Auto Shutdown Circuit - Page 1
Forum section: electronics / beginners

author: Bskitter
author_rank: Regular Contributor
timestamp: January 02, 2018, 11:23:43 am
content: 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.Here is the circuit I have come up withThe 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.I have slight doubt to the switch detection circuit some feedback on this and the circuit in general would be great !Byron

author: Rerouter
author_rank: Super Contributor
timestamp: January 02, 2018, 11:52:50 am
content: The diode on the power switch? this will half rectify the mains to the power relay, i am assuming it is meant to be in anti-parrellel with your optocoupler diode behind the resistor.You could do what most other smd stations do, have a reed switch in the holder, when the hot air nozzle is placed in the holder, turn off the heat, and keep the air running until it is cool enough for your design,What you are describing is not an auto-off, but rather a delay off, to accomplish this you only need to control 1 relay, when the device is turned on (detected by the optocoupler) you switch 1 relay, you have normally open connected to live, normally closed connected to the switch output, and common connected to the devices switch outputYou have a second relay then connected so while the switch is on the heating coil is connected, you switch off the switch, the heating coil is disconnected.By doing so, the device no longer turns off when the switch is flicked, because the relay holds it on, the other relay breaks the heating coil connection, and its left running air while no heat is applied.To better accomplish this, you will want to tweak how the optocoupler is wired, instead it should be between the switch output and ground, this way when its switch on it pulls low, and with the relay switched, If the switch is released, it stops pulling low. you then either wait X seconds for it to cool enough, or just measure it.using appropriatly rated resistors (you may want a few in series for a voltage rating),

author: Bskitter
author_rank: Regular Contributor
timestamp: January 02, 2018, 05:35:09 pm
content: Quote from: Rerouter on January 02, 2018, 11:52:50 amThanks for pointing this out.Here is a modified version with a capacitor should reduce the amount of heat given off by the resistor.Maybe add a second capacitor in parallel to the LED to keep the diode constantly active instead of it flickering at 50Hz.