555 timer for switching off the soldering iron - Page 1
Forum section: electronics / beginners

author: amateur_25
author_rank: Regular Contributor
timestamp: April 30, 2014, 10:32:53 pm
content: Hithere are two images to illustrate how I intended to implement this. The switch in the schematic is the switch on the stand.Some help on how I would get the trigger output to be pulsed low. Right now the input is pulled low as long as the iron is inthe stand hence the output of the 555 timer never goes lowThis issue aside, the 555 timer was going to be driving a suitably beefy N channel mosfet which is connectedto the DIN connector on my soldering station. This is so that the iron remains on for a while but then switches off unless I pick the iron up. When the iron isn't in the holder themosfet is turned on via the NC contact on the switch otherwise my iron won't be working when it's in my hand becase the mosfet isn't switched on. What use would that be!

author: HalfSpace
author_rank: Regular Contributor
timestamp: April 30, 2014, 11:59:48 pm
content: Hi amateur_25,The attached 555 timer design will time out after about 50 seconds by increasing R1, C2 or both you can extend the time out. W1 is the switch in your stand and must have open contacts when the iron is in the stand.Question, is the iron running on low voltage AC or DC as a mosfet may not be suitable for switching the iron?HalfSpace

author: Phaedrus
author_rank: Frequent Contributor
timestamp: May 01, 2014, 12:14:06 am
content: Note that Mosfets only block in one direction, so to block AC you need two, connected collector to collector (or emitter to emitter).You might also want some isolation between your 555 and the mosfet.

author: amateur_25
author_rank: Regular Contributor
timestamp: May 01, 2014, 12:32:26 am
content: Quote from: HalfSpace on April 30, 2014, 11:59:48 pmThanks for the schematic. I was using such small values because am new to ISIS and don't know how to use it properly. If I use small values I don't have wait long for the timer to expire.Am I not correct in saying the trigger output needs to be pulsed low to activate the timer? The switch in the schematic would pull the pin low but it would stay low as long as my iron is in the stand. The only way I know the iron is in the stand is via the lever switch. Also it's definitely AC since I found 14V ac on one of the pins. Actually in that case Ii can just use a relay. I do like the clicking of a relay. I find it therapeutic.Just out of curiosity if I did use just one transistor, would I just emit magic smoke then when it changes polarity?

author: HalfSpace
author_rank: Regular Contributor
timestamp: May 01, 2014, 02:09:31 am
content: Hi amateur_25,The switch needs to close when the iron is removed from the stand.When the iron is removed from the stand C2 is discharged quickly via R3 and when the voltage on the TRIG pin is below 1/3 of Vcc the output pin go’s high turning on the iron. If the iron is placed in the stand the switch is then open allowing C2 to charge up via R1. The iron is still on at this stage but when the voltage on the THRS pin reaches above 2/3 of Vcc (After about 50s) the output go’s low turning off the iron.A relay should work ok.HalfSpace