Dave's EEVblog #714  Harbour Freight handheld metal detector   - Page 1
Forum section: electronics / general

author: HiTech
author_rank: Regular Contributor
timestamp: February 13, 2015, 04:29:30 pm
content: http://www.eevblog.com/2015/02/14/eevblog-714-metal-detector-reverse-engineering/Here's a schematic I found on the Internet of the detector that Dave similarly drew on paper. It details a couple of options he didn't cover:http://www.findmall.com/file.php?22,file=218436I know Dave humorously refers to it as a cheapie item, but actually it's not a bad little "pin-pointer" detector once a few simple mods are done to it. I bought one awhile back for around $10 w/ a coupon. I'm not sure if HF even carries it in their inventory anymore these days.The detector has two inherent problems that plague it as reported by many owners who rely on using a pin-point detector (PPD) to narrow their search once the large detector has located an area of interest. The HF PPD triggers falsely (tones) during hot summer days even if you turn down the sensitivity. The side-mounted power switch is crap and affects reliable operation overall. Replace the factory push-button switch with something of better quality (heck even Radio Shack has one that's better!).To remedy the temperature instability, the pcb has pads to install an NTC 100 (100 ohms). Once installed, set the sensitivity control mid-point and then adjust the small blue trim pot on the pcb to barely a trigger a tone through the speaker, making sure there's no metal nearby. The detector should now work under a wide variety of temperatures without false triggers - yea! HF must have reached a price point whereby they couldn't profit by adding one more worthwhile component- and a crucial one at that!The pcb also has pads for a vibratory motor and a STDT switch to select either speaker or vibration. Radio Shack sells the small motor or you can scalp one from a pocket pager or cell phone. Some treasure hunting folks want to keep their finds private without alerting others around them so a vibrating motor is the ticket for that versus a squealing speaker.Another slight improvement is to make note that the plastic tube holding the coil is not water tight. The end cap will allow moisture/water to enter and gunge up the coil. Remove the cap and reinstall it with a sealing adhesive like silicone based glue or similar. The coil leads are just long enough to allow you to move the coil closer to the tip of the tube for slightly improved results -- gains about an extra 1" depth sensitivity. Some owners have even gone as far as to remove the coil, solder the 4 leads to a short length of CAT5 wire so they can hold just a "wand" while the electronics housing is clipped to their belt or to their larger detector.Just keep in mind that this HF detector is designed to locate small metal objects (it's not a discriminator type) at distances of around 3" + or -. That's what the $$$ name brand pin-pointer (PP) detectors yield for the most part and some owners swear this littlegem in the roughperforms equally to the popular brand name PP detectors selling near $100 or more.

author: JoeO
author_rank: Frequent Contributor
timestamp: February 13, 2015, 10:47:52 pm
content: HF USA still lists the PP on their website.

author: HiTech
author_rank: Regular Contributor
timestamp: February 24, 2015, 04:46:39 pm
content: Here's a link I found that has pics of a modification:http://www.allaboutmetaldetecting.com/2011/07/cen-tech-hand-held-metal-detector-mod.html