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Multiplexed inputs
Multiplexed inputs
There are more sensor-inputs than a singlePICAXE18X can handle. Rather than use an additional PICAXE, or one with more input pins, an analogue multiplexer HEF4051 chip has been used.
The HEF4051 has 8-channels selected by address lines A0 to A2. These lines are controlled by the PICAXE, and the output Z from the HEF4051 fed into PICAXE input.
Currently only 3 inputs are used to the HEF4051, so there is potential for additional sensors (e.g. rainfall sensor, additional temperature sensors). A 10K resistor is between Z and GND ensures that when power is switched off, the input to the PICAXE isn't left floating (which would use more power).
All unused inputs to the HEF4051 are tied to GND via a 10K resistor rather than a direct connection to GND. Although probably not strictly necessary this follows best practice: the resistor protects the input pin by limiting the current from high going variations in the supply which could damage the input to the device.
Currently the signals multiplexed are all digital signals. However future inputs may be analogue, and so the input to the PICAXE, Input 0, has deliberately been chosen to be one with an ADC to offer this flexibility.
The DS18B20's are both in standard arrangements with 4K7 pull-up resistors. The ground temperature DS18B20 is actually connected into this circuit via a 3-pin Molex connector.
The HEF4051 has 8-channels selected by address lines A0 to A2. These lines are controlled by the PICAXE, and the output Z from the HEF4051 fed into PICAXE input.
Currently only 3 inputs are used to the HEF4051, so there is potential for additional sensors (e.g. rainfall sensor, additional temperature sensors). A 10K resistor is between Z and GND ensures that when power is switched off, the input to the PICAXE isn't left floating (which would use more power).
All unused inputs to the HEF4051 are tied to GND via a 10K resistor rather than a direct connection to GND. Although probably not strictly necessary this follows best practice: the resistor protects the input pin by limiting the current from high going variations in the supply which could damage the input to the device.
Currently the signals multiplexed are all digital signals. However future inputs may be analogue, and so the input to the PICAXE, Input 0, has deliberately been chosen to be one with an ADC to offer this flexibility.
The DS18B20's are both in standard arrangements with 4K7 pull-up resistors. The ground temperature DS18B20 is actually connected into this circuit via a 3-pin Molex connector.
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