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Topic: Using Arduino Pro 3.3V with step up converter (Read 2143 times) previous topic - next topic

Priit

I am trying to build a datalogger, where are included several sensors. One of them CO2 sensor(K30 Sensirion), which needs too much energy to log data several weeks and therefore I decided to turn the CO2 sensor ON and OFF after some interval.

As I am using Arduino Pro 3.3V board for reading Analog values from CO2 sensor I need Step UP converter for CO2 sensor as it works with 5V.
Step up converter link:
http://www.sparkfun.com/commerce/product_info.php?products_id=8290

To turn ON Step UP converter and CO2 sensor I am using transistor BC639.

CO2 sensor needs 150mA(peak) for a measurement. Altogether Step UP and CO2 sensor need I guess over 200mA.

Problem is that voltage after switching with arduino digital pin and bc639 the voltage after Step UP converter raises only until 2V, not until 5V.

I guess that problem is about selecting the right transistor for this work, but I don't know which transistor would be good enough.

I hope that someone can help.

Grumpy_Mike

Quote
I guess that problem is about selecting the right transistor for this work,


No the problem is wiring it up right. How have you wired it up?

Priit

Here is the schema:


I also tried without DC/DC converter and then I could switch CO2 sensor ON and OFF.

Grumpy_Mike

A bit of a puzzle that as you have a block labelled 7805 VPack Step up. the 7805 is in fact a voltage regulator.

In any case you do not switch an electronic circuit like this by just trying to switch the ground, as you have found it won't work.

The best way to switch the power to this is to use a PNP transistor (or FET) with emitter to the + of the battery, collector to Vin of your device, base resistor to + of battery. Finally another resistor from the base to the arduino output. A logic high will then turn it off and a logic low will turn it on.

Priit

Thank You for clarifying the usage of PNP transistor.
Could You help me also to select the transistor?
I have chosen a BC327 transistor, but the Vin voltage raises only until 1.2V.

Rb = 1.8k
Rvin = 10k

CO2 sensor and step up converter current can be up to 300mA.

MarkT

Basically to select a transistor you need to work out the voltage, current and gain requirements.  Here voltage is 5V, that won't be an issue, current say 300mA, so call it 0.5A to be on safe side, and just have to make sure the base is driven with enough current to saturate it (drive current * current gain must exceed the load current).  Then check the power dissipation just in case.   The BC327 looks OK.  Just remember emitter to the +5V, collector to the load, base via a 470ohm resistor to Arduino (needs at least 8mA to drive as minimum gain is 40)

BTW where is the 5V going to come from if the battery is 3.7V?
[ I won't respond to messages, use the forum please ]

Priit

The problem with 3.7V I wanted to clear out also.

As I wrote before, the battery voltage is 3.7V and I am using Step UP converter to raise the voltage.

Now, if I construct the schema for sensor only and supply voltage is 5V, then everything works fine.

After trying to use voltage converter between battery and sensor voltage after step UP converter stays low. voltage on sensor raises only until 2V high.
Otherwise step UP converter is working fine.

Priit

I also add here current schema for better understanding


I hope it helps.

Grumpy_Mike

Not sure what is going on here. You have a sensor that you are powering from the step up converter, however you don't show how that sensor is fed back into the arduino.

Also you have the arduino powered off the battery, it won't run correctly like this according to the data sheet. First the clock is too fast and second the brownout might kick in.

However the circuit for controlling the step up looks right. If you short out the collector / emitter does the output go to 5V?

Priit

Thank You for the answers.
I have managed to put schema together and the final solution was like this:


One question I still have. The step UP converter is whisteling when the CO2 sensor is turned on.

Maybe there is an solution. The current seems to go very close to 300mA for a moment, but is it a problem?

Priit

Grumpy_Mike

Yes step up converters do make a noise, it is the inductor vibrating. It get louder the more current is drawn.
I still don't see how the sensor is read.

Priit

I didn't draw the sensor on the circuit as it was just informative circuit. Sorry for not clarifying that in last post.

Sensor is read with analog port as the CO2 sensor delivers 0..4V analog.

About the noise:
I also noticed, that when input voltage is over 3.8, then converter is making much more noise, than when it is under 3.8V.

Grumpy_Mike

#12
Mar 03, 2010, 11:27 pm Last Edit: Mar 03, 2010, 11:28 pm by Grumpy_Mike Reason: 1
Quote
delivers 0..4V analog.

If that is 0 to 0.4V then you are OK, if it is 0 to 4.0V then you are not.

So if you feed that back to the arduino being powered by 3.3V, you are exceeding the arduino ratings and damaging it. That is why I asked.
To a first approximation no input to a chip is permitted to be above it's supply voltage.

Quote
then converter is making much more noise, than when it is under 3.8V.

The noise is what you would normally expect.

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