LDR failure over time

Terry Pinnell · 2012-03-19T10:19:08+00:00

One of the circuits I made a decade or three ago was a simple'Dark-activated Porch Light'. A potential divider with an LDR in its uppersection triggers a 4001 Schmitt which, after inversion by one of theremaining 4001 gates, switches an NPN with a relay in its collector, thuspowering a mains lamp outside my front door. It then switches it off whenit's light in the morning.It works OK, but I'm getting tired of having to change the LDR every 9-12months, due to its gradual loss of sensitivity. What seems to happen isthat its dark resistance drops with time, eventually getting too low towork properly.I've just bought another couple of packs of LDRs:SILONEX - MPY54C569 LPRS - N5AC-50108I was wondering if there was any merit in putting a couple in series orparallel this time? Although I don't really see how that would help, asthey would presumably age together. Maybe mixing the two types would help?Alternatively I could consider re-making the circuit. A major task, as apre-requisite would be clearing up my shed/workshop! But worthwhile ifthere was a simple way of making a significant improvement.Any suggestions gratefully received please.-- Terry, East Grinstead, UK