Tuesday, June 7, 2011

Variable intermittent wiper control - Part 1

One of my bugbears with cars I've driven is that their wiper system either don't t have an intermittent mode or if they do then they're fixed, i.e., the wiper sweeps the windshield approximately once every couple of seconds and there's no way of adjusting that time interval. Having an intermittent setting is better than none but having a variable intermittent mode is certainly much better. With a very light drizzle the windshield may need only need be wiped every 10 to 20 seconds, hence having a wiper control that has a user-adjustable wipe interval from one second to say one minute eliminates the hassle of having to turn off the wiper and then on again every now and then.

Some 8 years ago I designed and installed my first variable intermittent wiper control. It was a simple circuit based on the 555 timer configured as an astable multivibrator. It employed a potentiometer to allow adjustment of the wipe interval. The control circuit switched a relay which in turn switched the wiper motor on and off. It even had green and red LEDs indicators showing when the relay was on/off. By 2007 I had a preliminary microcontroller design but never got around to working on it.

Here's the Honda Civic's wiper stalk switch schematic. As indicated the relay contacts were inserted in series after cutting the parking position wire.

And here's the 555 circuit to implement the variable intermittent control. I used an aluminum electrolytic capacitor for the timing cap. But as is known aluminums are leaky. Using a tantalum would be better.

The circuit provides a momentary pulse to the wiper motor (less than a second) after which the motor latches--it powers itself through the parking position circuit--and then stops when it reaches the parking position where the wiper blades are back at the bottom of the windshield..

R3 is a 1Mohm potentiometer. Because of D1 (1N4148 signal diode) charging rate is much faster than discharge rate, charging only through R1. Theoretically, output high and low times are as follows:

Thigh = 0.693(R1)(C1) = 0.693(7.5Kohm)(100µF) = 0.52sec

Tlowmin = 0.693(R2 + R3min)(C1) = 0.693(15K + 0)(100µF) = 1.04sec

Tlowmax = 0.693(R2 + R3max)(C1) = 0.693(15K +1Mohm)(100µF) = 70.34sec

D3 is a flyback diode to dissipate the reverse emf developed across the relay coil when power to it is cut off.

An intermittent wiper control is relatively easy to design. The problem and headache is the cars themselves, specifically, figuring out which wires of the wiper switch go where and control what, and how sophisticated or primitive the installed wiper switch module is. A non-electronic wiper control as is found in the 90's Honda Civic is relatively straightforward. As can be seen above it only has mechanical switches and has very few settings--low speed, high speed, water spray, and momentary wipe (a spring loaded switch lets the driver turn on the wiper which turns off once the stalk is released).

Designing a control for a wiper system that already has an intermittent mode and employs electronics and which has a rear wiper-mist is a tad more difficult--because it takes more effort to analyze and reverse engineer the lot of switches and circuitry--something absolutely necessary because we don't want to fry the car's electronics and cause some major short circuit!

I've been meaning to install a variable intermittent control on the Toyota Innova for some time now but for fear of ruining the wiper system, or worse, zapping some other part of the electrical system--say, the very expensive car computer!--I've procrastinated even inspecting the wiper system to determine whether the project would be feasible. To add to the concerns, this car has an airbag on the steering wheel. Inadvertently triggering it was certainly on my mind. But last week, with trepidation, I finally dove right in.

To begin with I had to get to the wiper stalk's base--which was hidden within the steering column--to get to the wires leading in and out of it so as to be able to test them. As it turns out, removing the plastic covers surrounding the steering column was easy--the top and bottom halves simply snap onto each other and are held down on the steering with just two screws (one of which actually screws onto the detachable wiper switch module--strange I thought).

With the base of the stalk exposed two wire connectors became apparent. Pushing down on the retaining clips on the connectors permitted their removal from the wiper switch module. After some time studying the module I was able to remove it from the steering column--It had plastic "rails" onto which it slid in and one retaining clip was all that held it in place. Love that simple plug-in design. Here's a pic after I'd taken the wiper switch module off. The two white rectangular wire connectors dangling beneath the steering wheel are conspicuous. Those red and black wires and alligator clips are from my DMM.

Close up of the connectors:

And here are shots of the the module:

I was able to partially disassemble the module and here's how the sub units look:

Turning the parts over we have:

See all the copper "tracks" embedded in the plastic? They're thin copper inserts some of which lead to the headers and some to PCB on the other side.  The yellow-greenish goo is lubricant for the sliding contacts.

And the headers/pins (into which the connectors plug) on the module:

Close up of the circuit board. There's one 3-terminal SOT device near the center of the board. No markings to identify it. Other devices are surface mount resistors. Values are readily discernible.

The PCB cannot be removed because there are some seven copper "posts" (embedded in the black plastic module) which are soldered onto the PCB. Here's how those copper posts look:

The PCB is at the top portion of the pics. They're overexposed in the images so you won't see the telltale green solder mask, just the edge of the phenolic? board.

In those two images notice the large black rectangular block. That's a relay although I could not see any markings on it. There's also at least one aluminum electrolytic capacitor in there. I could see the temperature rating but not its capacitance. The only way I'll be able to view all the components would be to desolder the posts. I'm not about to do that given my dodgy de/soldering skills. Maybe if I ever get a surplus unit I can do a teardown all the way.

In Part 2 I'll have the results of my electrical tests of the module.

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