On 13 November 2011 12:10, IVP wrote: >> 3) bigger bypass? Ferrite beads? Would they help for the EMI? Sure >> filtering the power wouldn't help > > Feed-through caps and chokes on power and signal lines are common > in car audio systems to filter out ignition noise. For your application i= t > would be a LPF or BPF based on the duration of the closure > > Is it possible to add a low-Z driver down at the reed switch ? Like > some paralleled low-power CMOS gates ? That would also allow > you to extend the pulse (eg one-shot), which would help discriminate > between signal and noise > > It can still be two-wire (or one if the frame is used as Vss) by super- > imposing the closure signal onto Vcc. Or smart switching between > the line as Vcc to the reed (held up with a reservoir cap) and a > conditioning circuit for it and then as signal back > > Joe > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > What sort of cable is used to the reed switch? I realise that you can't change it but you may get an improvement by terminating it appropriately. If it's a single wire)with frame return) , then there's not much you but a twisted pair will provide a level of immunity if operated into a differential detector. Basically the load resistance and capacitance for both legs should be the same. A simple opamp configured correctly should be enough. Correct earthing as you suggect and as much filtering as you can put in will also improve things significantly. RP --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .