LOHAN's cold heart beats beneath silicon breast Our crack Low Orbit Helium Assisted Navigator (LOHAN) electronics team has made some cracking progress over the past few weeks, putting together the beating heart that will control our audacious Vulture 2 spaceplane mission. Click here for a bigger version of the LOHAN graphic A couple of months back, we decided to fire the …
R5 to R9 are 10k, and so the LED current with a 12V source will be about 1mA and you probably won't be able to see them in the sunshine. I suggest a magnitude less.
I would be concerned that the frequency of the fail-safe timer is rather temperature dependant. I would recommend a crystal oscillator (32kHz with +/-20ppm spec over -40 to +85 are tens of pence).
There seems to be a lack of local decoupling on the micro or any of the other ICs/Modules.
Fuse, or otherwise protect, the incoming supplies, otherwise a loose ground wire accidentally touching of the through hole components will take tracks off you circuit board.
IC5 will dissipate significant power. Make sure it has a decent heatsink (useful to keep the electronics warm) or use a switch mode module (about a fiver in small volume) that are nominally drop in replacements for the old 7805 devices.
Keep up the good work!
1) Very conservative design. For the day job I build robots that steer drills three miles underground...
2) The LEDs are 2mA high brightness types. They'll be shaded when we start. One of them goes through the relay coil and I don't want that current to persuade the relay to trigger.
3) Failsafe timer isn't critical. We expect the electronics pack to remain close to room temperature. The oscillator is around a hundred hertz.
4) Decoupling is there but there's not a lot of it; I may add more.
5) IC5 dissipates milliwatts; it won't need a heatsink though it will need a mica insulating shim.
6) Relay because that way I can get a physical separation between the coil and the contacts, and guarantee that the immediate aftermath of powering up the processor doesn't send a random glitch to the igniter.
7) BJT rather than FET because FETs are not too happy under five volts. I'm constrained to 3v3 by the other hardware, or I need a lot of level conversion bits - extra complexity.
8) Two transistors not because I expect a transistor to die but in case an output from the processor GPIO fails.
Hope there's no moisture in the relay can as it might be frozen in position when you try to fire the igniter.
Strange choice, using a transistor to switch a relay, _ALL_ modern pyro systems use mosfets to fire igniters. Igniters usually fire with <1 amp for a few 10's of millis. I have used SP8K4 dual mosfet in a small sop8 package. Make sure you don't knock the flight package when you launch, you don't want to bounce the relay contacts.
Have you checked your relay for susceptibility to vibration/shock? It's not common, but since they are mechanical devices it _is_ possible to get a momentary contact if the shock is in the wrong direction.
You're not trying to switch a particularly heavy load, so I would have thought solid-state stuff would be adequate.
>So you have two transistors for redundancy switching a single relay?
I don't see a problem with that. Years ago, I made some boards to control model railroad snap-relays from a PIC, and for each port, I used two pins of a nice darlington transistor chip (with on-chip back emf protection, tailor made for this) to source the relay. Relays are noisy.
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