DSP-100 User Manual
Page 24 of 25
DSP100_MAN_A
The best way to check for any of these issues is to use a megohmmeter (commonly referred to as a megger).
Disconnect the loop wires in question from the vehicle detector and any other electronic equipment. Connect
one lead of the megger to one end of the loop wire and the other lead to earth ground. Measure the resistance.
For accurate measurements the ground and the loop should be wet or at least damp (use a hose or a bucket of
water if needed to get the area wet). The reading should be at least 100 megaohms. If it is less than 50 megaohms
the insulation is compromised and the loop circuit should be replaced. Between 50 and 100 megaohms, the loop
may or may not work properly and reliably.
Electrical Interference
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There are several possible sources of electrical interference: loop cross-talk, power lines,
electric motors, and insufficient twisting of the loop wires, just to name a few.
Other loops in the area that are connected to a different detector are prone to cross-talk (when the magnetic
fields from different loops interfere with each other). Adjusting the loop frequency of one or both of the loops in
the same area will usually allow you to find a setting that both loops will work reliably with.
Anything that uses electricity is a possible source for electrical interference depending on its proximity to the loop
and the amount of energy being used. If you believe the loop is experiencing electrical interference, turn off the
device believed to be the source of the interference and see if the problem goes away. Sometimes this is not
possible and more technical means are needed to help identify the source. Call Technical Support in this case.
If the electrical interference is occurring in the wire from the loop to the detector, additional twisting should help
mitigate the issue.
Moving Objects in Proximity to the Loop
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Objects that can move and are metallic or somehow electrically
conductive, may cause detection issues.
A common issue is movement of a slide gate or gate arm in close proximity to a loop. The best solution would be
to move the detection area further away from the moving gate. We recommend that all loops should be at least
4 feet from a slide gate. Try lowering the sensitivity one level so that the desired vehicles are still detected, but
not the moving gate. NOTE: Do not lower the sensitivity too much or vehicles will no longer be detected.
Another possibility is metal objects in close proximity to the loop. Utility manhole covers are objects that may
move slightly when vehicle tires drive over them, especially if the vehicle turns while a tire in on the cover. Most
manhole covers can be bolted in place. Contact the owner of the manhole to see what can be done to mitigate
the cover movement.
Detect LED Will Not Come On With a Vehicle Present
The first thing to do is verify that the LED in question is still working. This is accomplished by a quick lamp test.
Reset the detector by temporarily changing DIP switch 1. Both LEDs should turn off and turn on. If the Detect LED
does not illuminate, replace the detector.
If the red LED illuminates, then perhaps the sensitivity setting is too low. There are many variables in determining
overall sensitivity: loop size, number of turns, loop lead-in, percent coverage, etc. In most cases, a sensitivity
setting of 4 is the correct setting. However, to compensate for some unusual loop geometries, this setting may be
inadequate. Adjust the sensitivity one level higher and recheck the detector for proper detection.
