# .4 V DC to 5 V DC possible?

Discussion in 'General Electronics Discussion' started by zach48191, Apr 14, 2012.

1. ### zach48191

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Apr 14, 2012
I have an electronic project I am working on and we are almost done with it. I had a question of raising a low DC voltage to 5V DC so we can finish our second part of our project. I have looked into 741 OP amps non-inverting and I read that they require AC to get them to work. Also I have looked at DC to DC convertors, but all of them require atleast 3.3V DC to work. I was mainly wondering if it is possible and if so what can I do to make it work.

Any help would be appreciated and I really need to find an answer to this problem, because it is the final problem we are having with our project.

2. ### Wabajig

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Apr 14, 2012
Can you explain your project and the type of supply you are using now? What voltage are you trying to get to and with what input voltage?

Last edited: Apr 14, 2012
3. ### CocaCola

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Apr 7, 2012
What he said, and what is the end purpose of the 5v out?

Likely extreme overkill but Linear Tech makes a DC to DC converter that goes down to 20mV, you might even be able to use what has become called the 'Joule Thief' circuit, or one of the better designed derivatives of that circuit in some cases...

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The question is "why?"

If it's a signal, then a 741 would be fine. You can operate it from a (say) +/- 9V supply rail and adjust the gain so that 0.4V is amplified to 5V.

If it's for power (e.g. getting power from a single solar cell) then you need some sort of DC-DC converter, but ones which operate at low voltages like this are not easy to find. We'd also need to know the power you ant to get out of it and what the available input power is (for sanity checking purposes)

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,449
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Jan 21, 2010
Does that require an external power supply for this?

What's the minimum voltage it can start from without external power?

6. ### zach48191

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Apr 14, 2012
I dont have a schematic drawed up yet on the CPU its mainly on paper but a transitor is going to power up a relay with the 5v to turn on a 60W light. We have separate power to the transistor and anything else, but the main function is that with the .4 VDC coming in it needs to be 5V so it will flip the relay and turn on the light

7. ### CocaCola

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Apr 7, 2012
http://www.linear.com/product/LTC3108

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,449
2,809
Jan 21, 2010
very interesting, and very exotic!

OK, so a 0.4 volt signal.

Simply find a comparator which will operate from a 5V supply and which has inputs that can go to ground. Use this to compare the input to (say) 0.2V and switch on a transistor when the voltage exceeds it.

If your 5V is well regulated you can use a resistive divider to get approx 0.2V

Have a look at comparator circuits and see what you can figure out.

9. ### CocaCola

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Apr 7, 2012
I agree, and at only about \$5 I might just have to pick one up to screw around with after this...

10. ### zach48191

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Apr 14, 2012
AWESOME I am going to buy thatLTC3108 btw which one should I buy for the breadboard?? Also is there a website other than linear that you can buy one and get it overnighted?? I tried to buy one from linear and it keep not letting me select one it was weird? The circuit we built is a LED Thermometer that at a certain temperature it will turn on a heating light. This application will be used to control the temperature of lizard pets. As the temperature goes higher it will turn the light off and will turn the light on when it goes past .4 V. We are using this http://www.aaroncake.net/circuits/ledtemp.asp as the temperature sensor

11. ### zach48191

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Apr 14, 2012
After we complete the project I am going to solder it to a board and map the board out inclose the case and move the LM34 DZ in the middle on the bottom of the cage so it can detect the temperature correcttly. Took us awhile to calibrate the LM3914 as the directions on that website and not correct and will not function the circuit like intended.

12. ### CocaCola

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Apr 7, 2012
www.findchips.com

Get the SSOP package unless you are experience with DFN as it's one small bugger to solder to on a bread board

Also consider reading up a little on the circuit and application of the chip so that you can pick up any support parts on the same order before you spend money on overnighting it...

13. ### CocaCola

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Apr 7, 2012
Now, I'm confused... Why not just use the outputs of the LM3914?

Using the circuit you linked to, coupled with a transistor and relay you should be able to simply use the appropriate outputs of the LM3914 to trigger on/off status...

Last edited: Apr 14, 2012
14. ### TedA

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Sep 26, 2011
zach48191,

So you plan to use an LM34 temperature sensor, and need to use its output to switch your relay at a particular temperature?

If this is true, you do not need the LTC3108 at all. It is a neat part, but does not do what you want.

If you are building the LED thermometer described in the link, you already have most of what you need.

The linked thermometer project uses an LM34 temperature transducer to drive an LM3414 bar graph display driver to indicate a range of temperatures.

You can easily interface the LM3914 outputs to drive your relay. A single transistor can be used to do this.

Note that the LM3914 contains 10 comparators and a voltage reference; you won't need to add another of either of these to the project.

You might see if you can figure out how to add the transistor; we will provide suggestions as needed.

Ted

PS: I just saw CocaCola's last post. So what he said…

15. ### CocaCola

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Apr 7, 2012
PS don't forget to research flyback diodes as well, when looking up transistors...

16. ### zach48191

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Apr 14, 2012
We are using the voltage at a certain led but the voltage output is only .4v that is why we are trying to boost up the voltage to 5v so we turn on the transistor which take s 5v to turn on the relay

17. ### TedA

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Sep 26, 2011
zach48191,

OK, sounds as if we may now understand what you are doing.

Starting with the linked circuit, you want to take the LED driver outputs from the LM3914, and use one or more to make the relay go on and off.

The linked circuit will have only a small output swing at each LM3914 output, just enough to steer the current among the LEDs to the one(s) that you want to light.

A simple modification of the circuit between the LM3914 and the LEDs should fix you up.

The LM3914 has open collector LED driver outputs, that can swing up to 25 volts, and the full LED current can also be used to control something, depending on how the outputs are utilized.

It would be useful for you to let us know a few more details, such as the relay coil specifications ( particularly, we need the coil resistance, and the nominal coil operating voltage ), and what transistors you have at hand that might be used to drive the relay.

Would you prefer for the relay coil to be turned on when the LED lights, or turned off?

And does the whole circuit actually run off a 9VDC power source, as seen for the linked circuit? Is there any other change you might have made to the linked circuit? Helps to be on the same page!

By the way, where are you located? If we send you out for parts, is it to the store or to the shop? More to the point, we can try to suggest things that we imagine you might be able to get nearby.

Ted

PS: I trust that you have the data sheets for the ICs you are using. They are to be found at the TI website, since TI bought National. http://www.ti.com/

18. ### zach48191

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Apr 14, 2012
Im in basic digital electronics and had the teacher help us on the transistor ans relay appli ation. what is questioning is how you can use the led output to get the voltage up? We are placing a wire on the middle led and its voltage varies as more lights go on or off . At higher temperature the voltage decreases, we want to set the parameter so that once it reaches a certain voltage it will not have the 5v to flip the relay. This is our problem, I think that the low voltage chip will work but nothing is certain unless you actually put it in the circuit

19. ### TedA

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Sep 26, 2011
zach48191,

So all of my stated assumptions are correct? I had a bunch of questions for you that still lack answers.

I'm trying to avoid a long succession of suggestions each of which you counter by "I have no PNP transistors", or, "In Bagdad we can get only 5V relays with 10 ohm coils" sorts of answers.

If you want help specific to your situation, you will have to keep us from making too many guesses as to what it is.

Most of the initial posts were not helpful because it was not at all clear what you need.

It does appear that no extra chip is needed, although one could be made to work. A few transistors and resistors will suffice.

Your latest post mentions a voltage. Is this perhaps the cathode voltage of the chosen LED, perhaps D5, or D6 on the linked schematic?

Here are a couple of hints:

If you have the hardware at hand, a voltmeter, and a resistor of say 4.7k or 10k, try this: connect both the resistor and the voltmeter across the LED of choice. Now see what the voltage does as the LED goes on and off.

This should give you more than enough voltage to turn a transistor on and off.

Just thought of more questions:

What color are your LEDs? The color and LED forward drop are related, through the energy of the photons, and how they are generated. Do your LEDs have a reverse voltage specification?

Have you got pin 9 of the LM3914 connected to +9V, while R8 is the 15 ohm value? ( As shown on the linked schematic. This gives the bar graph mode.)

( And does the 15 ohm resistor get pretty hot when all of the LEDs are on? )

Sorry to be a bit testy, but this thread has had a strong component of "guessing game".

Ted

20. ### CocaCola

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Apr 7, 2012
One quick question for you to ponder that might set you straight and on the right path...

Is the LM3914 sourcing or sinking the LED? Think about that for a minute

When you get the answer revisit what you said here...