engineer-in-wonderland
Rooting around in the fascinating stuff at the bottom of a draw labelled 'Engineering - Junk Miscellaneous'. Delving amongst the delightful...

An Engineer in Wonderland – Slug-O-Cutor

Like many people last year, I had a slug infestation. Now, I claim not to be a cruel person, but the little buggers were eating just about everything in the garden and I wanted them dead. As I am not keen on the lingering death induced by slug pellets, or their effect on the food chain, an electronic slug-o-cutor had to be the answer. I couldn’t buy one, so one had to be invented. At this point, don’t get too excited, because I never did build anything for it seems that digging over most of the plot in preparation for laying a lawn also kills slugs. But I enjoyed the design challenge, and this was my thinking. slugocutor1.jpg

It had to: a, be solar powered b, get on with its murderous task without intervention c, kill instantly – as I have no wish for invertebrate suffering. So batteries and lots of volts. The engineer part of me thought about getting a box of slugs and experimenting – but frankly, I am too squeamish and still fell faintly sorry for them. Instead, I just guessed that 300V from a 1µF capacitor should do the job – inspiration coming from an inverter I built for a vintage stationary engine ignition system (the magnetos die and are expensive to repair). 300V from a powerful inverter (don’t try this at home) would also do it the job, but be far too dangerous to leave lying around in the garden. With power limited when running from a few AA cells, the inverter had only to run when needed. This is my answer. slugocutor2.jpg Once low resistance (slug) connects it self across the contacts, transistor T2 is turned on via diode D1. This feeds power to the self-oscillating flyback converter built around T1 – one of Zetex ZTX power transistors that need very little base current to switch a couple of amps. The auto transformer charges the 1µF capacitor via D3 during the flyback periods. Once the capacitor voltage reaches 300V, the zeners conduct and fire the thyristor, discharging the capacitor into the slug. D1 protects T2 from the 300V spike. D1 and D3 are fast devices – With the ignition system, I found that 1N4007s are way to slow for this kind of application. The gate capacitor may need to be returned to 0V rather than the cathode to better protect the gate from false triggering through the capacitance of the zeners and pulses on the 300V rail. If the slug moves away mid charge, the oscillator simply switches off. Once zapped, the slug is expected to fall away from the contacts – parallel wires across a slight slope. The battery is charged by solar cells and standby power consumption is essentially zero. Any comments? – there is a box for these if you scroll down a bit. Also: Any modifications or better ideas? – I can’t immediately think of a way to stop the circuit repeatedly delivering jolts to the same victim if it fails to move. Does anyone happen to know the minimum energy that quickly kill slugs? – and at what voltage. And, am I a monster for even thinking about automated gastropod doom? ‘Alice’ 07may08slugocutor3cropped.jpg [Design Sketch]

Tags: amps, design challenge, laying a lawn, little buggers, stationary engine

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11 Comments

  1. Acme Fixer
    July 02, 2010 23:59

    I think that keeping the contacts clean, dry and clear of dead slugs will become an intractable problem. I think the contacts would need some kind of mechanical device to do this. Perhaps a motorized squeegee or scraper. Whatever, the case, I think that without it, the slugs will die on the contacts and disable the device.

  2. 'Alice'
    June 11, 2008 12:13

    Thanks for all those suggestions Auntie G.
    A chance aside by a Gardner’s Question Time [www.bbc.co.uk/radio4/gqt] panellist about ferrous phosphate and slugs got me googling and I am now on the hunt for Growing Success Advanced Slug Killer [www.monrobrands.com] which is apparently based on the stuff.
    So much as I feel guilty about slugs dying slowly, I will be searching this out as reports suggest it is:
    A, effective and
    B, harmless to the food chain, whereas common metaldehyde is not.
    More unnecessary anorakine googling also turned up the information that inside is re-branded Ferramol which farmers buy in bulk.
    There will still be some kind of electrical activity in the garden as once I get my greenhouse, I will wrap two parallel copper wires 10mm apart around each leg of the staging, and connect a 9V battery between them.
    This, it appears, is a dead cert way to stop our slimy friends getting past.
    By the way, does anyone have recommendations for a lean-to greenhouse under 2m wide and 1m deep?
    I was really quite excited about making the slug-o-cutor, just to see if the 300V trigger and discharge circuits work together more than once, but with ‘safe’ slug pellets around I suspect I can use the time more usefully.
    ‘Alice’
    alice@electronicsweekly.com

  3. Auntie G
    June 09, 2008 14:13

    I have been investigating this further and found an article I took from the Observer newspaper some while ago which had a list of slug busters – here they are: -
    NEMASLUG – a lumpy beige paste that has to be kept refridgerated and is a leading biological control against slugs – deadly to them but harmless to everything else – expensive though, £18.95 for 40 sq m
    COPPER SLUG RINGS – contact with this gives them a shock, £16.75 for 6
    SNAILAWAY (don’t you just love these original names) – 2 silver bands on a self-adhesive strip act as a mini electric fence, the pest is diverted rather than killed – £8.45 for 2 metres
    SLUG-GONE (another brilliant product name) – pellets that contain grit, salts and scratchy wool fibres.Spread around plant and they expand to form a protective mat that lasts for 8 to 12 months. The champion veg grower Joe Maiden swears by them.
    Hope this helps your problem if you have not already zapped them with your Slug-O-Cuter

  4. Rob
    May 21, 2008 13:34

    Latest from Gardener’s Question Time….
    You should leave patches of rough grass near your veg patch, about 0.5 – 1m diameter if you can. These provide a habitat for several types of beetle that include slug and snail eggs, amongst other pest eggs, in their diet.
    Now we just need to build something that measures how busy the beetles are.

  5. Auntie G
    May 06, 2008 16:27

    Okay, someone states that gravel and egg-shells will not work, well I will not argue…..(memo to self must remember to listen to Gardeners Question Time on Radio 4 more)…..however slug pellets are not kind or safe to other forms of wild-life, hence me trying to find an ecologically safe alternative, other than ‘frying’ the little blighters.
    However I have to agree with Mr Gillatt, once you got these critters, there is no easy way of getting rid of them – hope for a dry spell and try planting marigolds near what you’re trying to protect, they will act as a sacrifical plant – the problem with that solution is that they can munch their way through dozens in one damp night.
    As for strange uses of slug goo – the Japanese have also come up with a rejuvenating face cream for women made from slug slime. It was featured a couple of weeks ago on the TV show ‘How to look good naked’ (I need all the tips I can get) :)

  6. May 02, 2008 13:30

    Sorry but eggshells, gravel, soot w.h.y. just don’t work… and a slug’s “foot” is far from delicate. A slug can quite happily “walk” over the edge of a razor blade. In fact the mucus excreted is such a unique substance that research has been done to improve products like adhesives. (Japanese of course)
    I think 3kV would be more realistic. That ought to explode the little b*****s properly. Maybe another pulse a second later to keep the electrodes nice and clean.
    Biggest problem after hedgehogs is moisture. How long could you leave the apparatus outside (at 300V or 3kV) before it started arcing during the run-up?
    And no, you are not a monster. Slugs, like rats, are virtually ineradicable. The only antidote is either slug pellets or collecting by hand and (look the other way) flushing down the loo.
    If anyone tells you that gravel, eggshells etc. work then they haven’t collated the results of their efforts. (Re. Gardener’s Question Time for confirmation). The only humane alternatives are hedgehogs and chickens and then you would need to protect the garden from the chickens somehow.

  7. Auntie G
    May 01, 2008 14:51

    Okay ‘Alice’, I see you have an aversion to gravel – sorry I only meant small amounts around the base of the plants – however you have not addressed the broken egg-shell solution – or maybe you should just import a hedgehog, they’ll get rid of them for you, put a small ad out.
    Presumably the hedgehog and cat potential problem with your ‘zapper’ would be solved by making the aperture that the slug uses to enter too small for a prickly or furry friend’s snoat/nose.

  8. 'Alice'
    May 01, 2008 13:56

    Thanks John Scrivens, you have a good point.
    My original concept diagram has a hat on it, with the intention of protecting the innocent.
    The diagram didn’t make it in to the original posting, but has now been added.
    I like hedgehogs and have yet to find one in the garden. Apart from a surfeit of slugs, are there other ways of attracting them.
    There is a toad.
    ‘Alice’
    alice@electronicsweekly.com

  9. 'Alice'
    May 01, 2008 12:06

    Thanks Auntie G, but I have a natural aversion to gravel in the garden, mine having been covered from end to end in the stuff by the previous owner.
    A certain amount of mental baggage has been accumulated from sieving out four tonnes of 12mm aggregate to leave the topsoil behind
    Lending weight to your argument, there were no slugs – although this may have been because there was no vegetation either.
    And surely these dissuasion techniques just move the problem elsewhere?
    No, it will be electronics if there is an infestation this year, or I will buy a duck.
    alice@electronicsweekly.com

  10. John Scrivens
    May 01, 2008 11:41

    I think the MOST important question here is not how much energy will kill a slug but how much energy WON’T harm a slug hunting hedgehog, or indeed the neighbours cat.

  11. Auntie G
    May 01, 2008 11:13

    Now why oh why hi-tech……..broken egg shells will deter the little blighters, delicate feet you see, (eh feet no I mean slippery under-belly, do slugs have bellies…?); anyway they don’t like sharp things to slither on, so this will do the trick. However if you don’t want your garden to look as if there has been a fight in the hen-house, then use sharp gravel, not sure the exact name for this stuff but in stead of lovely rounded pebbles you get sharp pointy little stones and scatter these around the base of of your plants.Any questions? :)