Windscreen wipers
Electronics projects Original wipers Home Julian Rogers Home

The windscreen wipers on my TR7 always seem a bit weedy (it’s the same on my SD1 too). All the motor current passes through the stalk switch on the steering column and the park switch in the motor itself also gets a hefty dollop of current when it shorts out the motor to apply a magnetic brake. Perhaps there is some resistance in the switch contacts (or in the original wiring and plug connections?) leading to voltage drop and heating of the contacts which in turn leads to a downward spiral towards failure (or even fire!)

Also he multi-strand wires used in the loom are not tinned and can be subject to tarnish even under the insulation or under the crimping on the plug connectors etc. (I always solder my connections after crimping them to ensure the best electrical connection.)

The upshot of all this is that I would like use the existing switches and wiring just to control mosfet transistors that would actually handle the high current to the motor with very few chances of electrical losses on the way. Also the original TR7 arrangement doesn’t have an intermittent or delay setting and that would be very nice to have. (Every time it rains, I curse my wife’s Hyundai i10 which, being poverty spec. has no intermittent setting.)

I went a bit overboard on the specification, perhaps. The control is by an Arduino which takes input from the stalk switch, a six position programming switch, the park switch on the motor and a motor current sensor. The current sensor would protect the motor if the blades jammed of otherwise got stuck. I thought it could also be used to automatically adjust the intermittent timing according to the amount of rain falling - a drier wind screen means a higher motor current etc.

The motor is a two speed device. It has two positive supply terminals, fast and slow, which connect to different arrangements of the internal wiring of the motor. I could have switched the supply to either terminal with a couple of mosfets but instead, I decided to supply the fast terminal only with pwm and so have the possibility of variable speed (I’m not sure with hindsight that that is really necessary but extra complication seems to be my middle name!)

Before I could start, I had to figure out how the stalk switch worked and the original connections to the motor. This is on the next page.

Wind screen wiper motor control