Fisher's Impulse Metal Detector
By Andy Sabisch
From Page 46
July, 1994 issue of Lost Treasure

Fisher Research Laboratory, founded in 1931, is the worlds oldest manufacturer of metal detecting equip­ment. Under the direc­tion of president and general manager Jim Lewellen, Fishers en­gineering staff has consistently devel­oped innovative cir­cuits that have kept the company at the fore­front of metal detec­tion technology. Features such as

silent-search motion discrimination and the S-rod handle design intro­duced by Fisher in the 1 980s are now used by nearly all major metal detec­tor manufacturers. Their philosophy of producing detectors that are truly improved models rather than simply re-packaging existing technology is evident in their latest release the Impulse. In development for over 9 years, Jim and his staff refused to release it until all of the specifica­tions they had established for its over­all performance had been met.


Pulse-type metal detectors by de­sign ignore ground mineralization such as black sand and hot rocks as well as the conductive matrix cre­ated by salt water to provide maxi­mum detection depth in virtually any area the detector might be used in. While there are a number of pulse detectors currently on the market, the Fishers new Impulse features a revolutionary new and patented cir­cuit, which results in exceptional per­formance while addressing some of the weaknesses that existed in other pulse units.

The Impulse does not require the loop to be in motion for a target to produce a signal. This allows users to sweep the coil at virtually any speed ranging from "competition speed" down to no motion at all and still receive a clear, repeatable signal on any target the coil passes over. Pinpointing multiple targets and searching confined spaces such as rocky areas and around old pilings are just a few examples of where this feature will be beneficial.

At first glance, the Impulse looks very similar to the well known and time proven 1280-X Aquanaut. The control housing is shared between the two units, and considering the impressive reliability record set by the 1280-X since its inception nearly 10 years ago, the engineering decision to do this should be commended. The Impulse is certified leak resistant down to a depth of 250 feet making it well suited for both divers and waders alike. The control housing slides easily from the handgrip assembly and it can be hip mounted with the belt clip that comes with the detector. The shaft extends from 31 inches to 50 inches thereby eliminating the need for an extension piece of optional assembly when converting from a diving to wading configuration.

Since the Impulse was designed primarily for use in or near the water, it does not have a built-in speaker but rather, a set of high-quality, wa­terproof headphones are permanently connected to the rear of the control hous­ing with a durable coiled cable. The ear cups are well padded and will remain comfortable even after hours of use. There is a small plug on each of the earpieces which can be re­moved when diving with the Impulse to help equal­ize the pressure felt in ones ears.

The Impulses operation is controlled through the use of two knobs located on the front face of the control housing. The knob on the right is labeled VOLUME and serves a dual purpose. Turning the unit on and off, it also adjusts the volume of the signal heard through the waterproof headphones. This control allows divers, waders, and land hunters to each set the volume to a comfortable level while ensuring that even the weakest signals are not missed. The knob on the left labeled TUNER also serves a dual purpose. It is used to set the detectors audio threshold and, when turned fully counterclockwise, checks the condition of the batteries both audibly and visually.

In the center of the control housing is an LCD panel that provides continual visual indication of battery strength and a bar graph, which depicts the relative strength of the signal, pro­duced by a target. Above the VOL­UME control is a red LED that will increase in brightness as the target signal strength increases. When the audio signal, LCD bar height, and LED brightness are used together, targets can be pinpointed with a high degree of accuracy.

The 8 AA penlight batteries power the Impulse. The battery com­partment, located in the rear of the control housing, is actually sepa­rated from the electronics section. Since users can only open the battery compartment, the potential for get­ting water into the circuitry and thereby damaging the unit has been eliminated. As a matter of fact, over the years Ive neglected to check the tightness of the battery compartment door on my 1 280-X several times and as a result, the compartment flooded.

The only damage that re­sulted was the need to replace the batteries and rinse off the plastic holder a job that took less than 5 minutes and I was back in the water with it. One of the most significant technological advances of the Impulses circuit design is its im­pressive battery life. Historically, pulse detectors have required a greater amount of power than most other detectors, which reduced over­all battery life. The engineers at Fisher have corrected that shortcom­ing and the Impulse can operate for up to 100 hours on a single set of alkaline batteries!


Before taking the Impulse out in the field, I decided to try checking the units response to various targets such as coins, rings, gold chains, and larger brass/iron artifacts. Setting the VOLUME control to the mid-posi­tion and adjusting the TUNER until a slight threshold was heard, I began to pass the items in front of the coil. I was quite surprised at the overall sensitivity the detector had on the targets even the smaller pieces of gold jewelry that are usually diffi­cult for pulse units to detect came through loud and clear.

Taking it out into my test garden, I found similar results. One aspect of the Impulses operation that I found was unlike other detectors Ive used in the past was that if the threshold was adjusted with the coil held even slightly above the ground, the thresh­old would increase if the coil was lowered to the ground (the ground in my area is highly mineralized). I found that by laying the coil lightly on the surface of the ground and adjusting the TUNER until a slight audio tone was heard through the headphones (and 1 or 2 bars ap­peared on the LCD display), this problem was eliminated. The threshold would remain constant as the coil was swept across the ground and even deeply buried targets were easily detected.

I live a short distance from a large lake in north Georgia that contains several beaches which are quite crowded throughout the summer. Unfortunately, treasure hunt­ers also hunt these beaches extensively from as far away as Tennessee and Alabama as soon as they close for the season. Since the Impulse was designed to find targets at depths exceeding those that conventional detectors could, I felt that spending some time at a few of these hunted out beaches would be a good test of its performance.

Parking at the locked gate, my 4-1/2 year old son Paul and I walked down to one of these beaches. Arriv­ing at the beach, I noticed that sev­eral loads of new sand had been spread around making any remain­ing targets even deeper, and hence more difficult to find. Placing the coil on the surface of the sand and adjusting the TUNER until a faint threshold signal came through the headphones, I started searching near the outer edge of the swimming area which was exposed due to the low lake level.

After several sweeps, I received a faint, but distinct signal. Removing several inches of sand I rechecked the hole. The Impulse in­dicated that the target was still there so I continued digging. At a mea­sured 18 inches I found a small childs cap pistol about 5-inches in length. My son immediately appropriated the gun and told me that he was sure that a cowboy had lost it fighting Indians or hunting buffaloes many years ago. Realizing that my first find with the Impulse wouldnt wind up in my display case, I continued searching along the ropes.

The next few signals produced coins of various denominations. What were impressive to me were not only the depth at which they had been found, but the composition of the soil that they had been in as well. The beach consisted of about six to eight inches of sand followed by two to four inches of dense black mud on top of a hard clay layer. All of the coins had been at an inch or two into the clay layer well beyond the reach of most of the detectors that had already searched the beach.

On several occasions I received a weak signal and then wound up spending a considerable amount of time trying to find the target that produced the signal. In each case the target turned out to be a very small piece of metal such as a wire tie, scrap of tin foil, or buckshot from a shotgun shell. Since an extremely deep target and a small piece of trash will produce the same weak signal, its important to be able to tell the two apart.

When I received a weak signal I would move an inch or two of the sand away with my foot and then recheck the area. If the target was still there, I knew it was a deep target and would take the time to recover it. If the signal moved and was now in the loose dirt, I would give up and continue searching since the target was more than likely a small piece of trash that had been just under the surface of the ground.

By using this technique, I was able to ignore virtually all small trash items that littered the beach which resulted in more good finds winding up in my goody bag. (NOTE The only time this method should not be used in determining if a target is worth recov­ering or not is if you try using the Impulse for electronic prospecting and are looking for gold nuggets.)

After a little more than an hour, my son had grown tired of exploring the area so we decided it was time to leave. As we sorted out the days finds on the tailgate of the truck, I was more than satisfied to see that in addition to the cap pistol I had recov­ered three quarters, a dime, two nick­els, five pennies, a bullet, three hair berets, an earring, and a zipper. All of the targets had been at least 8 inches deep, and had produced clear, easily distinguishable signals. While no gold rings were found on this trip, I did find several pull-tabs at depths of up to 11 inches. Since pull-tabs are electrically similar to many of the gold rings treasure hunters find, this showed me that had there been any rings present, the Impulse would have found them.

The next site I took the detector to was a beach near Hilton Head, South Carolina that is quite popular year-round. Arriving at the beach on a cold morning in mid-February, I was somewhat surprised to see two other treasure hunters already at work searching the beach. Waving to them, I adjusted the TUNER control and proceeded to hunt the wet sand be­low the high tide line. Almost imme­diately I received a soft signal that sounded like a good target. Remov­ing three scoops of sand, the Impulse indicated that the target was in the pile of loose sand. Spreading it out I picked out a well-corroded copper penny that had been almost 9 inches deep in the wet sand.

Continuing to hunt parallel to the ocean, I started to recover coins fairly regularly. After 30 minutes or so, one of the other hunters came over and asked how I was doing. Some­what surprised when I showed him the handful of coins I had found, he began asking me about the Impulse. He and his brother had been search­ing for almost two hours, and had found fewer coins together than I had making a single pass in the wet sand. Calling his brother over, they asked if they could try using the Impulse and compare it to the brand they were using. I gave them a quick demonstration on how it operated and they began searching the same area I had been working. Their suc­cess mimicked mine, and after nearly 45 minutes, they stopped and emp­tied out their pouches.

The Impulse had located 17 coins compared to 5 found with their nor­mal beach detector. Almost all of the coins showed signs that they had been there for some time with many coming from depths of 8 inches or more. As they reluctantly handed me the Impulse, they both said that they had to find a way to get their wives to let them buy a new detector.

Over the next few weeks I tried the Impulse at several other sites throughout Tennessee, Alabama, South Carolina, and Georgia includ­ing two lakes where I dove with it down to depths of 60 feet.

In all cases it worked flawlessly and I was able to locate a wide as­sortment of items ranging from coins dating back to the early 1900s to jewelry to Civil War artifacts. Many of these items came from depths that were beyond the capabilities of other detectors that I have tried under simi­lar conditions.


The Impulse has been designed to provide treasure hunters with maxi­mum detection depth under even the most adverse ground conditions. While it was intended to be used primarily by beach/shallow water hunters and divers, it can be used effectively for relic hunting and elec­tronic prospecting in areas where mineralization may have limited the effectiveness of detectors in the past.

Some of the detectors features such as the LCD bar-graph, fully adjustable shaft, non-motion search mode, equalizing plugs in the head­phones, and extended battery life show that the engineers at Fisher listened to the input provided by experienced treasure hunters during the Impulses development phase.

The Impulse can be purchased with either an 8 inch or 10.5 inch low profile, open spider search coil, with the larger coil costing $20 more. As an added bonus, both versions of the Impulse come with a locking hard-shell carrying case designed to pro­tect the detector when traveling. As with all of the other detectors in the Fisher line, the Impulse comes with the standard 5 Year Gold Seal war­ranty.

If you are in the market for a detector that not only ignores ground mineralization but is equally at home both on dry land or 250 feet below the surface of the water, you should stop by your local Fisher dealer and take a close look at the new Impulse before making your decision. For more information on the new Im­pulse and the name of your nearest factory-authorized dealer, give the company a call (209) 826-3292 or write them at 200W. Wilmott Road, Dept. LT, Los Banos, CA 93635 and be sure to mention that you read about their exciting new detector in Lost Treasure!

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