Skip to the last paragraph if all you want is the instructions on how to make your own screamer.
After fitting an Escort Radar detector to the GS1200SS, I was disappointed with all the hassle of plugging and unplugging an earpiece, and it was only on the second ride that the very flimsy wiring decided to break. I rode a couple of times with just the detectors built in sounder for a warning, but at anything over 110 kmh it was next to useless, and at completely legal speeds who needs a radar detector anyway? The solution was to fit a screamer.
US$99.95 (from http://www.cyclegadgets.com/Products...sp?Item=RADSCR ) seemed a bit expensive for what is little more than a piezo and a triggering circuit, so I decided to build my own. The actual Piezo alarm is available from Dick Smith for a little over $8.00 so I was very confident that I could design a trigger that would work off the earplug outlet from the escort. However, while I was sketching out a possible circuit diagram I happened to glance at some junkmail from The Warehouse, and there it was: A smoke detector alarm for $7.49. So here in one package was a piezo, a triggering circuit, and a case.
I took a quick ride to the nearest Red Shed and soon found their smoke detectors; $7.49 each (battery included) or a pack of 2 for $15.49. Hello Warehouse, Do your maths a bit better if you want to sell the twin pack to anyone.
Back home, and I quickly had the smoke detector case disassembled so I could track the circuitry. This was the first disappointment. The trigger circuit was all encased inside the ionisation chamber, and I couldn't tell from looking at the circuit board whether it was triggered by a low voltage input, or by a change in resistance.
But the GOOD part was that under the circuit board was a pair of test pins, and these pins were lined up with a hole in the case that was covered by a paper label. I set my multimeter on the diode setting and touched the pins with the probes. Instant noise, and much louder than the radar detector's sound level. I then reversed the polarity of the probes to the test pins. for no response. I also checked to see if there was any voltage generated across the test pins at any stage, and as far as I could tell there was virtually none. This meant that there was very little chance of any signal being sent back from the smoke alarm to the radar detector. So from this little experiment I determined that a very weak signal can be used to set off the piezo in the smoke alarm, but that the signal must be DC and the correct polarity.
Next I had to determine how the output from the Escort could be used as a signal. As the remote output is designed for an earplug which has a very high impedance and almost no current drain, I suspected that the output voltage would be very small, and probably AC. However to be safe I made the first reading with the meter set at the 0 - 20 V DC scale. I was very suprised to get a reading of +6.3 V DC. I switched the meter to 0 - 20 VC AC, and got an almost identical reading. This suggests that the output is close to a square wave with the peaks at full battery voltage when there is no load. I dont know what internal protection there is in the Escort, but I would suggest that users be very careful to avoid a short circuit in the audio output.
The internal audio in the Escort is not loud. but I would suspect that as long as the output is limited to less that 1/16 W (5 mA) no damage would be caused to the internals. I therefore decided that a 3.3 K resistor in the line from detector to the smoke alarm would be most suitable. But first I wanted to test much higher values of resistor to see just what signal would be the weakest that would work. I started with a 33 K, but that wouldn't trigger the smoke alarm. Then I tried a 10 K, still with no result. Next I wanted to try a 4.7 K, but despite having 100's of resistors in my parts box, that was one value I didn't have. So straight on to the 3.3 K that was my original estimate, and YES. It triggered perfectly.
I therefore made up a lead to go from the radar detector to the smoke alarm test pins with the 3.3 K resistor wired into the positive lead. I painted the smoke alarm black to match the bike (ignoring the Do Not Paint label on the smoke alarm), and attached it to the bike with Velcro strips. Riding to work this morning I pressed the battery check button at various speeds and could very clearly hear the 85 db alarm at every stage.
Parts list:
General purpose smoke alarm (The Warehouse) $7.49
3.5 mm mono plug. $0.45
3.3 K 1/16 W resistor $0.20
Dual pin socket (for test pin fitting) $0.20
Shielded audio cable. 0.2 m $0.50
Consumables: Solder, velcro, paint. $1.00
Make up a lead using the shielded audio cable, using a 3.3 K resistor in line with the center pin of the 3.5 mm mono plug. Connect a dual pin socket to the other end of the lead.
Underneath the smoke alarm, peel back the paper label to reveal the test pins. These are not labeled as to polarity, so you will need to test the system and determine which way around the socket will go. Connect the lead from the Radar Detector and power up. If there is no audio signal, reverse the socket and try again.
edit: For spelling mistakes
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Originally Posted by skidmark
Superb piece of work
Did you also discover the immense fine that went with it? 
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