Sunday, September 22, 2013

What to do on a Cold and Wet Fall / Winter day???

Why, Design something – of course!

I have to say, the Nerd in me will just not die.  And doing this ‘retired’ thing gives me time to go back to one thing I really enjoy – designing small electronic things…   Like the Kubota DC generator controller and remote, and its off-shoot the stand-alone regulator (still in progress).   Well,  I have also been thinking of something that would really be nice on Viking Star – intermittent wipers.    Often times when we are in heavy water (or light rain) the forward windows need a quick swipe.   And of course, just as I put up with it long enough to stand up, hit the switch and clear the window - a new splash will come along.  Just like most cars these days, would it not be nice to have delaying wipers?   Well, there are two problems:

  1. Doing the delay
  2. Getting the wiper motors to return to their rest position after doing a sweep.

The motors we have are simple ones, and do not have an ‘auto-park’ feature.   Turn the wiper switch off half way through a sweep and that is where the wiper arm stays, half way done.  It is not too hard to time manually turning off the switch at the correct time, and at one time I had picked up some fancy Auto-park capable wiper motors which would continue their travel to the completion of a swipe - even if the switch was turned off.  But their shaft was too short at 3” to fit through the window header (I need 4-5” it looks like..).  Guess I could have new shafts made, but I have been playing with a design in my head that would allow me to  use the existing un-modified motors.   A design in my mind that would not only do delayed sweep, but also figures out somehow to park the wipers using the existing motors.  And here is an idea:

Click for Larger View

Be careful, semi-Deep Stuff follows here…..

On the right one of the 555 times will time out the delay period between cycles, while the other 555 delivers +12v to the wiper motor long enough to complete one cycle.  This part is simple, and if I had auto-parking wipers it would work great.  However, the challenge is one will NEVER be able to get the 2nd timer set just right to complete an entire sweep and return to the parked position – and if one did manage to dial it in, it would change once the engine was started (and the battery voltage was higher due to recharging), or resistance on the window changed from say rain, or temperature, or…..    So – the middle of the schematic are two op amps.  The left hand one senses the amount of current being drawn by the motor (using the Ron resistance of the FET),  and the 2nd op-amp is configured to detect a change from a falling slope of current to a raising one, ala:  the change I am EXPECTING to see when the wiper motors gets to one side of the window, slows down and changes direction.   Via the two Diodes in the middle the wiper motor is initially powered from the 555 timer which is adjusted to ‘time out’ somewhere mid-sweep of the wiper blade, and then the Op-amp takes over continuing to power the wiper motor until it sees a dip in current.  Q3 acts as a kind of one-time switch to make sure once the motors are stopped, we do not restart them based on current draw.

And there you have it – self parking of standard non-autopark wiper motors!

Possible PCB layout
OK, I have NOT built this.  Only designed it, and ran some SPICE modeling.   It looks like it should work, and maybe this winter I will build up a few…  Or, perhaps I will play with an Arduino based wiper controller!   Wow, that might be overkill, but to be honest, it also might work out to be about the same cost and more reliable...  Hum. . . . .

 Until then, I will now return you to your regular Blog…

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