Dual-Stage Radar Sensor

I started with the excellent article on adding a motion sensor to the BMW alarm. I encourage everyone to read that article first, as I’m not going into enough detail to do the whole job. Consider this as a set of “release notes”. Also, I assume both the BMW alarm and the motion sensor have been installed at this point. Read over all of this first, since I make comments like “connect with to power” then later talk about “put a switch in”. It’s better to do this all at once, but easier to explain using several passes at it. Remember: measure twice, cut once, measure again.

Changes desired from the existing article:

I wanted to use a dual-stage sensor, thus the car would “chirp” when someone got too close to it, before setting the alarm off

I wanted to know which stage sensor had been set-off, by visual inspection (i.e. LED)

I wanted a cutoff switch for both the motion sensor, and the outer stage “chirp”

I wanted to have the sensor be easy(er) to adjust [i.e. not have to pull the car apart to adjust the sensitivity]

I purchased a dual-stage sensor from www.autotoys.com. This sensor is basically the same as the one mentioned in the previous article except that it has two stages of detection and comes with a small piezo buzzer to use as the outside “chirp” sound.

The first thing I did was to decide how I was going to wire everything and and measure everything. Based on other articles and talking with various people, I decided to put the sensor in the center dash, between the seats (under the cassette storage area). After removing the cassette storage bin, I was able to use my pocket knife to lengthen the hole already there to be just long enough to slip the sensor in. I put velcro tape on the sensor and the underside of the dash, and attached the sensor there. This had the advantage of making the screw to adjust the sensitivity easily accessible. I just remove the cassette bin (no tools needed), then use a small screwdriver to adjust the sensitivity. Thus, desire #4 was satisfied.

Next, I wanted an LED to show the status of the sensor. The sensor has an LED already attached to it, a bicolor red/green LED. I simply desodder it, sodder in extension wires, and added the LED to the end of those wires. I was able to snake that new cable (I used heat-shrink tubing to hold the wires together) through the gear-shift area (the shift boot cover comes off easy) and into the blank punch-out button area (which the light for the alarm itself and the glass-breakage sensor already were). I drilled another hole, and used one of the radio-shack LED black plastic covers to give it a professional look. It’s a little crowded there, but still looks good. OK, desire #2 is done.

I needed a place to put the buzzer that was going to serve as my outside warning. I decided it needed to go in the engine compartment, since that was the place the siren was, etc. It’s small so it can go anywhere. I choose up by the drivers wheel, inside the engine compartment. I tie-strapped it into place.

Now I had to find a way to get a couple wires into the interior of the car. If you look where the battery used to go in the ’96 model Z3, in the M roadster there is what looks like a bunch of cables doing just that, but they are fully covered and have a rubber grommet around them. I was able to move the rubber grommet just enough to snake the cables through to the interior. They come out just above the kick-panel speaker in the passenger wheel-well.

OK, this part is from memory (so PLEASE test all your connections before listening to me). I believe I hooked the red wire on the buzzer to the unswitched power supplied to the alarm. The black wire (ground) from the buzzer was attached to the green trigger wire on the motion sensor. I believe when motion is detected, this wire to connected to ground, thus the buzzer goes off.

Now I wanted to install cutoff switches, so I needed a good hidden location. I found one beneath the glove compartment. There is a piece of plastic that goes out from the firewall at a 90 degree angle, that’s what I used. It’s the piece of plastic that has the 2 or 3 plastic turn-screws to keep it on. In the center I was able to cut the holes for the switches themselves. I picked up a couple neat-looking blue auto-switches from a local electronics place and test-fitted them there. Make sure when the switches are in the plastic that there will be enough space to put the piece back on. Some of the tolerances were pretty tight. Now, cut the wire that supplies power to the motion sensor and connect one side to one terminal on one of the switches and the other side of the wire to one of the other terminals. Do the same thing for the cutoff for the “chirping” buzzer. OK, item #3 on my list is done.

OK, one last problem. When the car is running, so was the motion sensor. This has two side effects: the green light keeps going off indicating movement and the outside buzzer is also going off! While driving! This was unacceptable to me.

The solution was to use a relay, like is often used for fog lights and such. Make sure you get a real relay that has both an “87” and “87a” connector (the first one I bought said it had both, but really only had two “87” connections). Basically, you hook power up to the relay and based on whether power is applied to a third connector (or not), passes or does not pass power. The difference between “87” and “87a” is that they are the inverse of each other (one has power when the other doesn’t, and vice-versa).

I don’t remember the details (I’m a computer guy, not an EE), but it should be easy to understand based on the diagrams with the relay. Basically, hook ground an unswitch power directly to the relay. Then the terminal that determines whether power should run or not is hooked to the car’s “switched” power (the violet wire with a white stripe in the alarm harness). Then the terminal that has power with the terminal just mentioned does not have power should be feeding power to the motion sensor. Now, the motion sensor (and the outside buzzer) are only on when the car is switched off!

That’s it!

If you have any questions, you can email me at caslis@netcom.com