View Full Version : 3.3 volts

10-24-2009, 01:36 AM
I have a stamp activity board with bs2.
·Has anyone ever run both or just a bs2 on 3.3volts
I would like to do some serout to another module for a quick test.



10-24-2009, 05:55 AM
The supply voltage range for the BS2 is 5-15V.


Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

10-24-2009, 06:18 AM
Thats going through the regulator! can't I bypass the regulator and go direct to the output pin with 3.3v

Mike Green
10-24-2009, 06:24 AM
You could, but it wouldn't work. The BS2 is designed to operate with either a regulated 5V supply or an unregulated 6-15V supply. There's a brownout detector on the module, used to hold the processor in a reset if the supply voltage drops below 4.7V or something like that. That's to prevent the processor from becoming unreliable at lower supply voltages and accidentally erasing the EEPROM or some other mishap.

Post Edited (Mike Green) : 10/23/2009 11:51:58 PM GMT

10-24-2009, 09:17 AM

Use a voltage divider on the output of the BS2 to drop to 3.3volts. A singal 2N2222/or MPS3904 can amp up 3.3volts to 5volts for a BS2 input

The Truth is out there············___$WMc%___······················ ······ BoogerWoods, FL. USA

11-13-2009, 07:22 PM
I was looking for a way to go the other way, to use the new pressure sensor with my BS2, looking to go down to a max of 3.3 volts with a zener diode and resister. I didn't realize that it wouldn't work out of the box with my BS2... Any problems with this approach? Still new to the Stamp 2.

I know I am not the only one; any points on the subject might help others...

Thanks in advance

Larry Stroud

PJ Allen
11-13-2009, 08:19 PM
Zeners, amping up?· What's that all about?

On page 43 of this·PDF (http://www.nxp.com/acrobat_download/literature/9398/39340011.pdf)·(see Fig. 45) you will find the recommended circuit for interfacing 3V circuits to 5V circuits.· I'm sure that somebody will pop off about using a resistor or something.· There's the right way to do things and then there are short-cuts, quick-fixes, et al. ·(N.B., the road to Hell is paved with good intention.)

11-13-2009, 11:20 PM
There is also a circuit for this in the sticky at the top of the propeller forum since the prop is 3.3 and needs to talk to 5v sensors. (Parallax approved method)

- Stephen

11-13-2009, 11:34 PM
Here is what I ended up doing SparkFun . Com
sparkfun, bob-08745, logic level converter, 1.95 each
I ordered a few its easer than trying to put the smoke back in!
These things are about 1/2 inch long x 1/4 inch and there four channels in one package
Two low voltage to high voltage and two high voltage to low voltage
It would cost more to buy a transister than this complete package.

Tracy Allen
11-13-2009, 11:34 PM
Larry Stroud,

The Parallax pressure sensor (VTI SCP1000) implements the SPI interface, and there are separate pins for input and output. So for the stamp output to VTI input you can use two resistors to step down the voltage, to the SCLK and MOSI signals:

3.3V device inputs SCLK and MOSI
10k | 20k
p0, 5V ----/\/\/\---o-----/\/\/\-------Vss

p1 -----------/\/\/\--------------- 3.3V device output MISO

or replace the 20kOhm resistor with a zener. On the input side, from the VTI MISO, the Stamp will respond fine to 0 to 3.3 Volt inputs, because the Stamp switching threshold is near 1.4 Volts. You will need a voltage regulator to supply the 3.3 volts to the VTI chip. There are also chips that can do the level translation, such as the CMOS CD4050, which can be powered from 3.3 volts and give 3.3 volt outputs, but also take 5 volts on the inputs.

Other more modern chips implement essentially the circuit shown in the pdf that PJ mentioned. That is suitable for I2C level translation, where there needs to be bidirectional signaling with pullup resistors.

There are some Stamp products that can be set to work directly at 3.3 volts, such the MOBO Stamp, or our OWL2pe data logger.

Tracy Allen
www.emesystems.com (http://www.emesystems.com)

Beau Schwabe (Parallax)
11-14-2009, 12:25 AM
I agree with PJ Allen.

Just about any resistor solution relies on using the ESD process diode and will inject unnecessary noise into the IC. Other methods might incorporate a diode to "simulate" an open collector driven output and this works fine, but only if the voltage levels are still the same.

The diagram that PJ Allen refers to is the one that should be used... If you would rather a Bi-polar version instead of the MOSFET version, then the below schematic is basically the Bi-polar equivalent of what PJ is talking about.

Bi-polar version of the Bi-directional 3.3V to 5V Emitter Follower (http://forums.parallax.com/attachment.php?attachmentid=58334)

Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 11/13/2009 5:30:55 PM GMT

Tracy Allen
11-14-2009, 03:45 AM
The watchwords are "bidirectional" and "open collector (drain)" or "pullup resistor". That appleis to I2C signaling, which has pullups and is always bidirectional on the dta line, but only rarely bidirectional on the scl line.

If the signaling is unidirectional and/or if there is no pullup resistor, things can be simplified. For example, with SPI the simple voltage divider from 5V to 3V, or series resistor from 3V can work fine. The Stamp has plenty of noise immunity when presented with a 0 to 3.3 volt signal at its input.

There should always be protection resistors in case the power supplies come up at different times. That applies also to the mosfet or bipolar circuits that PJ and Beau posted , because of the currents that can flow through the protection diodes if one power supply comes up before the other.

Tracy Allen
www.emesystems.com (http://www.emesystems.com)