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Switch-closure rain gauges, C terminals on CR300 and the 100 k Ohm resistor

rainyday Jul 5, 2021 02:04 PM


Can anyone provide some guidance re the above please?  I want to connect an ARG100 rain gauge to a C terminal on a CR300 logger.  The P_SW terminal is already in use with another rain guage.

*Do I connect the resistor between the rain gauge and the C terminal, or between the gauge and the Ground terminal?

*Will any 100 k Ohm resistor suffice, or do I need a high precision one (e.g. low temperature sensitivity)?

*In the manual it says [Section] "Switch-closure measurements on C terminals require a 100 k Ohm pull-up resistor to 12 V."  Is that last bit short-hand for 'for a maximum of 12 V?' - anything I need to do about this?

Any advice gratefully received.

Many thanks, Andrew

Sam Jul 5, 2021 11:51 PM

The purpose of the resistor is to bias the C port high. The expectation is that when the tipping bucket tips, the internal switch will close and pull the C port low. The high-to-low edge transition will be detected and counted. It does not need to be a precision resistor, it only needs to be a current limiting resistor that will bias the C port to a >3.3V state.

rainyday Jul 6, 2021 12:27 AM

Hi Sam, that's very helpful and gives me the reassurance I need.  Many thanks for your assistance.

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