Utility locator design - Page 1
Forum section: electronics / projects

author: antenna
author_rank: Frequent Contributor
timestamp: June 17, 2025, 11:18:12 pm
content: I want to build a receiver for detecting 512Hz for locating a sewer line at a depth of about 1.5m. I was considering two coils out of phase for null detection when right above the transmitting line. Would it be reasonable to go with 4-pole active bessel filters on each receiving loop, or would I need 6-poles due to the harmonics of nearby 60Hz power lines? I am planning on summing the two outputs in another op-amp for null detection.I know this is top secret stuff the big companies don't like being talked about, but I am not going to market with it, just trying to save a thousand bucks. It's the one topic that I cannot find any schematics or existing designs for. Any advice would be greatly appreciated.

author: moffy
author_rank: Super Contributor
timestamp: June 18, 2025, 12:11:17 am
content: If you have two coils that should null directly over the signal, just connect them together in a way that they null, and treat the output as from a single coil, that way filters won't be causing phase shifts or gain imbalances between the coils. If you have a doubt about 4th or 6th order Bessel, then that is something for experimentation. Just design your board for both or go with the conservative choice of the 6th order. I don't know if the LM567 tone decoder chip might be of benefit:https://www.ti.com/lit/ds/snosbq4f/snosbq4f.pdf?ts=1750205413146&ref_url=https%253A%252F%252Fwww.google.com%252F

author: antenna
author_rank: Frequent Contributor
timestamp: June 18, 2025, 02:10:22 am
content: Thanks.This was going to be for work, but I just discovered something looking at locators online. I saw a clamp for a Rigid SL-20 locator on ebay. Almost crapped myself when I came across the picture because I knew I recognized that clamp. I went out to the grounds garage and sure enough, it was the very same clamp sitting on the shelf. So, that means this place either has one or had one at some point.We are going to contact our electrician and hope someone didn't steal it because those things are $3600USD. If it is gone, then I will come back to this project. I just don't see how the electrician would leave the clamp behind. It's been here for at least a year. Thats the trouble with big businesses, the operators don't pay for anything and don't care what happens to stuff.

author: antenna
author_rank: Frequent Contributor
timestamp: June 18, 2025, 02:33:28 am
content: I'm probably going to do this whether they find their locator or not, that way I have one, but it might get put on that back burner where projects die. The phase error was the reason for the bessel filter as opposed to other filter types, but I really don't understand it anyhow. I like the idea of connecting the coils and treating them like one. Tempted to add a third coil for peak detection.My major concern is that I will be driving a metal tape that is not grounded at the other end, so there won't be measurable current flow. This tells me that I might struggle detecting a magnetic field. I didn't want to go with E-field receivers because they are less accurate. There is so much to consider when designing these that I can see why the good ones cost so much.

author: moffy
author_rank: Super Contributor
timestamp: June 18, 2025, 03:41:18 am
content: Quote from: antenna on June 18, 2025, 02:33:28 amCoils won't work so well for that unless it is some form of metal detector, you could generate some current by placing a grounding stake in the ground and using that as one of the excitation terminals, but I would imagine that would give a wider area to detection.

author: antenna
author_rank: Frequent Contributor
timestamp: June 18, 2025, 05:58:24 am
content: Im thinking I would need to tape a return wire to the fish tape for a return path to establish a magnetic field. But then my concern is it acting like a transmission line with wave cancellation.

author: moffy
author_rank: Super Contributor
timestamp: June 18, 2025, 11:01:48 pm
content: If you can create your own loop then you will get some leakage flux, maybe there is a clever way to wind the wire/coil like a loose Rogowski being asymetrical on the way out and back.e.g.https://www.eevblog.com/forum/projects/mini-rogowski-coil-current-probe/msg5942258/#msg5942258Rogowski coils are used to measure current, so symmetry being what it is I expect that putting current into the coil would radiate also.

author: antenna
author_rank: Frequent Contributor
timestamp: June 18, 2025, 11:58:50 pm
content: Thats cool, thank you! I am going to try that!

author: MathWizard
author_rank: Super Contributor
timestamp: June 21, 2025, 08:03:30 am
content: So what's the transmitter? Do they just hookup a 512Hz signal to a big metal pipe in the ground ?

author: antenna
author_rank: Frequent Contributor
timestamp: June 21, 2025, 08:04:42 pm
content: I haven't built it yet. I figured on using a 1-5w audio amplifier into a transformer to raise the impedance for unterminated line tracing and have a low impedance tap for driving loops.The electricians did have the SR-20 locator, and they were extremely happy to hear we found their signal injection clamp. I guess they have been searching for that for years.