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I'm trying to design a wireless temperature sensor powered by one (or maybe two) rechargeable NiMH AA cell(s). The sensor should read and transmit the temperature every couple of seconds and remain in low power consumption mode the rest of the time.

I'd like the sensor to be built with the following components:

To power these components with the batteries I chose, I suppose I'll need a boost converter that generates 4 to 5V out of 1.2 to 1.5V input (or 2.4 to 3V if I use two cells).

My problem is that I can't find the right boost converter that is simple and cheap enough for me to build, and that I can easily get parts for. For example, I've found these really cool resources online:

From App Note 30 above, I could select a couple of circuits that I think could meet my requirements, such as:

  • Figure 9. Boost Converter (1.5V to 5V)
  • Figure 11. Single Cell Up Converter (1.5V to 5V)

The problems I have with those circuits is that I don't have easy access to some of the components, such as the LT1018, LM10 or the TRIAD SP-29 (I don't even know what that is), for example.

So, my question is what would be an appropriate boost voltage regulator circuit for my specs?

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    \$\begingroup\$ Single NiMH cell power for microcontroler is not that simple. NiMH cell near discharged state can be only 1V. I was looking for DC/DC converter for single NiMH too, but there are just few on the market and they are not available everywhere. You should use diffrent chemistry (LiIon? LiPo?) or few smaller NiMH cells. \$\endgroup\$ – Kamil Jun 1 '14 at 19:33
  • \$\begingroup\$ @Kamil - Thanks much for the info! Well, if it is that difficult to use a single cell, I guess I can go for two cells. That certainly makes the design a bit easier. \$\endgroup\$ – Ricardo Jun 1 '14 at 20:11
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    \$\begingroup\$ Difficulty is caused by voltage drop on diodes in converter. When you have 1V (discharged NiMH) - voltage drop on single diode (with 0.2V drop) is 20% of whole input voltage. That makes converter inefficient/harder to build. \$\endgroup\$ – Kamil Jun 2 '14 at 0:34
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    \$\begingroup\$ Isn't it a lot easier to chose for the smaller variant of AA batteries and just use a 12V battery? These batteries are like halve the size of an AA battery and since they're 12V you can simply use a 7805 IC to get a 5V out of it. Or just use some resistors to lower your voltage to 5V. (A23 batteries) \$\endgroup\$ – Handoko Jun 5 '14 at 22:27
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    \$\begingroup\$ Triad SP-29 is a transformer made by Triad Magnetics. \$\endgroup\$ – Nick Alexeev Jun 6 '14 at 20:41
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Why are you not going ahead with a standard DC/DC boost IC? Starting at $0.9, that should be fairly accessible to you. No reason to reinvent the wheel here. A quick search through digikey gives plenty of options with input ranges for this search going down to at least 1V input with 5V output.

Here's one of the datasheets on a cheap one. All you need to add is two caps and an inductor. You state that you're designing a wireless sensor module, not a DC/DC converter, so I assume my answer is valid for your requirements.


EDIT - Now that I know why you're designing a boost converter rather than buying:
I would go for a boost converter driven through a low voltage 555 if you can find one of those easily. If there's no low voltage 555 timers, you could also just use an oscillator to drive the boost converter. One oscillator that I've used previously is a ring oscillator with an RC addition for control knobs to drive a boost converter. I was able to achieve 18V out of 5V at ~80% efficiency. Another simple oscillator that could likely be usable is a basic astable multivibrator.

If you don't want to go with a standard oscillator based boost controller, you could try the the joule thief or something similar.

With any of these multi-chip or discrete component solutions, make sure you add in a 5V linear voltage regulator after it to ensure stable operation after the boost stage.

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  • \$\begingroup\$ Thanks for the links, they are helpful. My problem is finding boost IC at the local (national) shops here in Brazil. I've selected plenty of them already, but none of them are readily available here. Examples are: LT1302, NCP1402-5V, LT1615, L6920DC, REG710 etc. When I posted the question I was hoping that among the vast set of options that are out there, someone would be able to point me to one or two DC/DC boost IC that stand out as more popular at those voltages, like the LM7805, for example. But I guess that no single IC stands out enough for Brazilian shops to hold them in numbers. \$\endgroup\$ – Ricardo Jun 5 '14 at 22:24
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    \$\begingroup\$ Actually, I've purchased a few MC33063 from Farnell-Newark in Brazil which takes 3V \$V_{in (min)}\$and can be adjusted to output 5V, for example. But 3V is 3 NiMH AAs, so it just saves from 1 AA (with 4AA I can just toss the converter). \$\endgroup\$ – Ricardo Jun 5 '14 at 22:30
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    \$\begingroup\$ Thanks for the update! But going through your DigiKey list, I think I found one IC that meets my needs and is actually available at Br Farnell: The LTC3525-5. So you're winning!! \$\endgroup\$ – Ricardo Jun 6 '14 at 10:34
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    \$\begingroup\$ @Ricardo Glad to hear that you've found something for your issue! That chip looks like a good simple chip for the job. \$\endgroup\$ – horta Jun 6 '14 at 13:42
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    \$\begingroup\$ This has been the most helpful answer, so I've assigned the bounty to it. Thanks! \$\endgroup\$ – Ricardo Jun 7 '14 at 23:29
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Your problem comes from the use of the LM35 temp sensor. You try to use that instead of a more appropriate choice, which causes you to include more circuitry.

  • Replace it by a sensor that works at 3V (LM75 clone? NTC?) and you can use a single LiSOCl2 cell...

  • If you also select a uC that works down to ~ 2.5V you can use two AA cells

  • if you really want to boost a single 1.5V: google "boost 1.5V" (what else?) for some suggestions

  • or google "boost 1.5V dealextreme" ...

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    \$\begingroup\$ (answering a comment that just disappeared about working near 1.0V) Dunno. Maybe not down to 1.0V. But my main point is that (from a systems design point of view) the question is too narrow. It should be more like 'I want to design a small, rechargeable-powered, temperature transmitter'. (With quantification of small, and of the lifetime and tyransmit interval.) That would allow much wider and better answers. \$\endgroup\$ – Wouter van Ooijen Jun 1 '14 at 19:45
  • \$\begingroup\$ Thanks for the answer. It's already widen my search vocabulary a lot. The question is narrow because those are my requirements. I'm pretty sure if I let you guys design the sensor, you'd come up with something sleek, but it would be nowhere near my build capabilities. And also, you'd be left with all the fun... Now seriously, I'd drop the LM35 in a heartbeat if I could find a decent replacement for it, but I can't. It's the first time I heard about LM75, but it has 2 to 3°C accuracy, against .5°C of the LM35, and it's 3V against 4V of the latter, so no big voltage saves there either... \$\endgroup\$ – Ricardo Jun 2 '14 at 2:51
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    \$\begingroup\$ Microchip's TCN75 works down to 2.7V. Watch out when comparing accuracy: that 0.5C is typical (at 25C), the 3C is guaranteed (over the entire range). And reading an LM35 requires an absolute voltage reference, which adds to the component cost & the inaccuracy. \$\endgroup\$ – Wouter van Ooijen Jun 2 '14 at 6:44
  • \$\begingroup\$ 2.7V is more within the range of two NiMH AAA cells... \$\endgroup\$ – Ricardo Jun 2 '14 at 11:32
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If you run at 3V, you can use a 555 timer to create a Voltage doubler or look up the setup for other Voltage Multipliers like Dickson charge pump might work and could potentially be driven by the ATtiny rated at 1.8V if you operate at 3V.

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  • \$\begingroup\$ Thanks for your answer. Yes, I've been looking at voltage doublers as well, but I was unsure about whether they would deliver the current I need (around 50mA, I guess) and what would be the voltage drop if I draw that current. Do you think I can get that kind of current out of any of these supplies? \$\endgroup\$ – Ricardo Jun 5 '14 at 22:18
  • \$\begingroup\$ I got to those multipliers pages you linked, but dismissed them on the impression that they are designed to work with AC inputs, not DC. Isn't that the case? \$\endgroup\$ – Ricardo Jun 5 '14 at 22:36
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    \$\begingroup\$ Look at the Dickson Charge Pump section which is has DC input and is driven by two pulse trains with opposite phases \$\endgroup\$ – Kvegaoro Jun 5 '14 at 22:39

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