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.
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. 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’ [Design Sketch]Tags: amps, design challenge, laying a lawn, little buggers, stationary engine