How could a thermistor be used in a circuit so that if something got in between it and the heat source it could make something stop working? Ie: in automatic doors, when something is between them, doors do not close. A diagram would be worderful, thank you x

A thermistor is a device whose resistance is directly proportional to its temperature. There are two kinds, those with a positive coefficient and those with a negative (all this does is determine whether resistance increases with temperature, positive or decreases with temperature, negative.)

This probably wouldn't be a very effective way of preventing a door from closing, however, since an obstruction between a heat source and a thermistor wouldn't cause the temperature of the thermistor to drop very quickly, unless the heat source was extremely concentrated and intense. Also, the calibration required would be extensive.

That issue aside, it is possible to make something like this work depending on the operation of the door...

For instance, if the door is held open by electromagnets, it might be possible to include a normally-open dry contact on that circuit which maintains a closed circuit so long as the level of current passing through the relay that controls the dry-contact (on the thermistor's circuit) exceeds the 'tripping' point of the relay. If the current through the thermistor (and hence the relay) fell below that certain point (the relay's 'tripping' point) then the dry contact would open and the electromagnet would release the door; the current would fall when the resistance of the thermistor increases due to the heat source being blocked (assuming a negative coefficient thermistor).

This is just one of many possible scenarios.