Underground locator generator for Radiodetection using PIC16F628


I obtained an early vintage Radiodetection® RD400 “LLTS” Precision Line Trace unit for a very low price through eBay. The RD400 is the receiver portion of a transmitter/receiver pair that comprise an underground cable locator system. Because the matching transmitter was not available (hence the low price,) I had to design one that matched the two audio frequencies detected by the receiver.

Underground locator generator

“LLTS” means Long Line Tracing System. Used with original transmitter, this unit was said to be capable of tracing signals in cables up to 45 miles. One of the most useful home-based applications of an underground locator is for locating breaks in the perimeter cable of underground dog fences!


This transmitter is a simple design. The only control is a three-position rotary switch or center-off DPDT toggle switch. The transmitter is housed in a case that contains the 120VAC power cable, test cables, ground rod, and current clamp (not shown.)


  • Power: 8-35VDC such as from a wall adapter for 120VAC 60Hz
  • Output: 512Hz or 8192Hz modified sine wave, 50-80VAC


Switch the unit on to either 512Hz or 8192Hz. The LED flashes slowly (1Hz rate) if 512Hz is selected; fast (16Hz rate) if 8192Hz is selected.

Underground locator generator schematic

The receiver detects the electromagnetic field generated by the transmitters signal in the underground cable. Because magnetics are involved, and because magnetic fields are induced by current flow, and because current cannot flow without a complete circuit, it requires that whatever means of connection to the underground cable is used, signal current must be caused to flow in the cable being detected. Therefore, some thought may be necessary to get a useful signal into an underground cable.


This program is a transmitter for an underground locator that operates at 512Hz or 8192Hz.

Transformer coupled, modified sine wave


               |      RA0 |-17-- output 1
  MODE   ----7-| RB1      |
               |      RA1 |-18-- output 2
               |          |
               |  16F628  |-1--- LED
               |          |
               |      RB6 |-12-- PGD
               |      RB7 |-13-- PGC
 20MHz XTAL-15-|     MCLR |-4--- MCLR
       XTAL-16-|          |



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For more detail: Underground locator generator for Radiodetection using PIC16F628

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