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When out of town I'm thinking of using an LDR with a 555-relay driver to periodically toggle the lights in a few rooms at night, and a thermistor near an incandescent bulb to kill the circuit when the bulb is sufficiently warm/hot.

This will involve placing the LDR, and the thermistor at some distance from the board which will be near the switchboard; a length of 8-12 feet.

In the case of HF/VHF circuits a pair of leads this long are not advised. Are there similar restrictions applicable to a simple driver circuit too?

How long a lead may be used to connect a component to a circuit?

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As I understand it you want to use this as a presence simulator to keep burglars out, right?

The thermistor is not a good idea: the bulb will reach its steady temperature within a second or so, then it will only light very briefly. Better use a timer to switch the lights on for a certain (variable) time. You can use 555 timers to create switching events and on-times, but I would use a microcontroller for that; that will allow you to create any switching pattern, even replicate actual switching it recorded previously from real use (though you'll need extra hardware to detect switching events).

A wire length of 12 feet/3.5 m is not problematic. It may pick up some noise, but I don't think high accuracy is a requirement. The noise will be less if the impedance is lower. For instance you can use a 10 kΩ thermistor in series with a 10 kΩ resistor, or a 1 kΩ NTC in series with 1 kΩ, both will give you the same output. But the latter has an impedance of only 500 Ω and the former 5 kΩ.

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