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I'm building a model rocket launcher with my son. The circuit is very simple as you can see. We want to add a buzzer and a LED/bulb when it is turned on as a warning to stay clear of the rocket.

How would I light the bulb and sound the buzzer without launching the rocket? The buzzer and bulb would need power before we press the launch button. We have purchased all the components we need I think and are ready to build.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ This question is showing the classic problem of people who have made no effort to familiarize themselves with the safety practices long and successfully used in an area, jumping in with ill-considered armchair designs that are as likely to introduce new, unfamiliar and non-intuitive hazards as make anything safer. \$\endgroup\$ – Chris Stratton May 21 '16 at 21:39
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    \$\begingroup\$ @ChrisStratton It appears to be nothing more than a simple novelty circuit right down to the "warning" light/buzzer. One they could have fun and experiment with without the need to worry about hazards or being intuitive. Definitely sounds like a a good project for a parent/son bonding time especially if both are new to electronics. \$\endgroup\$ – Bacon Brad May 22 '16 at 6:45
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    \$\begingroup\$ The issue appears in the responses brought out by the question. \$\endgroup\$ – Chris Stratton May 22 '16 at 7:15
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    \$\begingroup\$ @ChrisStratton the comment seems unnecessarily surly. This isn't that dangerous, and the question was reasonable and even had a nice schematic. What's the beef? \$\endgroup\$ – Dov May 22 '16 at 9:19
  • \$\begingroup\$ I used model rocket igniters to detonate gunpowder for a birthday party after watching our electrician do it in the lab for a model boat simulated sinking. When my son asked for a repeat a couple of years later I had just found out that we could get steel wool strands to burn up with a hand-cranked 5V generator, so I tried with a 6V battery pack, and the improvised fuses worked really well. 6 rocket igniters cost $7 or so, whereas I could make hundreds out of a single steel wool pad. I don't know that this would work for rockets though. \$\endgroup\$ – Dov May 22 '16 at 9:22
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Power-on indicator and buzzer additions.

Simply add them in parallel as shown.

BTW, congratulations on an excellent first post with correctly drawn and oriented schematic.


schematic

simulate this circuit

Figure 2. Time-delay function.

As Wouter suggests, a time-delay adds some additional safety. Because semiconductor failure modes are not predictable (they could fail open-circuit or short-circuit) the time delay should receive its power from the launch button. You could also add another LED in parallel with the igniter to indicate that you have power at that point in the event that the igniter fails.

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    \$\begingroup\$ My problem with this circuit is that nothing prevents the user from clocing the key and immediately pressing the launch button. A enforced delay would be advisable. \$\endgroup\$ – Wouter van Ooijen May 21 '16 at 10:15
  • \$\begingroup\$ Thanks for the suggestion of the delayed launch, I had not considered that. I will accept this answer as I really appreciate the design. Many thanks \$\endgroup\$ – user1226884 May 21 '16 at 10:27
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    \$\begingroup\$ I'd put the LED and launch button after the time delay, so the launcher can see when the delay has expired. The point is to delay switch -> launch enabled, not launch btn -> launch. \$\endgroup\$ – Wouter van Ooijen May 21 '16 at 11:07
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    \$\begingroup\$ On the contrary, a time delay is more likely to make things less safe. The absolute last thing you want is a system that launches the rocket without active human involvement at the instant of launch. If you are going to do that, at the least make the on/off switch a button that must continue to be held down and consider giving the timer some progress output. Even then, lifting a finger to cancel is less safe than actively pushing a button to launch. The safest place for a delay would be an arming delay before the launch button is active. \$\endgroup\$ – Chris Stratton May 21 '16 at 17:09
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    \$\begingroup\$ The delay as drawn is quite hazardous; the "arming" delay is not actually beneficial, as the area should be completely safe for launch before the key is turned, and it's not demonstrably safer for the system to sit "disabled" for 5 seconds, 5 minutes or 5 hours after the key is turned before it will operate. \$\endgroup\$ – Ecnerwal May 22 '16 at 1:07
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Having used commercial circuits and built a few myself...

The standard model rocket igniter takes a fairly hefty current to work. In the olden days, an incandescent bulb was placed ACROSS the launch button. In the modern era, an LED and resistor can be in the same place. The igniter will not ignite (and if, somehow, it did, it would only do so when you armed the system - but it won't.)

schematic

simulate this circuit – Schematic created using CircuitLab

The reason for this arrangement is that the launchpad is a somewhat filthy environment as soon as one rocket has been launched - so the connections TO the igniter are prone to flaky-ness. With the light across the button, the continuity of the igniter is assured (it's not assured that you haven't shorted the igniter below the active area, but continuity is at least confirmed - or not.)

My default arrangement has a keyed power switch and an additional arming switch. The arming switch on the model I've been using for some decades has to be pulled UP to arm.

While you can put a buzzer there with the arm light, I would not. It makes it harder to communicate while the system is armed (background noise) and the reason you give for it is "to warn people to stay clear of the rocket" - it is the job of the person in control of the key and arming switch to warn people (harder to do with a buzzer going off) and to shut the system down (disarm) if someone gets too close (ie, well BEFORE they are "in hazard range.") If person, dog, cat, whatever gets within ~25 feet of the pad, the system is disarmed - and that's somewhat impractical (envisionable, but cost-prohibitive) for any means other than the person controlling to be looking.

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  • \$\begingroup\$ Comment only: Person control (RCO?) supervision essential but a simple Doppler RADAR or ultrasonic rangefinder (under $5) would make a nice techo add on to the launch prevention system. \$\endgroup\$ – Russell McMahon May 24 '16 at 19:29
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All excellent suggestions;

Here is a further refinement. This circuit keeps the leads shorted until just before firing to prevent unattended launch through an ESD event.

You'd use a DPST (Double Pole Single Throw switch with momentary contact.

Don't for get to fuse the system, no need to melt every thing.

enter image description here

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  • \$\begingroup\$ Why the -1? This is not an answer, but it doesn't claim to be one. It is simple but probably usefull addition. \$\endgroup\$ – Wouter van Ooijen May 21 '16 at 18:24
  • \$\begingroup\$ This is a great addition indeed. +1 \$\endgroup\$ – 0xDBFB7 May 21 '16 at 18:40
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    \$\begingroup\$ ESD won't set off a standard model rocket igniter. They are not finicky, high-strung devices. They require a large slug of power to actually ignite. \$\endgroup\$ – Ecnerwal May 21 '16 at 21:33
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My suggestion: use the on/off key to power a circuit that consist of the buzzer and a 555 that is wired for a delayed output. The delayed output powers a LED (to indicate to you that you can launch) and the launch butten and igniter. (You might need a power transistor or relay, depending on the curerent required by the igniter.)

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    \$\begingroup\$ For this application any electronic circuit needs to fail in a safe manner. If using a timer I would recommend that the timer circuit is actually powered via the pushbutton so that if it or the transistor fails short-circuit the igniter can not be energised while the button is released. \$\endgroup\$ – Transistor May 21 '16 at 9:34
  • \$\begingroup\$ Yeah, that is what I suggested. The 555 is powered by the switch, the output of the 555 (or maybe a transistor powerd by the 555 output) powers the launch button and igniter. So the switch and button are still in series. \$\endgroup\$ – Wouter van Ooijen May 21 '16 at 10:13
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    \$\begingroup\$ The timer serves no useful purpose - it's just adding a "feeping creature." Either the person who holds the key is qualified to hold the key, or not. Accidental activation while setting up the rocket is prevented by taking the key to the pad. The key is not turned until the area is clear & safe, or "you're doing it wrong." Having an additional delay adds nothing to "safety" - it just adds a delay for no good reason (and complexity for no good reason.) \$\endgroup\$ – Ecnerwal May 22 '16 at 1:00
  • \$\begingroup\$ The timer makes sure that the buzzer buzzes some time before the rocket can be ignited. Seems sort of usefull to me to have a warning signal and some time to act upon it. \$\endgroup\$ – Wouter van Ooijen May 22 '16 at 7:48

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