Help needed with designing a safety circuit to ignite a pyrotechnic device

AnalogKid

Joined Aug 1, 2013
10,987
To prevent this thread from being closed (like your thread on another forum), maybe you should clear up what this is for. Paper exercise, model rockets, NASA...?

ak
 

sailorjoe

Joined Jun 4, 2013
364
Wow, that's a lot of stuff. Ih, this is probably an exercise in thinking more than designing. So think about the problem this way. First, what does it take to fire a squib (pyrotechnic ignitor)? Why did they give me a switch that is normally a short circuit? How can I use that to ensure the squib can't fire accidentally (safety is paramount)? What does the short pulse tell me? Why is it there? Maybe I can latch it so I know it happened, but it doesn't have to drive anything. What does the logic signal tell me? Could it imply a push button that has to be held down long enough for the squib to fire? Could it be a timing window gate? And lastly, when it comes to safety and reliability I have a simple formula....the only thing that will never break is the device that isn't there. So think about how the arming system could be made with the least number of parts, arranged so they are as safe as can be. Hope that helps, and good luck with the interview.
 

djsfantasi

Joined Apr 11, 2010
9,156
Of course it's also possible they want to see how you perform under pressure, whether you are willing to sacrifice good practice and safety just to satisfy management. Just sayin'.
If you had the time and motivation, provide two designs. Requested and Recommended. I know that would blow me away. My philosophy is I always appreciate alternate solutions, but I get to pick the final design.
 

Bernard

Joined Aug 7, 2008
5,784
This assignment is not too far fetched, back in the 1950's I devised a system to fire a blasting cap synchronized with the 1/10 second timing line on oscillograph record. The shooter would press a SW to charge a capacitor in the blaster, when ready operator would say FIRE, shooter would hit Fire button, nothing happens until 1/10 timing line came on then Bang. The first electronic switch was a Ne strobotron which required 300 V on anode, 90 V trigger but later when they became available, SCR's. No IC's or LED's, just vacuum tubes & germanium
transistors.
 

Thread Starter

ih96

Joined Dec 24, 2015
8
To prevent this thread from being closed (like your thread on another forum), maybe you should clear up what this is for. Paper exercise, model rockets, NASA...?

ak
I've already explained that it's for an interview for an internship. The task is purely to design the circuit, no construction involved.

Of course it's also possible they want to see how you perform under pressure, whether you are willing to sacrifice good practice and safety just to satisfy management. Just sayin'.
If you had the time and motivation, provide two designs. Requested and Recommended. I know that would blow me away. My philosophy is I always appreciate alternate solutions, but I get to pick the final design.
I'd been considering this and I think as it's a task for candidates who aren't expected to have a detailed knowledge of electronics they have probably simplified the problem and are just expecting us to address the brief. However, they could well be expecting stronger candidates to suggest improvements. I think I'm going to try to tackle the given problem first, and if I manage that I could try also talking about changes which could be made to the circuit to make it safer.

Wow, that's a lot of stuff. Ih, this is probably an exercise in thinking more than designing. So think about the problem this way. First, what does it take to fire a squib (pyrotechnic ignitor)? Why did they give me a switch that is normally a short circuit? How can I use that to ensure the squib can't fire accidentally (safety is paramount)? What does the short pulse tell me? Why is it there? Maybe I can latch it so I know it happened, but it doesn't have to drive anything. What does the logic signal tell me? Could it imply a push button that has to be held down long enough for the squib to fire? Could it be a timing window gate? And lastly, when it comes to safety and reliability I have a simple formula....the only thing that will never break is the device that isn't there. So think about how the arming system could be made with the least number of parts, arranged so they are as safe as can be. Hope that helps, and good luck with the interview.
Thankyou! That's such a helpful response. I'd already tried to break down my plan so that each aspect was addressed individually, but I'd not even considered the details you'd mentioned and I think they'll be really good things to talk about. :)
 

Thread Starter

ih96

Joined Dec 24, 2015
8
Here is a sketch of what a firing circuit might look like.
A Manual NC pushbutton SW.
B Pos. pulse triggers 30 sec. OS.
C Lo to hi logic signal triggers a 30 sec OS

All 3 states are monitored by red & green LED's. If all green we should hear the bang.
Select transistor for current required by squib.
Note that 74AC series can source or sink 24 mA.View attachment 97206
Thanks so much for this but I'm far too new to this to fully understand that circuit. :( I was going to try to design the circuit so that it had 3 latches that would be set separately by each event. Then, all 3 inputs would go to an AND gate which would initiate the device only when the latches had been set. Does this sound about right to you? I've been doing my research on latched circuits and I think I've got a grasp of how they work and how they can be used, but I'm still unsure about how I would put 3 of them together to get the result I want.
 

Bernard

Joined Aug 7, 2008
5,784
We can add latches, but I strongly suggest not doing it; re read comments by joyed999, post # 3.
You might use the latches but explain the hazards of doing so & go to one shots the 555's in post 20.
OS timing is adjustable to fit requirements. If you would like an explanation of the circuit, just ask.
 

Bernard

Joined Aug 7, 2008
5,784
If we can assume that all inputs are active until last input is triggered, then we might use the simplified firing circuit shown. A memory device is needed for B a single positive pulse, such as a flip-flop, FF or a one shot, OS. All three events are combined in a hard wired and gate, AND, consisting of 3 diodes & a load resistor.
In initial state all inputs are low causing current to flow thru resistor making the output low. Only when all 3 inputs go high will the output go high. If inputs A or C do not stay high during the time frame then a memory device is needed for the short input.
A reset- set, RS, flip-flop, FF, such as a 4013 could be used a memory device. Input signal processing might be needed so that R & S are not high at the same time like capacitive resistive coupling to the inputs.
If the RS FF would automatically reset after a given time we would have a OS.Squib # 6 00000.jpg Squib # 7 00000.jpg
 
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