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15544Electronics advice needed

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  • mmilway
    Oct 12, 2013
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      I am working on some automation that uses magnets, reed switches 555 timers and relays, all running on a 12V regulated supply. I believe I am getting noise problems because the 555 timer randomly triggers without a magnet going near the reed switch. The reed switch is on the end of a metre or so of cable.

      The reed switch will pull the input trigger to ground when it is activated. What does the group consider to be a reasonable value for the pullup resistor. Should I have other filtering such as a small capacitor across the reed switch connector? If so, what value?

      I have previously found that while the data sheets claim that a 555 timer can drive a small relay (say 70mA), I have found the switching transients cause the timer switching functions to fail. I now use a small NPN tranistor to isolate the 555 from the relay. This seems to work well.The board has a 220uF Electroletic and a 10uF Tantualum capacitor across the power supply. Each chip also has a 0.1uF ceramic capacitor across its power pins.

      Michael Milway
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