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Apologies if this is simple/rule breaking, it is my first electronics post and I am a newbie.

I wish to build an simple motion sensor / pir circuit, to control a relay. For controlling hard-wired heaters, this sensor to be installed "inline".

Timing to be user adjustable, so relay should stay on between 0.5 to 3.0 hours after trigger.

Question Is it possible / advisable to keep the whole circuit HV/110v? (goal: small circuit size, and build simplicity) Or should I step down to LV? I realise it is a very basic design question.

Thankyou.

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    \$\begingroup\$ The motion sensor and control for the relay would most likely need DC power, low voltages like 5 or 12 V. So I would use an AC-DC converter to power those and the timing circuit, and use a 555 timer or a microcontroller like arduino. \$\endgroup\$ – tokamak Mar 8 '16 at 4:40
  • \$\begingroup\$ Thanks @tokamak. For purely academic reasons, (learning) I am trying to build the circuit without any microcontrollers. I have found 110v pir's like this, but I lack the understanding as to if I should make this project 110v or 12v. i.e are there notable pros and cons to each path? \$\endgroup\$ – Mtl Dev Mar 8 '16 at 6:01
  • \$\begingroup\$ A 3-hour timer using a single capacitor will be impractical and even if it were practical can you imagine the tedium of setting it up? A better solution is to make a 'fast' clock - 1 Hz would be good enough - and use a binary counter to count pulses and turn off the light when the time has expired. You can do your calibration by testing on a low count before switching back to high count for normal operation. You need to consider what's supposed to happen at power-on. e.g., Is it OK if the lamp turns on for 3 h after a power failure, etc. \$\endgroup\$ – Transistor Mar 8 '16 at 7:06
  • \$\begingroup\$ @tokamak: I would like to accept you answer, if you could please post it as such. Yes you are correct, it will be much easier if I use LV. \$\endgroup\$ – Mtl Dev Mar 10 '16 at 0:33
  • \$\begingroup\$ Sure thing, tried to write a helpful answer. For sure the benefits of DC (safety, ease and many options of design) outweigh the cons (power converter inefficiency). \$\endgroup\$ – tokamak Mar 10 '16 at 17:34
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Although you may be able to find PIR sensors that run on 110VAC, that's because they contain an AC-DC converter inside the shiny white box. Very few plug loads run directly off of the AC; most convert it to DC. It's much easier to design a DC circuit to accomplish what you're looking for, there are many parts and kits and design tutorials out there already. The adjustable timing feature really lends itself to a microcontroller application.

That being said, old plug-in clocks used the 60 Hz from the AC sine-wave as their clock signal to move the hands on the clock. You could try a similar technique, but be careful messing around with AC, I would still step down that 110VAC to 5 to 10V with a transformer or optoisolator... you want your circuit to be galvanically isolated from the mains power for safety. It would require a pretty complex DC register/counting circuit scheme to accomplish this, counting the 60 leading-edges per second for 3 hours. (https://www.maximintegrated.com/en/app-notes/index.mvp/id/1994)

Because of the long durations desired on the order of hours, a 555 timer implementation by itself wouldn't be accurate as transistor noted, since that is a 1 capacitor method. It could serve as an adjustable clock signal feeding into a counter circuit, though.

There are many ways to skin a cat here that can't all be discussed, and looking into the various methods is a fun way to learn a lot.

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