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Following my yesterday post which was a break wire alarm based on logic gates to drive a siren for the protection of a camp site, I have decided to work on a second concept that uses completely different circuit based on a simple transformer design. Considering that the main uninvited visitors of the camp site are stray dogs and wild boars, I've decided to use squib (firecracker) as the main load simply because I've found it extremely more effective than siren, which doesn't seem to scare them anymore.

In the following circuit the primary of T1 is energised upon closure of switch 1. The role of R2 is to limit the current coming from a 9v battery. When the wire is broken by animals, the sudden collapse of primary will induce a transient in secondary, which in turn, heats up the extremely thin nichrome wire (0.02mm) of the squib. The switch 2 is a safety measure which is closed at first to bypass any transient when the circuit is powered up first and then it's opened by the operator. My question is regarding the transformer itself, what type of transformer would work best? My assumption is step down type with a very high number of thin wire turns at primary and a secondary with thicker and fewer turns to provide sufficient current for the squib. Please share your suggestions.

enter image description here

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  • \$\begingroup\$ What an odd idea.... R u planning on going out and fixing the wire in the middle of the night after the first boar... or camper.. breaks the wire? Even if that is the case, the whole thing could be done very simply with a resistor, a battery, and a transistor, or better, a MOSFET. \$\endgroup\$
    – Trevor_G
    Commented Mar 11, 2017 at 18:43
  • \$\begingroup\$ Hi Trevor, as you know I already have a circuit which you helped to improve yesterday. This is however, came to my mind when user287001 suggested a closed loop system. I know it's odd but it's interesting if it works. The firecracker will makes an easier sounding system than a siren. As long as it wakes us up it should be good. \$\endgroup\$
    – Baphomet
    Commented Mar 11, 2017 at 18:50
  • \$\begingroup\$ YOu know..if you are serious about all this you may want to think about something a bit more reliable that doesn't need to be reset / have someone walk the perimeter to find the break, and resets itself..... \$\endgroup\$
    – Trevor_G
    Commented Mar 11, 2017 at 19:04

5 Answers 5

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I'd suggest you need to invert your architecture and get rid of the transformer altogether.
Perhaps something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

With this you'd have 90 uA loop current and plenty of energy to fire the squib even with only a small 9 V battery. I've included both your 'safety' switch and a loop test switch so you know that the break loop is ok before you toggle the safety.

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  • \$\begingroup\$ Hi, thanks for the this, I have two questions. Can the value of R1 be further increased to lower the power consumption? and how is the reliability of mosfet vs e.g BJT against noises ? \$\endgroup\$
    – Baphomet
    Commented Mar 11, 2017 at 19:08
  • \$\begingroup\$ Ya u need a filter cap across D1 @JackCreasy. That wire is one huge antenna. See yesterdays question .. \$\endgroup\$
    – Trevor_G
    Commented Mar 11, 2017 at 19:09
  • \$\begingroup\$ Yeah Trevor maybe combining your famous filter circuit with this circuit ? \$\endgroup\$
    – Baphomet
    Commented Mar 11, 2017 at 19:11
  • \$\begingroup\$ From yesterday electronics.stackexchange.com/questions/291510/… \$\endgroup\$
    – Baphomet
    Commented Mar 11, 2017 at 19:19
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    \$\begingroup\$ @Trevor. I don't think you need a capacitor across the gate at all. That's a power FET with a huge capacitance (approaching 1000 pF, and add to it the capacitance of the Zener diode (about 300- 400 pF). If it makes you feel safer....by all means ...but I don't think it's needed. Remember that I simply selected the devices offered by CircuitLab ....maybe there are better choices that could be a made. The zener for example could be a 1N5242 which has lower threshold current. \$\endgroup\$ Commented Mar 11, 2017 at 19:55
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You need electrical data on the squib before continuing. In particular, you need to know how much voltage and current must be supplied to the squib for how long to set it off.

I expect the total energy required would be significantly more than what can be stored in the magnetic field of a reasonable transformer. You're also only allowing 90 µA to flow thru the transformer. That's going to require a very large primary inductance to store a reasonable amount of energy.

Your circuit is also very wasteful of the limited battery energy because the energy has to be maintained in the transformer core all the time, and most of the time it won't be used. It would be much better to use a transistor to invert the current on/off in the loop. The squib would be actively driven directly from the battery whenever the loop is opened.

Again though, start with the squib specs. Making a circuit to drive it when a connection is opened should be trivial.

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  • \$\begingroup\$ Thanks for the reply , the squib seems to be sensitive. I charged a 100uf cap to 9v and it set it off. I haven't tried lower value caps. If I increase the resistor value to 1m, would that improve energy consumption? regarding the transistor, could you please show the schematic? thanks \$\endgroup\$
    – Baphomet
    Commented Mar 11, 2017 at 18:31
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Simpler to do something like this.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The gate is too exposed on this. \$\endgroup\$ Commented Mar 11, 2017 at 19:01
  • \$\begingroup\$ @JackCreasey, yup, you took it a lot further too. I like :) \$\endgroup\$
    – Trevor_G
    Commented Mar 11, 2017 at 19:03
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I'm a hobbyist but here's my view on the matter:

Using a transformer to match up to the very low impedance of your squib may seem like a good idea (and it is), but ignoring the details of your primary circuit for a moment (it's not relevant to me for now) the problem will be in transferring sufficient energy to guarantee an all-fire specification for your squib.

You've provided zero specifications for the squib. Estes provides very clear all-fire specifications for their rocket engine squibs, by comparison. You have to deliver a certain number of Joules (about \$\frac{1}{2}\:\ J\$) within a certain period of time (\$50\:\textrm{ms}\$) in order to meet their all-fire specification. Their specification is actually rather difficult to meet with a transformer arranged as you have it.

Review this answer I provided earlier regarding the basic energy equations for capacitors and inductors.

A capacitor's energy is \$\frac{1}{2} C V^2\$ and is stored as lines of electric force in vacuum regardless of the dielectric itself (which actually acts to counter the electric force in dielectrics), but it's easiest to imagine that once you've selected your capacitor the energy present is a matter of how much charge stored on it. Matching up the capacitor to a particular squib would be about meeting the specification I mentioned earlier. You could store a lot of energy on a super-cap, for example, with very low voltage. But it wouldn't deliver the required energy in the required time since the voltage present would be too low to induce enough current in a short enough time. Most of the energy would stay on the capacitor. So the selection of the capacitor depends upon the load. The pairing must be arranged to meet the all-fire specification.

An inductor's (transformer here) energy is \$\frac{1}{2} L I^2\$ and is stored as lines of magnetic force in vacuum space regardless of the core material used. (In your case, you may want a gapped core.) But again, it is easiest to imagine that once you've selected your inductor/transformer, the energy present is a matter of Webers (volt-seconds, and the equivalent of charge on capacitors.) Again, the selection of the inductor/transformer depends upon the load and the pairing must be arranged to meet the all-fire specification.

For inductors/transformers this is often more of a problem. You may have noticed that the volume of a capacitor (for a given manufacturing technology) is proportional to the energy it can store. The same is true for inductors/transformers.

If you had a specification (you don't, yet), it might be possible to compute the required core volume needed to store the energy you want held there. (You might read this post from me which is one place where I discussed core volumes.) But that can't be done just yet because you don't have a specification. And I suspect you will be disappointed at the sheer size required, once you are able to provide an accurate specification (considering that you are imagining a single pulse here.)

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  • \$\begingroup\$ Thanks for the reply. The squibs I am using are different from commercial type. They are ex military. They have much thiner nichrome wire, 0.02mm thickness and about 2-3mm long. I fired one with 100uf cap charged to 9v. They make a good bang for scaring animals. I haven't tried lower value caps. I know the commercial variety use much thicker wire. \$\endgroup\$
    – Baphomet
    Commented Mar 11, 2017 at 19:30
  • \$\begingroup\$ I think the specs are : 22uF, 12v. I also measured the resistance, it was 6 ohm. \$\endgroup\$
    – Baphomet
    Commented Mar 11, 2017 at 19:50
  • \$\begingroup\$ @Baphomet Thanks! That's a lot more resistance than the Estes squibs, by the way. \$\endgroup\$
    – jonk
    Commented Mar 11, 2017 at 21:09
  • \$\begingroup\$ yes because it's much thinner and requires less energy. \$\endgroup\$
    – Baphomet
    Commented Mar 11, 2017 at 21:34
  • \$\begingroup\$ @Baphomet Do you have any information about how long it takes to actually fire? Is it on the order of 50 ms? Hmm. Never mind. You gave me a capacitor. That's good enough. I can work it out. About \$1.6\:\textrm{mJ}\$ and \$\tau=132\:\mu\textrm{s}\$. \$5\tau\$ would be more than long enough. \$\endgroup\$
    – jonk
    Commented Mar 11, 2017 at 21:46
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This is the revised version of the logic circuit from yeserday including filtering components. I have also modified the load so it's using squib (firecracker) rather than buzzer. With extra safety clamping circuit. It works like a charm on breadboard.

enter image description here

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  • \$\begingroup\$ Just a few thoughts: Q1 off state Collector current may exceed current through R2. Q1 turned off you can exceed V(be) breakdown (unlikely to damage Q1,but will rapidly charge C1). C4 charge state limited by 2 * 2N6427 off state Collector current (about 1-2 uA combined), likely maximum 7 - 8 V on C4. TVS1 leakage can exceed current through R4 - R8. Your 'loop ok' condition has the voltage on C2 very close to the VT+ threshold of about 6 V. While the TVS leakage may mean you never see an open loop, if it is working it is too close to the limits. \$\endgroup\$ Commented Mar 11, 2017 at 23:35
  • \$\begingroup\$ Ok, first about Q1, I guess reducing R2 to 4.7m help. Or should I change the type of transistor? Also I should choose a transistor with high V(be) breakdown. I'll change Q5 to a NPN to help C4 develop more voltage. The TVS that i am using has 1uA leakage. Vishay SA12A-E3/54. Is that high? If you have any solution to above problems please share. Thanks \$\endgroup\$
    – Baphomet
    Commented Mar 11, 2017 at 23:52
  • \$\begingroup\$ Also TVS is 12v. \$\endgroup\$
    – Baphomet
    Commented Mar 12, 2017 at 0:04
  • \$\begingroup\$ Extreme low current design is almost as much art form as engineering. I hate to say that given the application you are designing for that a small MCU would appear a better solution and would get you to very long battery life ...approaching the self discharge rate. And... Yes 1 uA of leakage in any component is high for the resistor values you have. \$\endgroup\$ Commented Mar 12, 2017 at 0:49
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    \$\begingroup\$ @JackCreasey I have found a great article on how to reduce the power of Arduino mini pro (ATmega328) down to uA levels. home-automation-community.com/… \$\endgroup\$
    – Baphomet
    Commented Mar 12, 2017 at 11:22

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