Auto Shutdown Circuit

Hello Everybody,

I am currently building an SMD station. I have most of the electronics planned out expect for the auto turn off function...

Let me explain, when the user turns on the SMD station they will flick a switch and the system will power up, once the user has finished with the system they will flick the power switch to the off position.

Once this happens I would like the Arduino to register that the power switch has been put in the off position. This will then put the system into shutdown mode, it will wait for the temperature reading from the thermo-couple to go below a certain point and only then will it cut power to itself.

So I need a circuit that will turn on when the power switch has been put to the on position.

Some sort of detection should be present like an optocoupler.

And a separate power control should be present such as a relay which is controlled by the Arduino.

Could anyone please provide me with some example circuits that they might have ?


Perhaps a block diagram would help me understand what you have in mind. To me, SMD, is an acronym for "surface mount device". Are building surface mount devices? Seems strange, so probably not.

Are trying to build a temperature controlled soldering iron? They are pretty cheap and rather reliable, and readily available.


The way I'm reading this is he is looking to allow a cooldown of a hot air rework station before it is powered off by means of an arduino.

I think he's way overthinking the whole situation. My solution for this was to plug the rework station and solder station into a powerstrip, along with a simple desklight. The rework station wasn't a problem as much as the solder station, as the rework had it's cooldown while in the cradle and would go to sleep. The solder station only has a little led with no cooldown. The desklight was always left in the on position. If the light is on, I know the soldering equipment was also on, so it helped me from forgetting.


You got my point.

But I want it done automatically.

Here is the circuit I am thinking to implement.

The AC Relay (Relay 1) will turn the system when when the switch is flicked.

The DC Relay (Relay 2) will activate once the arduino has booted.

When The Switch is turned off, power is cut to Relay 1. The Arduino is sense that there is no longer a signal from the Opto Couple.

The system will not turn off since Relay 2 is still active.

Once the Temperature has fallen below a preset value the Arduino cuts the power to Relay 2, ultimately turning the entire system off.

No time to design a cct right now as per requirements.

I made this for a similar situation, just on/off as per a timer, now I have piece of mind ;).

PowerUnit.jpg Click on the image for full resolution.

Here is the circuit I am thinking to implement.

There are at least four serious mistakes in that circuit.

  1. The LED in the optocoupler will be instantly fried, unless you add a reversed diode in parallel to it or use an optocoupler designed for AC input.
  2. The 2N3906 needs a base resistor.
  3. The DC relay needs a kickback diode.
  4. It appears that when the DC relay is closed, it will connect Live and Neutral together.

Are you really up to this task?

you forgot 5: The line voltage is connected to pin 3 Of the optocoupler. In that circuit diagram pin 3 Is not internally connected to the LED. And even if it was connected to something, Line voltage across that switch will not give you any voltage potential, In that configuration.

Here is an updated schematic, excuse the mistake. It was a quick draw just to convey my idea.

@promacjoe2 Could you suggest another circuit which provide detection if the switch is flicked or not ?

D2 should be placed in parallel with the optocoupler LED, so that R1 limits the current through it.

As it is, D2 might destroy the AC relay coil, as the relay will be powered by pulsating DC when the switch is off.

Consider too that since the optocoupler is AC driven, it will give a pulsating output if no other hardware measures are taken. Not too difficult to handle in software, though.

@jremington Thanks for pointing that out. Another silly mistake.

I have modified the circuit.

@dougp Nice point. Im planning to use an Interrupt on this PIN. So maybe ill add a capacitor to keep the led on instead of it flashing at 50Hz.

Neutral is not supposed to have any positive potential. Your optocoupler diode has its anode, "+", going to neutral through an AC coupled capacitor. You also have a standard diode in parallel, In the same orientation, as the LED. The LED needs about 1.5 V. A standard diode will conduct at .6 V or less. Electricity flows in the path of least resistance. Where is your power going. That is considering if you have a positive potential on the neutral.

Do yourself a favor, You have a power supply for the Arduino. A relay can turn this power supply on and off. A pushbutton can bypass the relay contacts and also turn the Arduino on. Once the power is on the Arduino will turn on the relay, and hold it on until such time as the Arduino is programmed to turn it off. Once it is off, everything is off.

Simplify your circuit, and don't overthink it.