Rocket Timer circuit causes BS1 to drop command line.

yuright

I have included the code and a PNG with the schematic. This is a simple rocket parachute timer. At launch a momentary switch sets pin 6 low and starts the timer program. Pin 7 is set as an input with 0 written to it. After the delay, pin 7 is set as an output and holds it for 6 seconds then goes back to being an input. I have a 3k pull up res on the primary N Chan Mosfet which effectively keeps the igniter mosfet low. When 7 goes low it grounds the primary mosfet which ungrounds the gate on the igniter mosfet allowing it to close and fire the parachute charge.
I am using 2 cell Lipo batteries to power the circuit about 8.4 volts. The odd thing is when I use one battery which has around 1000mah capacity the circuit works fine. If I use the 850mah battery the program crashes when it gets hit with the load from the igniter. I have checked for a voltage drop but can't see anything with my meter. When I say it crashes what I mean is that it acts like a reset. Pin 7 won't hold low for the six seconds it just goes back to main immediately. One other thing is if I write a different program like low 7 pause 6000 the smaller battery works fine. Thanks

erco

You def have power supply issues, your tests prove that; any reset means the Stamp input voltage went too low. Igniters are a dead short until they self-destruct. Your big 1000 battery can supply the current without dropping voltage, but your 850 battery can't. And your 1000 is probably marginal. You COULD try a big electrolytic cap & diode in parallel with your Lipo to power the Stamp while the igniter burns, but it's a gamble. Best bet is to use a small seperate battery for your Stamp.

yuright

Thanks erco, I get what your saying and it makes sense. I am puzzled though by the fact that if I change the program to something simple like LOW 7, PAUSE 8000 it will burn the bridge wire without any problem using the smaller battery. I wonder if there is something wrong with the way I wrote this program? I guess I will build this thing on a breadboard and see what happens, should have done that in the first place. Thanks

erco

Try this and see what happens:

' {$STAMP BS1}
' {$PBASIC 1.0}


' Chute Timer
high 7

Main:

DEBUG CLS
DEBUG " Looping thru Main "

IF PIN6 = 0 THEN Start_Time
IF PIN6 = 1 THEN Main



Start_Time:

DEBUG CLS
DEBUG " Start Timer! ", CR
PAUSE 8000
DEBUG " Kapow! "
low 7
PAUSE 6000
high 7

END

erco

Of course, if a simple program like LOW 7/PAUSE 8000 works, it's because the Stamp is probably still resetting due to low battery voltage, but it keeps briefly triggering the igniter after resetting, because you eliminated your IF check on pin6. So there may actually be a pulsing signal going to your igniter each time the Stamp resets.

yuright

I think you may have something there. I don't have any way to record what the voltage is doing when this happens. On my meter I get only a 100th of a volt drop. I ran LOW 7 by itself and as you said, it is resetting to LOW 7 which keeps hammering the igniter . Add HIGH 7 after LOW 7 and it won't work.

Humph! I wanted to keep the weight down. Oh well, what's the smallest battery I could run the Stamp on? I definitely don't want a lawn dart.

erco

Depending on current draw and how long before your igniter burns thru and lets the voltage recover, you might be able to power the stamp off a big 5V electrolytic cap for a few seconds. That would get charged off your main battery continuously through a diode and resistor. Lighter and less maintenance than a secondary battery.

yuright

Thanks, I will try that. Is this what you are talking about? check attatchment

Note, Change 330uF cap to 1000uF. I tested this setup and it works every time.