Is there a dependence on fuel octane rating or ethanol content for the lawn mower hack posted above? I want to really nail this down accurately...

BTW, I'm bumping this thread because I'm new here and this gets me almost where I want to be. I'd like to recap a unit that I had get struck by lightning this year. Or I'd build one from scratch if I could figure out what freq my receiver is tuned for. Any suggestions?

 

Is there a dependence on fuel octane rating or ethanol content for the lawn mower hack posted above? I want to really nail this down accurately...

Are you joking? I can't imagine any mechanism how the fuel composition would affect the spark.

If your unit suffered lightning damage, it may be beyond repair. Is its power supply working?

 

So how exactly do you, or anyone else, know your invisible dog got out of the fence anyway?

 

I have about 2000 feet of buried fence which occasionally gets a break in it.

Here's my cheap solution. As a broad-spectrum frequency generator which you can pick up with any portable AM receiver as static, simply disconnect your transmitter completely, take a thin wire (awg 18 or so) and wrap it around the spark plug wire of your lawnmower (DO NOT CONNECT IT TO THE SPARK PLUG!). Then connect it to only one end of your fence wire.

Start the lawnmower. Tune the AM radio to around 600khz off any station and hold it near the ground by the start of your fence. You should hear static and a buzz at the spark frequency of the engine. Then just walk along following the radio static until it quits and voila, you found the break. To be sure, stop the mower and connect the thin wire to the other end of the fence wire, restart the engine, go back to where the static quit, and see if now it doesn't continue from where you left off.

In my case with 2000 feet of wire, I put a "manhole" (6" diameter by 8" long plastic pipe) about every 200 feet, so in my case I can move my lawnmower noise generator closer to the break if I need more signal strength (the signal tends to fade with distance and ground dampness).

Hope this helps (and it's free!).

Hey DonaldRobert,

I had to create an account here to tell that your cheap and brilliant solution saved me $50.

I got an RF Choke as described lots of places on the internet and an AM radio but it didn't work with my SportDog system. I was almost resigned to the fact that I'd have to pay $50 for the proprietary fix-it kit when I came across your post.

I did exactly what you described and found my break within a couple of minutes. Worked like a charm.

Thank you!

 

Anyway, here's a question: How does the load of the wire affect the oscillator? The oscillator is said to be able to drive 10 TTL loads. Is that a lot?

First generation TTL ICs had a common base input stage, so whatever was driving them needed to sink 1.6 mA out of the TTL input circuit for a guaranteed logic low signal. 10 TTL loads means the oscillator's output circuit can sink 16 mA and be below 0.8 V or 0.4 V DC. Because of the history, this is a very common output spec.

ak

 

Coming across this old thread ... I'm trying to find a break in my Invisible Fence (that brand, not generic) and keep coming across guides that say to use an AM radio as detector. However, the transmitter on my fence transmits at 7 or 10kHz, so it's not clear to me how an AM radio is supposed to detect that. Is it detecting harmonics ? Or are those guides intended for other fence brands that work in the AM band ?