I was going to use 4 AA batteries for the ignitor. I'm not sure
exactly how much current the ignitor takes. I know that 2 AAs isn't
enough, but 4 works. The 12V battery will just have to keep the relay
closed so that the AAs trigger the ignitor. I'll probably start the
timer using a magnetic switch that disconnects from the rocket when it
takes off.
---
Great! I misread it and thought you were going to use the little
12V battery (I think it's called an A23 or something like that) to
trigger the ignitor.
I went to a few of the model rocketry sites on the web and looked up
ignitors, and it seems like they need 3 amps or so to fire, so the
relay needs to be able to handle _at least_ that.
Here's a simple circuit that simulates nicely:
..
.. +----+-----+---------+----------+--------+
.. | | | | |K | COM O-->TO +6
.. | +->[200K] | [1N4001] [COIL]- - -|
.. O | | C | | O--> |
.. |NO | 2N4403 B--[1K]--+--------+ |NO
.. O |S1 | E | +-------->TO IGNITOR
.. | | | |
.. |+ | [1K] |
..[12V] | | C
.. | +---------+-----------------B 2N4401
.. | | |+ E
.. | | O [1200µF] |
.. | NO| | |
.. | S2| O | |
.. | | | |
.. +----------+---------+-------------------+
..
It starts timing out when S2 is no longer held closed (by your
magnetic latch?) and allows the 1200µF cap to charge up (through the
200 kohm pot) to the point where the 2N4401 starts to conduct. That
will start to turn on the 2N4403 whichwill turn on the 2N4401 even
harder, which... So you get a quick snap-in of the relay.
Here's the LTSPICE circuit file:
Version 4
SHEET 1 1268 852
WIRE -464 272 -464 64
WIRE -464 624 -464 352
WIRE -400 528 -400 496
WIRE -400 624 -464 624
WIRE -400 624 -400 608
WIRE -272 496 -400 496
WIRE -272 624 -400 624
WIRE -272 624 -272 544
WIRE -224 480 -224 432
WIRE -224 624 -272 624
WIRE -224 624 -224 560
WIRE -112 64 -464 64
WIRE -112 272 -112 64
WIRE -112 432 -224 432
WIRE -112 432 -112 352
WIRE -112 480 -112 432
WIRE -112 624 -224 624
WIRE -112 624 -112 544
WIRE 16 64 -112 64
WIRE 16 160 16 64
WIRE 16 288 16 256
WIRE 16 432 -112 432
WIRE 16 432 16 368
WIRE 112 208 80 208
WIRE 272 64 16 64
WIRE 272 112 272 64
WIRE 272 208 192 208
WIRE 272 208 272 176
WIRE 304 432 16 432
WIRE 368 64 272 64
WIRE 368 96 368 64
WIRE 368 208 272 208
WIRE 368 208 368 176
WIRE 368 384 368 208
WIRE 368 624 -112 624
WIRE 368 624 368 480
WIRE 368 672 368 624
FLAG 368 672 0
SYMBOL pnp 80 256 R180
SYMATTR InstName Q1
SYMATTR Value 2N4403
SYMBOL npn 304 384 R0
SYMATTR InstName Q2
SYMATTR Value 2N4401
SYMBOL res -128 256 R0
SYMATTR InstName R1
SYMATTR Value 200k
SYMBOL cap -128 480 R0
SYMATTR InstName C1
SYMATTR Value 1200e-6
SYMBOL res 0 272 R0
SYMATTR InstName R2
SYMATTR Value 1k
SYMBOL res 208 192 R90
WINDOW 0 62 58 VBottom 0
WINDOW 3 64 54 VTop 0
SYMATTR InstName R3
SYMATTR Value 1k
SYMBOL voltage -464 256 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL voltage -400 512 R0
WINDOW 0 -53 5 Left 0
WINDOW 3 -242 110 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value PULSE(1 0 2 1E-6 1E-6)
SYMBOL sw -224 576 M180
WINDOW 0 32 15 Left 0
WINDOW 3 32 44 Left 0
SYMATTR InstName S2
SYMBOL ind 352 80 R0
SYMATTR InstName L1
SYMATTR Value 100e-3
SYMATTR SpiceLine Rser=240
SYMBOL diode 288 176 R180
WINDOW 0 24 72 Left 0
WINDOW 3 43 36 Left 0
SYMATTR InstName D1
SYMATTR Value 1N4148
TEXT -488 688 Left 0 !.model SW SW(Ron=1 Roff=10Meg Vt=0.5Vh=0)
TEXT -498 708 Left 0 !.tran 0 20 0