This module uses an LDR (Light Dependent Resistor) to detect darkness.  In the dark, the resistance of the LDR goes up.  The LDR is in a Voltage Divider arrangement so that the output voltage will change with changing light levels.  The user varies the variable resistor to get the right output voltage to trip the rest of the circuit.  The circuit should work well indoors.  Out of doors or in bright ambient light, you may want to reduce the variable resistor to 100k.  This will make it easier for the user to adjust the switching point.  More sensors appear on page 2 Lots more sensors

This module has the LDR and variable resistor the opposite way round from the dark sensor.  Because the resistance of the LDR is quite low in the light you can use a 10k variable.  If the ambient light level is very bright you may need to reduce the value of the variable resistor in order to get a switching output from the module.  You normally want the output voltage from the voltage divider to rise so that it can be used to switch the next block which could be a transistor switch or comparator; see the Comparator Blocks and the Driver Blocks pages.

A thermistor responds to temperature; when the temperature rises, the resistance of the thermistor falls.  With the thermistor at the top of a voltage divider, that means that the output voltage will rise.  This rising voltage can be used to switch a following block.  The next block can be a single transistor switch, a Darlington switch, a Field Effect Transistor (FET or MOSFET) switch, or an OpAmp comparator - see the Comparator Blocks and the Driver Blocks pages.

The cold sensor block has the opposite arrangement from the Hot sensor, with the thermistor at the bottom. As the temperature falls so the resistance of the thermistor rises.  This means the output voltage for this voltage divider also goes up.  This rise in voltage can be detected in the following block.

This block uses a moisture sensor which consists of a small piece of PCB with parallel tracks on it.  These tracks are linked alternately as suggested by the Crocodile Technology graphic.  A dry sensor may be difficult to use since it must normally be wet.  If you are using it as a soil moisture tester you may wish to change the value of the 100k resistor or even make it a variable resistor.

A wet sensor uses the same PCB touch pad that the dry sensor uses but with the voltage divider the other way up.  The extra 1k resistor is just to protect the next block from a short circuit across the sensor pad.  You may feel you can dispense with it.  Again, you may need to change the 100k resistor depending on what level of wetness you are testing.  This circuit works ok with water but won't detect a damp finger.  See the Touch Block below for that.

Using a touch sensor is very easy.  Just put it into a voltage divider circuit with some high value resistors.  Touching the pad is equivalent to putting a large resistor in parallel so that the output voltage is pulled up to wards the battery voltage.  More touch devices appear on the Touch Blocks page.