3.3V and 5V sensor voltage requirements: Parallax customer input requested!

Ken Gracey

Hey there,

We're designing a small breakout board with the VTI SCP1000 Pressure Sensor (0-30K feet above msl). This sensor requires a 3.3V power supply.

Propeller users will have 3.3V·and 5V power supplies·on all of·our demo boards. BASIC Stamp·boards have only 5V power supplies. As time passes and we will produce more breakout boards·based upon·prior designs. My personal preference is to keep the designs simple and raw (no voltage regulator), clutter-free and with the minimum passive component/adapter board·circuitry needed to use the device.

To some·customers this isn't usable out of the bag because they need 3.3V.

Would BASIC Stamp programmers be happy if we had a·breadboard-friendly 3.3V regulator in our component shop and didn't include this in·the design?

Propeller customers okay without a regulator?

Your input is welcome and appreciated.

Thank you,

Ken Gracey



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SRLM

I'd vote for plain, no regulator. If you did have a regulator, I'd vote for a small SMD solution that has a jumper so that we can bypass it.

Oh, another thing. I suppose it should go into the new products page, but one thing that I'd like to see parallax carry are the small jumpers to jump headers. I know Mouser and other places carry them, but I'd prefer to order from Parallax...

John Abshier

As a Propeller user, I prefer 3.3V sensors because I don't have to worry about voltage mismatch. Leaving off the 3.3V regulator makes it cheaper, another plus. A 3.3V regulator for the BASIC Stamp would be more expensive for the first sensor. As a STAMP user I would be worried about breadboard space. Perhaps the 3.3V regulator could plug into the AppMod connector.

John Abshier

Phil Pilgrim (PhiPi)

Ken,

Here's how I handle that with my daughterboards: Each daughterboard has a 5V and a Vdd input. Sometimes Vdd is 3.3V; sometimes, 5V, and the board has to work both ways. There are three situations that have to be addressed:

1. Board contains components that will run on 3.3V or 5V interchangeably: use the Vdd input with no add'l regulation.

2. Board contains 5V-only components: power the board from the 5V input and interface with 3.3V-compatible (e.g. 74HCT) interface logic, if necessary.

3. Board contains 3.3V-only components: power the board from 5V and include an onboard 3.3V regulator. Use 5V-tolerant (e.g. 74LVC) interface logic, if necessary.

I definitely think you need to provide a canned regulator solution. I've seen way too many threads in these forums dealing with misapplied linear regulators, often due to poorly-chosen caps. Having a drop-in solution, with the proper caps, would eliminate a lot of tech support calls (and the expense associated with each one).

There are some regulators that use MOSFET pass transistors and have almost zero dropout voltage at modest currents. You could easily feed these with either 5V in or 3.3V in and still get 3.3V out — no jumpers required. That would simplify the user interface enormously.

-Phil

SRLM

Another slightly OT post, but one thing that I'd like to see on the Parallax website is a "New Products" page where the latest dozen or so products are displayed. I periodically print out documentation, and I forget what I need to print...

Since I've started down the path of OTness, I thought I'd add one more thing: The Propeller page http://www.parallax.com/tabid/407/Default.aspx lists the 1.0 version of the datasheet for download, but the current version is 1.2.

Post Edited (SRLM) : 6/8/2009 9:00:42 PM GMT

Lauren Davis

Dear SRLM,

All new products can be found on this page http://www.parallax.com/tabid/753/Default.aspx. You can navigate to this page from the front page of the Parallax site (beneath the three square images). You have the option of visiting a page displaying new products, featured products, or subscribing to the RSS feed.

Thank you very much for telling us about the datasheet. We have updated the web.

Kind Regards,
Lauren

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Lauren Davis
Marketing Manager
Parallax, Inc.
916-624-8333
ldavis@parallax.com
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John R.

I would suggest that with 3.3 volts becoming more common, leaving the regulator off would make sense. Those using the STAMP line (or SX at 5V) the need to supply 3.3 volts will become more common, not just for Parallax sensors, but for other devices too.

In addition to the cost factor for one sensor, as more 3.3 products become available, why should we have multiple 3.3 volt regulators on each sensor?

I'm sure the Parallax documentation team can make putting a 3.3 volt regulator on something like the BOE so simple that even a toddler can do it. (The human kind, not the mechanical version)

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John R.
Click here to see my Nomad Build Log

Mike Green

I like the idea of a 3.3V adapter for the BOE. As John Abshier suggested, perhaps this could plug into the AppMod connector. It could provide a 3.3V regulator and a few 5V to 3.3V logic level interfaces like the ones that use a MOSFET to provide a 5V to 3.3V bidirectional interface for an I2C bus. There would be a female header with the 3.3V supply and the 3.3V logic I/O.

Humanoido

I don't know if this helps but here is my idea and thinking. I'm a Basic Stamp lover, starting from the BS1 and up. Absolutely every sensor should work with this product line. I am also a beginner to the amazing Propeller. It would help if the sensor was one small unit and instantly workable out of the box for use with both Basic Stamps and Propellers to capture both markets. This end user wants a complete working product with no fuss, the most simple demo, and least power draw. It should work with the BS1 on up to the Propeller. If this is not economically feasible, one option is to make 2 versions, one for Stamps and one for Propellers. It would not be so convenient to add parts to a purchased sensor board unless it was intentionally marketed as a solder-needed kit. I personally like a finished working and tested board for rapid development - Plug n' play is always a good concept.

humanoido

Chris Savage

This is adding fuel to the fire I know, but from an engineering stand-point we’re faced with two issues when making a product compatible with both 5V and 3.3V systems. The first and obvious problem is the voltage level of the device. The regulator is the obvious part to think of as it supplies the 3.3V required by the device and can be gotten from the 5V device power supply. What may not be immediately apparent though is this is only half of the power problem. With the regulator the device now has 3.3V however you still need to connect its I/O to a 5V system. This can sometimes be as simple as resistors, but more often requires FETs or level translators.

The second issue faced when making a product compatible with both 5V and 3.3V systems in minor differences in operation cause by running the device at 3.3V when it has a 3.3V LDO regulator for 5V operation. Some devices are affected either in operation, calibration or both by the supply voltage. When operating from a 5V supply the 3.3V regulator drops the voltage down to 3.3V and everything is fine. But when you apply 3.3V there is a small voltage drop and the device is now running at anywhere from 3.0V – 3.27V depending on the LDO used. This can affect devices that use the supply voltage as a reference, such as modules containing an ADC. This then introduces potential calibration issues. Realistically some of these devices may work on the BASIC Stamp but really may benefit from a faster microcontroller such as the Propeller anyway. These are just some things to keep in mind on this topic. It is also why many experimenter and break-out boards don’t have this circuitry included and are very minimal.

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Chris Savage
Parallax Engineering

Phil Pilgrim (PhiPi)

Chris,

Take a look at the MIC5203. It's an inexpensive LDO with a PNP pass transistor, and it's stable with cheap ceramic caps. At the SCP1000's current requirement, for example, the dropout voltage is almost nil. Also, most 3.3V devices will run at 3.0V, so if consistent analog calibration is necessary, you will get better regulation at 3.0V with a 3.3V supply.

I've seen so many instances in the forum where regulators are misapplied, often due to inappropriate capacitors. Given the choice, I'd make a sensor module as drop-in easy to apply as possible.

-Phil

Whit

Ken Gracey (Parallax) said...

·My personal preference is to keep the designs simple and raw (no voltage regulator), clutter-free and with the minimum passive component/adapter board·circuitry needed to use the device.

To some·customers this isn't usable out of the bag because they need 3.3V.

Would BASIC Stamp programmers be happy if we had a·breadboard-friendly 3.3V regulator in our component shop and didn't include this in·the design?

Ken,

What you suggest here seems good to me - especially with Parallax's (always excellent) documentation.

On the product info page, a simple link to the 3.3v regulator for Stamp Users and those with 5v systems along with the diagram for proper hook-up would be great. I think that Parallax selling the regulator would be nice for folks who might not know how to go about·looking for this and the documentation could show them how to do it (and maybe some sample or test code).

The whole thing would also be educational and make novice users aware of the issues involved.

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Whit+


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