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best way to make an intermittent AC circuit...

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New Member
im trying to make an AC circuit which switches on and off intermittently after certain periods of time. basically what it needs to do is this...

1. it receives a power input of 230V AC

2. it outputs 230V AC for 120 seconds.

3. after 120 seconds the output stops for 60 seconds

4. after that 60 seconds output goes live again for 120 seconds

5. repeat

when the input goes dead the output needs to go dead. when it goes live again it needs to start the process afresh.

i know bits and pieces about DC circuits, but AC is something ive never messed with much, and i have no idea where to start looking on this one. if anyone has any suggestions then they are greatly appreciated. even better, if anyone knows of a product that already does this out of the box then it will save me the DIY effort!!!



New Member
a motor that runs a 3A fuse in the plug, but is actually probably not more than 100W
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New Member
it might be helpful if i list some more specific details about what it is im trying to do.

basically this is for a reptile enclosure to operate a water misting system. the misting pump is for intermittent use only, with 1 minute off for every 2 mins on.

i have a humidity controller which has a 230V plug on it. you plug the misting system into the plug, and the controller into the wall socket. it monitors humidity and when it drops below a threshold the plug goes live, powering the misting system, which then pumps water in until the upper threshold is reached, at which point the controller switches the misting system off.

the issue here is that the controller outputs constantly until humidity is reached. if it never reaches it or takes ages to get there, the motor could potentially burn out since it is only intended for intermittent use.

what i need is a circuit which ensures the motor is never allowed to run for more than 2 mins without giving it 60 seconds rest in between. a timed relay could work with the 2 minutes on part, but after the 2 mins are up it will switch off and then theres no way to switch it back on again unless the input goes dead and then live again. which it never will if humidity hasnt been reached within the 2 mins!!

im wondering if using 2 timed relays and a relay could work though....

controller feeds signal to timed relay 1. output goes live for 120 secs. when the output goes dead this then triggers a 2nd timed relay which goes live for 60 secs. when the 60 secs is up the output goes dead. there is then a relay on the main input signal to timed relay 1 which can only go live when timed relay 2 is dead. this means that so long as there is still a signal from the controller, the 2nd timed relay would re-trigger the 1st one. in my head this would work perfectly, in reality maybe not. any thoughts? id use the controller to supply the constant 230V feed, this way when the controller switches off (e.g. humidty is reached) so does the whole timing circuit, meaning it doesnt carry on for the full 2 mins even if humidity is reached after 20 seconds
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New Member
ok, ive found a solution, just need someone to advise on how i go about the relay circuit...

il use a mk111 interval timer circuit. this is 12V DC, but i can use a 12V regulated PSU plugged into the humidity controller to power it. when it receives power it will pulse on for 5 secs, off for 5 secs (or however i set it). this will then switch on and off a 230V relay to power the misting system. as soon as the controller shuts off, so will the interval timer, and thus the misting system!

can i simply use a power relay that allows 12V DC switching and thats it? or do i need anything else on the circuit for protection? against power spikes and what not
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New Member

I think your heading in the right direction with the 555timer. From this schematic it looks like that kit uses a vr15m121c relay. The specs listed here for that relay indicate that the contacts are rated for 7A at 220VAC. This is more than adequate to switch your load.

You can change R1 to a 470Kohm resistor and VR2 to a 470kohm potentiometer to reach you 120sec on and 60sec off time.



For the relay, try a 240 VAC, 10A, DC voltage controlled Solid State Relay ....
Ought to work. ... Usually available in electronics surplus stores ... web sites.
Should not be too expensive.


Well-Known Member
Most Helpful Member
Here is a 555 astable circuit tuned up specifically for your application. There is a network which makes the first On period the same as the subsequent ones.

Start with a surplus plug-in 12Vdc plug-in power supply powered by your humidity controller. It supplies 12V to the 555 & relay. The relay can be any 12Vdc coil relay that is rated to switch ~2A at 250Vac. The relay contacts are wired in series with your motor.


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