# Need help from somebody with electronics knowledge

### Help Support The Rocketry Forum:

#### shockwaveriderz

##### Well-Known Member
I want to post a diagram showing a mercury switch ignition system that is powered by the discharge of a capacitor....prior to launch you use a battery to charge the capacitor(this battery plugs into 2 holes, ie brass or copper small diamter tubing or I suppose you could use a micro plug od some sort?) and it has dual arming nd safety switches.... one of the switches is a SPDT 3 connector slide switch that has a On-Off-On..

slide it one way and you can charge the cap with an attached battery....slide it the other way and the circuit is armed/completed on one side ONLY ....

the circuit uses a burn string that is connected to brass/copper K&S tubing spring switch on the other connection side such that when the burn string is burned at booster engine ignition , this fail safe actually arms the system in flight not on the ground ...

Would somebody that has more and better electronic knowledge tha me be willing to verify this diagram for me? perhaps even draw a schematic with a parts list from say mouser.com ?

any help will be greatly appreciated.
TIA

The arming and laucnh sequence would be as follows...

1. You pull the burn string down and across the booster engine nozzle that already has an igniter in it for the booster engine. You can then use a piece of tape across the booster engine nozzle to hold the burn string down and in place. An optional step here is to place a small piece of paper or non conductive material across the spring switch just in case the burn string is accidently released prior to launch. This could a small red platic flag material that says "Remove Before FLight".....

2. Push the slide switch to the CHARGE position and plug the battery in and charge the Capacitor

I suppose you could have a LED in series for a visual CHARGE/No CHARGE warning.

3. After you charge the CAP, you remove the power source and move the switch to OFF...

4. After you have the model on the launch pad with the ignitor hooked up and you are ready to launch, you push the slide switch to the ARM position.

5. If you have a nonductive piece of material between the spring switch,remove it...

6. LAUNCH

7. As soon as the booster engine ignites, it will burn through the burn string releasing the spring switch which will then make a fully armed circuit ...

8. At booster engine burnout, the mercury will be thrown/pushed forward making contact with the 2 connectors in the mercury switch completing the circuit and the capacitor will discharge firing the igniter for the upper stage....

#### AlexNUMB

##### Well-Known Member

There are commercially available products that are safer and more reliable for igniting upper stage motors. Lighter and less complicated, too...

i.e. Gwhiz LC Deluxe, Pico timers, etc...

Just curious... Seems like more trouble than it's worth?

--Alex

#### shockwaveriderz

##### Well-Known Member
Perhaps it is more trouble than its worth, perhaps not....And yes I agree there are a number of electronic timers that can do the job as well if not better.... Actually it relates to an earlier thread about gap staging, a person what a CHeap And Dirty(CHAD) way to do staging ....

#### Missileman

##### Well-Known Member
I am wondering about the burn wire.
Wouldn't that interfere with a plug or cap or whatever that keeps your ingnitor in or even possibly prevent your ignitor from blowing out and overpresurrize your motor?
Perhaps a safety plug (ie. wooden dowel separating contacts) attached to a wire that you attach to your launch pad.
When the rocket launches it pulles the pin out completing the circuit.

#### shockwaveriderz

##### Well-Known Member
Its a burn string..... although I am sure it could be modified to have a burn/pull pin wire ......

#### Missileman

##### Well-Known Member
I am an Electrician by trade and this has got me thinking.
First off capacitors do not hold a charge indefinitly.
While your waiting for your launch there may not be enough charge left to fire the sustainer.
Intead of using a SPDT switch here is an idea.
Remember the pullpin?
If you have the pin actuate a microswitch you would have both a NC contact and a NO contact.
Use a SPST safety switch, when the switch is turned on it sends voltage through the NO contact (closed while the pin is in place) of your microswitch and to your cap.
On launch the pin is pulled which opens the NO contact (disconecting your charging circuit) and closes your NC contact arming your mercury switch.
This way you maintain a full charge on your cap. until the last second.
Just a thought.

#### bobkrech

##### Well-Known Member
Shockie

You can get a 2 gram PICO-TM1b capacitor powered timer for $20 from http//www.picoalt.com This timer uses a 1F capacitor as the power source and will run the unit for over 10min and still fire an ematch for DT. It comes with quick connects for the ematch so there is no weight increasing connectors to fool with and no time consuming soldering, just strip 1/4in off the ignitor leads and plug it in. It is setup with a header and shunt so all you have to do is pull the shunt to initiate the timing sequence. You can the interval you would like when ordering or if you will need to change it get an optional configuration cable. A standard battery powered version is also$20. The 5 gram PICO-TM1c comes with a 1000uF cap, quick connects for an ematch and a 3V lithium battery holder. It also uses a breakwire to start the timer.

The breakwire/timer approach is much more reliable and safer than a mercury switch. I wouldn't even consider it after looking at the Picoalt prices.

Bob Krech

#### shockwaveriderz

##### Well-Known Member
Bob:
I have one of those picotimers, and I think its great. In fact I'm also postive I was the first person to own one, as I helped Robert Dehate in its initial design and useage. I also have his picoalt altimeter which is cool too.

One of its strongest feature is also a major weakness: being le to program the timer via a windows program....I currently don't own a laptop so if I wanted to adjust the time at the field I can't...

Also we discovered during beta testing a bug in it, such that it will not work with certain serial ports.....so a special "smart" cable is needed....

plus it will only fire low volatge ematches.....with the mercury switch you can use either batteries or an supercap like the picotimer uses to fire regular igniters........

The mercury switch design as posted is extremely safe to use....you would have to have both safety and arming switchs closed and turn the rocket upside down to get it to fire........highly unlikely.....

#### shrox

##### Well-Known Member
How about a solar panel on the launch stand with an umbilical to keep the capacitor charged. A quick tug on a string could disconnect it.

shrox

#### Micromeister

##### Micro Craftman/ClusterNut
TRF Supporter
Shockie:
I've been flying a very similar mercury switch capacitive discharge system or years in my Upscale Laser-X. It does incorporate a pull-out plug instead of the burn string but other than that is almost identical.
Works like a dream. Missileman is correct no capacitor hold a charge forever but after charging with a 9 volt battery the leakage has not been a problem with on pad armed waits of more the 20 25 minutes. some of the new Super capacitors in the 4.5 farad ..yes I said Farad not mfd. can be set up to hold more than enough juice to pop an estes igintier after more than an hour. Bench tests have run all the way out to 2hours and 15 minutes
Steve Humphrey did an excellent article for Z0G-43 recently on the who, what, where and whys of supercap discharge systems. I'm sure by now it will be up an the Narhams.org web site. he's done a lot more work than i with these new HI power components. OBTW they will fit in a BT-5 easily or a 10.5mm tube with a little packing

#### shockwaveriderz

##### Well-Known Member
Cool Micro.... I hope that article is up! I did that circuit form memory from maybe 25 years ago when I used to play with mercury switch CDI systems a great deal....back then it was of course considered "state of the art" and "pushing the envelope".... The dual arming/safe system does indeed may it a very relaible and safe system to use.

I still wish somebody that knows electronics would do a circuit diagram for me B(

and yes I would use those new supercaps, in fact, I was the person that turned Steve Humphrey onto to those aerogel supercaps..........

They are also used with Robert Dehate's picotimer and I got some to use with my pico altimeter

#### Hospital_Rocket

##### Well-Known Member
Shockie

Your capacitive fire system has one design flaw. Energy stored in a capacitor is not measurd in volts or amps, rather it is in joules. Most small capacitors will not store any significant amount of energy and a s such the energy stored will not generate ebough current (which is what heats and igniter element) to be useful.

The type of capacitor which stores enough energy to do what you want will be electrolytic in design, and these tend to be rather large. I think any electrolytic capacitor which is physically large enough to store enough energy to do what you want will be way larger than any battery based solution you could use.

The rated voltage on a capacitor is an indicator of how much potential you can place across it's structure before it fails.

I'm not sure how far your math skills go however this article will give you a peek at what you need to understand to complete your design.

https://www.eecs.wsu.edu/~ieee/sumobots/capacitors.html

By the way, I have seen electrolytic capacitors that were over charged fail rather violently. Also a large elecrtolytic can store a fair amount of gotcha! for quite some time. In fact, I have seen them pick up a charge from atmospheric static. Make sure that your design has a provision to safe the system by shorting the capacitor before use.

If you proceed, please be careful.

#### Hospital_Rocket

##### Well-Known Member
That should do it. I have used similar capacitors in audio amps to handle peak current demands.

Ultra low leakage current (can hold a charge for several weeks)
Keep this point in mind. A charged electrolytic knocked me on my a once and I was numb in my arm for hours.

Another point I forgot to mention. A capacitor discharges over time. If your firing circut burns the lead thru before the capacitor is fully discharged It will be able to provide an unpleasant surprise.

The charge/discharge time of a capacitor is a logrithimic curve calculated by

R X C

Where R is the circuit resistance and C is the rated capacitance.

Using this curve can help you detemine how much capacitance you need to create the firing current over time period.

As I said earlier, please make sure your design has a feature which shorts the capacitor leads when you are not in actual use.

#### Hospital_Rocket

##### Well-Known Member
Bruce,

The math works, however I've never worked with these specific types.

At a couple of dollars it might be worth an experiment.

A