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# Battery suggestions for weather balloon

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#### elsey.jack

##### New Member
Hello everyone,

I'm working on a weather balloon for a research project. It will be moored to the ground and reach a height of 100 meters. There will be two modules. One module will be attached to the balloon and will transmit the temperature (via a 434 Hz RF transmitter) to a hand held module where it will be displayed.

The range of the RF transmitter is dependent on the voltage supplied to it. A maximum of 500 ft through open space is achievable with 12 V as a supply voltage. The PICAXE 18X microcontrollers that I am using to control the modules runs off of a 4.5-5.5 supply voltage.

The prototype that I have built of the balloon module uses about 25-30 mA with the RF transmitter at 5V.

I need a lightweight battery that will power the balloon module for about 1-1.5 hours. I was looking at lithium polymer batteries but each one of those only gives me only 3.7 volts.

I could buy more than one lithium polymer battery and link them in series to get a higher voltage, but I would have to get smaller batteries with a shorter battery life.

Could I possibly use a transformer to boost the voltage from a 3.7 V battery to 12 V? Can transformers only be used with AC voltages? Do they make transformers light enough for my purposes?

Thanks,
Jack

Last edited:

#### blueroomelectronics

##### Well-Known Member
Since PICAxes are just PICs I'd try running them off 3.7V

#### AllVol

##### New Member
Hello everyone,

I'm working on a weather balloon for a research project. It will be moored to the ground and reach a height of 100 meters. There will be two modules. One module will be attached to the balloon and will transmit the temperature (via a 434 Hz RF transmitter) to a hand held module where it will be displayed.

The range of the RF transmitter is dependent on the voltage supplied to it. A maximum of 500 ft through open space is achievable with 12 V as a supply voltage. The PICAXE 18X microcontrollers that I am using to control the modules runs off of a 4.5-5.5 supply voltage.

The prototype that I have built of the balloon module uses about 25-30 mA with the RF transmitter at 5V.

I need a lightweight battery that will power the balloon module for about 1-1.5 hours. I was looking at lithium polymer batteries but each one of those only gives me only 3.7 volts.

I could buy more than one lithium polymer battery and link them in series to get a higher voltage, but I would have to get smaller batteries with a shorter battery life.

Could I possibly use a transformer to boost the voltage from a 3.7 V battery to 12 V? Can transformers only be used with AC voltages? Do they make transformers light enough for my purposes?

Thanks,
Jack

Just thinking out loud, couldn't we use one output of the 18X to drive a boost circuit that would raise 5V to 10 or so? A few more small components would not weight that much.

Just a thought.

#### elsey.jack

##### New Member
Just thinking out loud, couldn't we use one output of the 18X to drive a boost circuit that would raise 5V to 10 or so? A few more small components would not weight that much.

Just a thought.

boost circuit?

##### Well-Known Member
A simple boost circuit, such as a capacitor charge pump, can boost voltage, but isn't good at boosting current, so such a thing is best used when you only need a milliamp or two. You can drive a voltage multplier (doubler or tripler) using PIC outputs.

#### elsey.jack

##### New Member
A simple boost circuit, such as a capacitor charge pump, can boost voltage, but isn't good at boosting current, so such a thing is best used when you only need a milliamp or two. You can drive a voltage multplier (doubler or tripler) using PIC outputs.

So, I would output a square wave from the PIC and run the AC voltage through a circuit like this?

Hmm. Would 1 uF capacitors work? Would higher capacitances correspond to a higher current capability?

Thanks,
Jack

#### kchriste

##### New Member
Forum Supporter
The higher the frequency you use, the smaller the capacitors can be.

##### Well-Known Member
So, I would output a square wave from the PIC and run the AC voltage through a circuit like this?

Hmm. Would 1 uF capacitors work? Would higher capacitances correspond to a higher current capability?

Thanks,
Jack

yes, that is one way and should work. From my experience, you choose the capacitor value to have a low impedance at the oscillator frequency (and you get to define what low means), and once you do this, most of the output impedance of this configuration is due to the impedance of the oscillator input plus the diodes. You always suffer the voltage loss of the two diodes, so make this low by using Schottky types.

For this configuration, my favorite approach is to use two logic outputs to drive it, each being 180 degrees out of phase with the other. This helps lower the source impedance and increase the amount of current you can draw for a given voltage droop.

#### elsey.jack

##### New Member
For this configuration, my favorite approach is to use two logic outputs to drive it, each being 180 degrees out of phase with the other. This helps lower the source impedance and increase the amount of current you can draw for a given voltage droop.

I don't know what you can do with a PIC, but I know that PICAXE microcontrollers cannot output more than one sound at a time, let alone output a serial signal also. Would two 555 oscillators, one rigged to go off slightly after the other, work if I wanted to get a 180 degree phase difference?

Also, the DC voltage coming out of the multiplier circuit is bound to be a little "bumpy". Would that mess with my RF transmitter?

Thanks,
Jack

#### Hero999

##### Banned
What about using thin copper wire rather than rope?

It would eliminate the need for RF and batteries.

#### blueroomelectronics

##### Well-Known Member
Do you need live data? You could add a simple EEPROM and record data.

#### elsey.jack

##### New Member
What about using thin copper wire rather than rope?

It would eliminate the need for RF and batteries.

True, but wouldn't that add weight? I was planning to use high-tensile kite string.

I would prefer to use 3 or 4 of those foil balloons you can get from a party store. If the module gets too heavy then I'd have to get a meteorological balloon, and those require a lot of expensive helium to inflate.

##### Well-Known Member
I don't know what you can do with a PIC, but I know that PICAXE microcontrollers cannot output more than one sound at a time, let alone output a serial signal also. Would two 555 oscillators, one rigged to go off slightly after the other, work if I wanted to get a 180 degree phase difference?

Also, the DC voltage coming out of the multiplier circuit is bound to be a little "bumpy". Would that mess with my RF transmitter?

Thanks,
Jack

It is a lot simpler to use one output from the PIC, if it is handy, and then invert that with a simple logic inverter to create the out of phase part. Then again, it is also easy to use a hex inverter IC to make an oscillator with a few inverters, then the remaining inverters to invert and buffer to two outputs 180 deg out of phase. This is cheaper and less parts than two 555s. But I think you may have missed the point of my first post, which is that this voltage doubler idea is a bad one if you need significant current as well. What do you plan to power with this?

#### elsey.jack

##### New Member
Do you need live data? You could add a simple EEPROM and record data.

That is a good idea, but being able to see the air temperature reading stabilize in real time would be preferred. I think I'll include that as a backup feature.

Looking back at using 555 timers and a voltage multiplier, that probably wouldn't work either. My supply voltage would only be 3.7 volts, so the peak voltage from the 555 timers would only be about 2.5 V. I would probably get only 9 volts DC if I ran that through a voltage quadrupler.

This may be wishful thinking, but does anyone know of any (non-SMD) IC solutions out there that do DC to DC conversion efficiently?

#### elsey.jack

##### New Member
It is a lot simpler to use one output from the PIC, if it is handy, and then invert that with a simple logic inverter to create the out of phase part. Then again, it is also easy to use a hex inverter IC to make an oscillator with a few inverters, then the remaining inverters to invert and buffer to two outputs 180 deg out of phase. This is cheaper and less parts than two 555s. But I think you may have missed the point of my first post, which is that this voltage doubler idea is a bad one if you need significant current as well. What do you plan to power with this?

I will need about 30 mA to power a radio transmitter. Even if I was able to get a high enough voltage out of it (see my post above), would I have enough current to run what I need?

##### Well-Known Member
At 30 mA, forget about voltage multipliers. Why the SMD fear? SMD is great and dead easy (with the right tools that is). Anyway, there are lots of DC converters around in "brick" form, with dip wire leads, like this for example:

but these are usually more expensive than I would like ($15 to 20 each). For an SMD part well suited to delivering 30 mA, consider the LT1317. Most DC DC converter ICs nowadays run at fairly high frequencies and demand low parasitic reactances . Surface mount does a much better job of minimizing lead and pcb parasitics than leaded parts do. #### Hero999 ##### Banned True, but wouldn't that add weight? I was planning to use high-tensile kite string. I would prefer to use 3 or 4 of those foil balloons you can get from a party store. If the module gets too heavy then I'd have to get a meteorological balloon, and those require a lot of expensive helium to inflate. How much would the batteries and transmitter weigh? Plus the extra complexity and cost. Isn't an extra balloon or two cheaper? Wire that carries 30mA doesn't have to be very thick, you could probably use it with your high tensile kite string if it isn't strong enough. #### elsey.jack ##### New Member At 30 mA, forget about voltage multipliers. Why the SMD fear? SMD is great and dead easy (with the right tools that is). Anyway, there are lots of DC converters around in "brick" form, with dip wire leads, like this for example: **broken link removed** but these are usually more expensive than I would like ($15 to 20 each).

For an SMD part well suited to delivering 30 mA, consider the LT1317.

Most DC DC converter ICs nowadays run at fairly high frequencies and demand low parasitic reactances . Surface mount does a much better job of minimizing lead and pcb parasitics than leaded parts do.

Ha ha, let's just say I've got shaky hands. (There is a reason why I am studying engineering and not brain surgery). I have never done any SMD work nor designed and ordered any custom PCBs (which I assume I would have to do to cut down on parasitic reactances). Not to say that I wouldn't want to learn to do that stuff, but I would like something more familiar so I can finish this project by the end of the summer without any more issues.

I think that I may just go with wiring up three of these 100 mAh 3.7 lithium polymer batteries to get 11.1 V. Their combined weight is minimal and I should get about a good 1.5 hours' worth of juice from them.

#### elsey.jack

##### New Member
How much would the batteries and transmitter weigh?

Plus the extra complexity and cost.

Isn't an extra balloon or two cheaper?

Wire that carries 30mA doesn't have to be very thick, you could probably use it with your high tensile kite string if it isn't strong enough.

True. That idea would probably work for my purposes.

Call it foolish pride, but I would really like to get the wireless functionality working. I've already spent two months and one pair of fried walkie-talkies trying to get it to work, and I think that I am close enough to go through with it.

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