I want to add an intermittent function to the wipers on my 40-year-old car without any additional switches. The details of the wipers can be found here [http://www.globalsoftware-inc.com/coolerman/fj40/5G.htm]. The wiper knob closes the path to ground allowing the motor to begin sweeping the wipers. (There is another ground at the motor that mechanically closes when the wipers are not parked. This allows the wipers to park regardless of when the wipers are switched off.) It seems that all I need to do is complete the path to ground whenever I want to sweep the wipers, but it seems too easy so I’m second guessing myself. I’ve drawn out a small circuit using a ATtiny85 to detect if the wiper circuit is closed (not shown on the diagram). If the circuit is closed (knob pulled out) for longer than one second, the wipers operate as long as the knob is out. If the knob is pulled out and returned in less than one second, the uC activates the relay long enough to start a sweep and repeats this at a predetermined interval.
Am I missing something? Is this a reasonable approach? I chose an automotive relay I’ve used before, datasheet here [http://www.farnell.com/datasheets/1809465.pdf] with a diode and RC snubber across the contacts.
[Edited from original to reflect accurate wiring.]
To be completed by OP.
| A | B | C | +-------+-------+-------+ Everything off |12.9 V |12.9 V |12.9 V | Bulb in, WIPER_LO closed | 0 V | 0 V | 4.9 V | Relay energised, WIPER_LO closed | 0 V |12.9 V | 4.9 V |
[Second schematic illustrates switch and two ground paths.]