Magnetic Rocket Altimeter Arming

[SEVANS]

I am currently building a Rocket, and the altimeter location makes the little screw switches I normally use not practicle. I have been looking at reed sensors and latching reed sensors and was wondering about the posibilities of arming the altimeter through the rocket body with a magnet.
I am a bit concerned about the latching ones staying latched at mach 2 and the shock of flight and the ejection charges. And also getting one that would meet the specs of the altimeter.
If the altimeter turned off, in flight, that would be a real bad thing!
Altimiter specs below.
http://www.perfectflite.com
Its the HA45. (click on HIALT45K)
We normaly jump the switch block and then switch the battery leads instead for safety.
When the electric match fires it draws 5 amps from the 9 volt battery. The sensors I have seen so far dont seam to be rated that high. ??? May be a combination of a reed sensor and latching relay. Any help would be greatly apreciated, schematics, part numbers.
I know how to solder, but my electronic knowlage is limited. I have built circuit boards in the past, but I had plans and part numbers.

 

[(*steve*)]

Oddly enough, for a forum called "Electronics Point", we're not all model rocketry experts.

Explain your problem again, but this time the way you might explain it to your grandmother.

 

[Laplace]

Can the 1.5 mA operating current be separated from the 5 A igniter circuit? If so then use a miniature reed relay to trigger a CMOS latch that will supply the operating current. Use a magnet to activate the relay before launch and then reset the latch when back on the ground.

 

[(*steve*)]

For triggering on launch, I would see if I could use a hall effect sensor. It may be lighter than a reed switch, and you could even have a magnet on your gantry(?) so that it automatically turned on at launch. Hall effect switches are pretty much immune to vibration and should have a long life in what may be a hostile environment.

 

[SEVANS]

LOL I can see a language barier here. The altimeter is armed on the pad manually.
In this case hopefully a magnetic switch.
It has a baramtric senser that sets itself to zero so to speak. (no matter what altitude you at) It takes a few seconds. (you listen for the triple beep) It waits to sense a barimetric pressure change to determin launch (about 180 ft.) Then it goes into progresive modes, mach delay, apogee detection. ect.
I have seen some schematics for isolating the ejection circuit from the operating circuit.
But that was a dif. altimiter.

 

[SEVANS]

Oh and Steve

The electric match, triggers a black powder charge that blows the rocket apart at apogee to release a droug chute. The altimiter continues detecting to say 1000 agl the to blow out a main chute.

 

[(*steve*)]

Thanks for that SEVANS.

Considering the presumably short pulse required to set off the electric match, you may be able to use a fairly small mosfet to switch 9V to it.

What sort of batteries do you use?

A friend of mine built up some devices for firing fireworks which used a capacitor charged to around 300V to light them. The capacitors can hold a charge for many hours, and weigh not a lot at all (probably less than a single AAA battery). Your 9V for the altimeter could then come from a separate (and likewise lighter) source.

If you use the same battery for the altimeter and the electric match, then it would be a good idea to decouple the power to the altimeter using a diode and a capacitor to ensure that the altimeter does not lose power as the chute is deployed.

 

[SEVANS]

OK
I tried some Laching Reed Switches I got from ebay. They seam to work, and fire the match. I may have very low current matches. Or as said above very short draw time.
Problem is, after I turn it on, when I flick the switch with my finger, the reed switch turns off.
I was afraid of that. In a rocket, I need a swith that can stay on with vibration an g forces involved in a rocket. May just have to go back to the simple little screw swiches.

 

[(*steve*)]

OK, you're using the reed switch to power the altimeter, right? And the altimeter provides the signal to fire the chute via the electric match?

I would NOT run the power to the match via the reed relay. Although I understand it provides some level of safety and isolation, I would use a mosfet to switch that and the altimeter to trigger the mosfet. In the absence of power a resistor could hold the mosfet safely off.

As for vibration turning the altimeter on and off, this is far more of a concern. However I would place a diode in series with the reed relay, and a capacitor following that. This would both decouple the altimeter power from the electric match, but would also provide power while the relay chatters open and closed during powered flight. Only testing would determine whether this was effective, but evidence from the recorded data should provide this information.

Presuming that there is a relatively vibration-free period of flight from the engine exhausting its fuel to max altitude, the altimeter should have time to stabilize and detect when to fire the chute. This would hopefully provide some level of insurance...

A lot of speculation, but maybe some of it will be of assistance.

 

[SEVANS]

Steve

I appreciate your imput.
The altimeter already has a 4 second power back up. (capacitor ?)
I actually used the reed to fire 2 or 3 matches directly ($1.25 a piece, cost more than the reed lol) Didnt seem to faze it. If a latching reed is prone to vibration and switches off, even with back up power, whats to switch it back on? The magenet is in my pocket, and the rocket is on the way up (and back down badly)

I have been reading a little.
What about a combo of a hall effect sensor and mosfat switch like you mentioned above. I need the a final swich that will stay on. Is there a latching mosfat? What about a solid state latching relay.
Seams like it would be a very very very simple circuit.
I looking for something that would end up small, and maybe only 2 wires sticking out of it, with the end product. I dont think the match has to be isolated with the right combonation.

Scott e

 

[SEVANS]

I found this.
http://www.valley-electronics.co.uk/index.php?productID=984

A little bigger than imagined. Maybe it could be made smaller.
Maybe subsitute a 9volt for power, and a hall effect instead of the rc reciever for the switch. ???