The cap doesn't have to go directly across the relay contacts - you could still have a pull-up to Vdd and then at the node where you would normally attach the PIC input pin, instead put a single pole RC LPF between the pullup+switch node and the PIC pin. This way, the reed relay never sees a high current pulse but you still slow down the edge as far as the PIC sees it so that the PIC will not respond to microsecond pulses but will respond to millisecond pulses (like the relay closure). Sean On Sun, Nov 13, 2011 at 11:11 AM, Dave wrote: > Definitely. =A0Even a 0.1uf will weld the contacts or wear them out prema= turely. > > Geo wrote: > >>Electron wrote: >> >>> Even the >>> original digital odometer may have one, in fact the "peak current of 40= 0uA vs >>> average current of 9uA" I measured may well be due to a cap in parallel= to the >>> reed input which gets charged by a high value pull-up resistor (math su= ggests >>> 330kohm), the pulse closes the reed only for few degrees of wheel rotat= ion, >>> so in the open state the capacitor is charged (albeit relatively slowly= ) by >>> the resistor, the input at the microcontroller raises (slowly, but as i= t's a >>> schmitt trigger this doesn't cause problems) and the capacitor opposes = a low >>> impedance to eventual high frequency transients. When the reed closes, = the >>> capacitor discharge (with relatively high current) and the digital pin = goes >>> low very quickly. >> >>I suggest being careful with the discharge current. I have replaced 100s >>of reed relays which have been cold welded in the closed position by a >>small excess current flow (they could be cleared by vibration). I think >>you need to work out the maximum frequency of the reed opening and >>closing (i.e. using max speed and wheel diameter) then choose a >>capacitor with an additional resistor in series with the reed to limit >>the current flow on closure. This resistor will also form a low-pass >>filter in conjunction with the capacitor which should help with your >>noise problem. >> >>George Smith >> >>-- >>http://www.piclist.com PIC/SX FAQ & list archive >>View/change your membership options at >>http://mailman.mit.edu/mailman/listinfo/piclist > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .