A HUD (Heads Up Display)

for Radar Detectors with an Earphone Jack

Mike Chaplin

The circuit shown below uses 4 special Light-Emitting Diodes (LEDs) with VERY bright, focused beams. Mounted in a small housing at the top of your windshield & with the LEDs pointed towards your face in your normal riding position, the HUD can get your attention under ANY lighting conditions and let you know that your radar detector is going off. Earl Minkler & I have used the original prototypes since '92 with excellent results.

The LEDs are powered by the earphone signal from your radar detector. On the Maxon RD-25 units that we used (12-Volt model), I measured a 10 volt peak in the waveform that the earphone would receive with the volume control fully CW. Initially, I tried 5 LEDs to "match" the voltage level; but the detector didn't deliver enough current for the LEDs to be bright enough. Reducing the count to 4 was the trick for the Maxon unit. The number that will work best for your detector is dependent on the voltage level it uses for the earphone. I used an oscilloscope to make the initial measurements. The LEDs are SO bright that at night you MUST reduce the volume control on the detector to near minimum to keep the LEDs from blinding you in the low-light conditions.

There are several brands of LEDs today that have models with similiar specifications. However, these special-purpose LEDs are not readily available at the neighborhood Radio Shack, or most hobby shops. Here is the info on the LEDs we used;

  LED MANUFACTURER
---------------------------------------
Hewlett Packard, Model H-3000-L
RED Focused LED for Daylight Use
Light Output  : 2000mcd @ 20 mA & 1.8 V
Viewing Angle : 25 degrees
Electrical    : I(max) = 50 mA

  LED DISTRIBUTOR (Info @ 1-1-96)
---------------------------------------
GILWAY Technical  Lamp
800 West Cummings Park
Woburn, MA. 01801-6355
          617-935-4442
Part # E184 (Hp H-3000-L)
$0.99 ea. (1-100 LEDS), 
$30.00 minimum order, 
Doesn't Accept VISA, etc...

There are brighter LEDs available today with even more focused beams-- some with view angles less than 10 degrees. I would NOT recommend using LEDs with viewing angles of less that 15-20 degrees. Any minor head movement up, down, or to the side may take your eyes out of the viewing "sweet-spot". With the units we selected, head movement is not an issue.

Below is a very_bad_ascii symbol of a diode and how to identify the polarity of the leads.

         Schematic Symbol         Side View of Physical Device
         ----------------         ----------------------------

                |  + Anode
                |                    + Anode   -------+-|--\
                |                    - Cathode   -----+-|--/
             \-----/
              \   /               The (+)anode lead is slightly
               \ /                 longer than the (-)cathode.
                v
             -------
                |
                |
                |  - Cathode

SINGLE LED TEST CIRCUIT : Take a 9V battery, a 200-Ohm resistor(or 12V & 250 Ohm), and one LED (all 3 in series as shown below) and checkout the red BEAM at night. Its impressive. You will also understand why a volume control to turn 'em down at night is REQUIRED.


              _____/\  /\  /\  /\  _____
             |       \/  \/  \/  \/     |
             |          200 Ohms        |
             |         (250 Ohms)       |          9v-1.9v 
       + ---------                   \-----/   I = ------- = ~36 mA
            ---     9V                \   /        200 Ohm
         ---------(12V)                \ /
       -    ---                         v
             |                       -------
             |                          |
             |__________________________|

With this circuit, you can test the individual LEDs and check their polarity. If the diode is reversed (known as "reverse-biased"), there will be no current flow, no light, and also no damage to the LED from the test circuit. This circuit will also give you a "reference brightness" to check the HUD against; i.e. is there enough current flow in the LEDs.

The trick with these LEDs is getting the right amount of current through them. Their approximate or "average" voltage vs. current characteristics are;

Voltage   Current    Comments
-------   --------   -----------------------------------------------
0-1.5 V      0 mA    maybe some low uA leakage current
1.8-2 V   1-50 mA    20-50mA is needed for the HUD
   >2 V     >50mA    Excessive heat generation & possible LED failure

As you can see from the voltage/current data above; (1) connecting a single LED across a 1.5-V battery will produce no light, and (2) placing a single LED across 2 1.5-V batteries in series will FRY the LED after a bright flash of red light.

Again, on our Maxon (12V) units, 5-LEDs got a little less than 10 mA peak in the pulses from the earphone jack. :-( But 4-LEDs got ~35 mA; PERFECT ! We didn't need any sort of series resistor to get a good current in the desired range; however, I can't say that this would be true about ALL detectors. Its possible to be too low with N LEDs and TOO high with N-1. The N-1 solution would then need a series resistor to limit the current. Most properly designed audio amplifiers have current-limiting circuitry to prevent internal damage when driving a short circuit; so a series resistor may not be required after all. You need to measure the current. This peak current can NOT be measured correctly with any volt- meter. You will need an oscilloscope, waveform recorder, or A/D system to measure the current peaks.

You want 30-50 mA peak current with the volume control set fully CW. Even 50 mA peak currents are OK from the detector because its a pulsed waveform with ~50 % or less duty cycle; so its not a continuous (dc) current.

When positioning the module on your bike; make sure you have it as high in your field-of-view as possible, and angularly positioned to point at your eyes while you are in your normal riding position.

I have a buddy with a machine shop (Earl) make the housings for us. He used a block of black nylon material, "delron" I believe. The blocks were precisely drilled on a mill to keep the LEDs aligned in the same plane, and the rear side was "hogged out" to allow room for making the electrical connections. After this, it was backfilled with RTV to secure everything inside and provide a strain-relief for the earphone-jack cable. The LEDs are wired in series, so they flash in-sync (together) at the same time. This appears best for getting your attention.

We mounted the HUDs to the backside of the air-dams that we have at the top of the fairing windshields and precisely pointed the unit so that the LEDs were pointing towards our eyes while in our normal seating position on the bike.

Its great knowing what your radar detector is doing, still have earplugs in both ears, and an un-wired rider! The maxon unit flashes the LEDs very slowly with a weak radar signal. As the signal strength increases, the flash rate of the LEDs increasing and is QUITE noticeable; even with sunglasses on heading towards the sun.

July '96 Update . . . . . . The 4-LED HUD has been successfully used with the ESCORT PassPort 4600 detector. We and others have gained some experience adapting the HUD to other detectors. Generally, I would recommend the following configurations as a starting point;

If you don't actually have the test equipment to measure the peak pulse currents in the LEDs of your HUD when connected to your detector, then you should always build one of the test circuit(s) shown above to allow comparison of the brightness of your HUD LEDs to the one in the test circuit. This is very important because at low current, the LEDs appear to work just fine; but in bright sunlight, they won't be bright enough to get your attention.


1995-96, Mike Chaplin catfish at endorphin-express dot com

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