12 volts from two 9 volt batteries

[sandmantoy]

I am trying to build a launch controller for my rocket hobby to reduce equipment carried in the field. One type of rockets use 9 volts to launch while others use 12 volts to launch. My plan is to use two 9 volt batteries a switch to select mode and verifie via an analog volt meter. There are a few 12 volt led's involved here too making the controller run on 12 volts. So with two 9 volt batteries wired to produce 18 volts reduced to 12 volts and be switched down the line from 12 volts output to 9 volts output. Is a regulator what I am looking for or two regulators? What about resistors? There are three LED's one to show continuity that my electric matches are connected one showing the fire switch is armed and one showing ignition voltage sent. My goal is that the 18 volts will last awhile when basically shorting it out at 12 volts and 9 volts every time a rocket is launched. This is normally done with 22 gage wire sent out to the E matches so I don't think there is a lot of amperage here. I will look further for this info to make sure. I have seen people use everything from car batterys, motor cycle and lantern batteries wired up to do this. A radio shack project box and some of there other gizmo's, a little creativity and some help from you guy's. I'm thinkin less than 2LBs. to carry and keep the kidds happy for hours... Sorry to be so long winded lol.

 

[bryan1]

Hiya Sandman,
If the max voltage you need is only 12 volts don't bother with 9 volt batteries as they are 1 of the most in-effieicent batteries for capacity around. Just look at nimh batteries where aa cells have reached 2400 mA where the 9 volt batteries are only 500mA or less. Personally I'd go for a 12 volt 7 ah SLA battery and a 7809 voltage regulator for the 9 volts. Checkout the datasheet for the regulator as it has plenty of circuit examples and for sizing your led's for each voltage here's a link to electronic assistant https://www.electronics2000.co.uk/ it's a small freeware program that's one of the most usefull programs I've found on the net. Just type the type of led and the voltage into the led calculator and it will give you the resistor needed.

Hope this helps

Cheers Bryan

 

[hyedenny]

Hi,
Unless theres something really tricky about your launching circuit, you dont have to worry about the voltage youre outputting to your e-match. A single 9volt battery should provide more than enough for a couple LEDs and their resistors, 2 switches and the ignitor. Even if you need a few extra components to show ignitor continuity, you dont need to worry about having a specific exact voltage. One 9v battery will last MANY launches.

 

[sandmantoy]

I was wondering how important the voltage is in firing the ematches. Like I said I have seen car batteries out there firing the rockets. For the smaller rockets it may not matter with an extra 3 volts. Heydenny I am going to try the different size ematches or igniters to see how they react first with the 9 volts on the 12 volt igniters and see how the 9 volt igniters react to the 12 volts. Bryan 7809 regulator nice, I already have a 8- AA holder already too.

 

[Oznog]

Yeah since the objective is to burn the match in the first place, it seems that extra voltage is not a problem, even a plus.

A 9v reg might have trouble powering an electronic match anyways. Although it is only very brief, it may require more current than the absolute max a normal-sized reg could put out for any period, long or short.

 

[stevez]

You might also try AAA, AA, C or D cells in series - 8 of them get you 12 volts - tap off at 6 to get 9 volts. This works as long as you can make one side common.

 

[sandmantoy]

Just curious Steve if I tap the batteries at 6 on the AA's what do you mean by make one side common? I was thinking after about the voltage meter, does the ground for the 9 volt and the ground for the 12 volt have to come from there originating source. I was assuming they where common but Know I don't clearly understand how that works.

I guess it's good anything to do with electronics burns up before my rockets take off LOL!!

 

[Skyone]

If you really don't care about battery capacity, you can always just use a 7812...

 

[spuffock]

Use a sla and 12v. If you use too much voltage the ematch will burn out.
What did you want it to do otherwise?

 

[eblc1388]

For a few seconds use, just builds a string of 4 diodes connected in series. This would drop 2.6V~2.8V to the ematch if connected in series with a 12V battery.