Update 9/23/2007:
My speed pulse generator report has been updated with several changes. You can view it
here and then select report # 16 - NAV Speed Pulse Generator.
The first change, by popular demand, is an option to buy one of these if you can't make your own - see the Comments section in the new report for details.
The other changes are related to a new and improved circuit design. The circuit changes should make it easier to build the device and less likely for anyone to damage it during installation. On the previous design if you shorted a wire or hooked it up wrong then the 555 Timer IC would be fried thus rendering the device useless. It is a real pain to de-solder and replace the IC so I added an IC socket that will let you unplug the IC and replace it with a new one in just a few seconds. Most people shouldn't need to replace the IC but if you do something stupid then it is at least a cheap easy fix thanks to the socket.
I also found during additional testing that the three 100K ohm potentiometers were not truly needed since they slowed down the pulse quite a bit when set to the maximum setting, plus if someone turned them all down to the minimum setting at the same time then they would burn up the IC. So the revised circuit only uses one 100K ohm potentiometer in series with a 33K ohm resistor. This provides a more practical pulse rate range that is still adjustable and it prevents the IC from being damaged when the potentiometer is turned to the minimum setting.
I also replaced the 1K ohm potentiometer with a 1K ohm resistor since this doesn't need to be adjusted and the resistor is cheaper / easier to install.
Finally, I used a smaller circuit board and project box so the device would take up a little less space.
I have been using my device for a while now and it is great to drive around with all NAV functions available all the time. No more stupid lockout feature and I can upgrade to the new firmware whenever I want to without losing the bypass.
Several people have asked if this will work with other NAV units and vehicles. Based on my research I think that it will work with many other NAV units and vehicles but obviously I only tested it on my 07 Tundra DC Limited so no guarantees. The mod is cheap enough at $15 for parts so why not build your own and try it out?
Enjoy.
