You are probably on the right track with the “few cm of ignition wire”. It would have very little capacitance and therefor very little loading. If it was connected to a low capacitance detector (a few pF) then the required capacitance would be fF (femtofarads) given the ratio required to step the voltage down.
My idle mind was thinking around the problem, why an indicator light? How difficult or expensive would it be to have a solar powered transmitter in some part of the spectrum available for such uses. Something in the GHz region so a small directional antenna can be used, pointed at a base station. It could be as simple as detect pulse, programmed delay, 1ms burst of radio. The electric fence pulse is virtually instantaneous at all points so having a different delay for each detector will result in a series of 1ms pulses received at the base station. The base station has the smarts to detect a missing pulse, sound an alarm.
Again - not costed. But to me it seems a more desirable idea, as the user does not have to go looking.
Hi Alan
I think the fence might be DC. Once again this is unknown to us mere mortals. in this case a capacitor will only pick up the on and off spike which is enough to trigger a 555 or something.
This is plural. I think there are about 6 sections of fence to monitor. So overcomplicating is a game changer and can get to be a bit over the top (or fence).
Anyway haven’t heard from Cath for a while so she might be looking down the commercial product path. The whole exercise was to save the expense of commercial devices.
Cheers Bob
I’ve seen older electric fence transformers. They look like bigger versions of old time car ignition coils and work the same way. Voltage (often 12V) applied to the primary to build up magnetic field. Open that circuit, magnetic field collapses, secondary produces a spike (I wouldn’t call it DC). The spike is quite short so it gives a good shock but doesn’t last long enough to kill someone. An oscilloscope will usually show a smaller pulse of the opposite polarity due I guess due to inductance in the load (fence).
A back of the envelope calculation says a solar powered wireless based system is practical if anyone wanted to take up the design work. A 2.4GHz transmitter (nRF24L01) with SMA connector less than $5. A directional antenna (basically a few bits of fencing wire) should give 1km line of sight range. A low power solar light around $2-3. A solar light has a solar panel, a battery (usually NiCd), and a boost circuit to convert the battery 1.2V to sufficient to run a LED. That would be adequate to run a low power CPU and a pulsed higher powered transmitter. A suitable CPU to run the show $2. Many PIC CPU have a comparator which could detect a pulse divided down by capacitive divider. I’m thinking given the tiny capacitance doing the detecting, the CPU clamp diodes on the pins would be enough to bypass over voltage and revers voltage. And the CPU would provide the delay. Should only need a few passive components and a waterproof enclosure. And a clamp to mount it on a star picket.
The base station doesn’t need to be expensive. Just a receiver (omnidirectional receiver antenna), another CPU, a power supply, and whatever alert mechanism is suitable.
Which makes one think such a device should already exist. Or there are design problems I haven’t thought of (Rumfeld’s ‘unknown unknowns’).
Alan
Yes, it is called the “Kettering” ignition system and the one I grew up with. Then along came electronic bits and the Capacitor discharge system. And it IS DC, the bit of spike at the end of the pulse is probably a bit of ringing and could even be put there by the CRO earth lead being a bit long or the probe itself. I have seen it all before (many times) where something that looks like it might be a problem actually goes away when the CRO is removed.
It won’t supply enough current to do human (or animal) harm. I have not seen any figures on this but it could be down to pA or µA at most. That is why I think anything hanging there permanently will upset the fence operation and if you consider multiple devices hanging off the fence you will probably render it completely ineffective.
Anyway I am going to drop this unless Cath comes forth with some more relevant info or ideas. I don’t have an electric fence to play with and to date Cath’s fence make is unknown (to me anyway).
Cheers Bob
