/sn74lvc1g14dbvr/ic-schmitt-trigger-inverter-smd/dp/1470878) but it’s not suitable for a breadboard prototype. You don’t know where I should be looking for a suitable prototyping product by any chance?

http://au.element14.com/jsp/search/browse.jsp?N=2101+203662+110122219+110112113&Ntk=gensearch&Ntt=schmitt+trigger&Ntx=mode+matchallpartial&No=0&getResults=true&appliedparametrics=true&locale=en_AU&divisionLocale=en_AU&catalogId=&skipManufacturer=false&skipParametricAttributeId=&prevNValues=2101+203662&mm=1001353||,1002627||,&filtersHidden=false&appliedHidden=false&autoApply=false&originalQueryURL=%2Fjsp%2Fsearch%2Fbrowse.jsp%3FN%3D2101%2B203662%26Ntk%3Dgensearch%26Ntt%3Dschmitt%2Btrigger%26Ntx%3Dmode%2Bmatchallpartial%26No%3D0%26getResults%3Dtrue%26appliedparametrics%3Dtrue%26locale%3Den_AU%26divisionLocale%3Den_AU%26catalogId%3D%26skipManufacturer%3Dfalse%26skipParametricAttributeId%3D%26prevNValues%3D2101%2B203662

If this link doesn’t work, search for “schmitt trigger” then select “Gates & Inverters” and then select “Logic case style” of “DIP”

Regards,
John

13

I’ve had a bit of a dig on element14, this seems to fit the bill (http://au.element14.com/texas-instruments/sn74lvc1g14dbvr/ic-schmitt-trigger-inverter-smd/dp/1470878) but it’s not suitable for a breadboard prototype. You don’t know where I should be looking for a suitable prototyping product by any chance?

Or you could use the part you selected and use this adapter:

http://au.element14.com/capital-advanced/33205/convert-from-5-sc-59-sc-74a-sot/dp/1654363

Regards,
John

14

I’ve had a bit of a dig on element14, this seems to fit the bill (http://au.element14.com/texas-instruments/sn74lvc1g14dbvr/ic-schmitt-trigger-inverter-smd/dp/1470878) but it’s not suitable for a breadboard prototype. You don’t know where I should be looking for a suitable prototyping product by any chance?

One more thing. If you do use this part with an adapter, don’t use any of the pins LCD_DATA[15…0] for GPIO or you board won’t boot. If you want to use any of these pins, you must use a schmitt trigger with an Output Enable (OE) pin, which you tie to the SYS_RESETn pin.

Regards,
John