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1  Using Arduino / General Electronics / Re: help with an LED dim / switch circuit, please on: September 26, 2014, 01:32:11 pm
actually, the state it's not telling me if the FET is on. Only the Arduino can control the FET, so it knows if it does. It tells me if the switch - relay combination results in on or off.
2  Using Arduino / General Electronics / Re: PCB Manufacture on: September 26, 2014, 10:15:49 am
I'm using DesignSparkPCB to draw all my schematics and have also designed PCBs with it and had them manufactured. It's quite easy to use in my opinion. But best of all, it's free, so can't hurt to give it a try. It can export Gerber files, which is what I had sent to the PCB manufacturer.

http://www.rs-online.com/designspark/electronics/eng/page/designspark-pcb-home-page
3  Using Arduino / General Electronics / Re: help with an LED dim / switch circuit, please on: September 26, 2014, 04:16:23 am
Do you mean when I'm testing it? Yes, I test with actual LED strips connected.
Or do you mean the state signal back to the Arduino? That is, in fact, only telling me whether the MOSFET is on. If it was just an on/off circuit I would have done it the way you described or just take the GND for the optocoupler LED at the drain, not the source. But I wanted to get the signal before dimming to avoid getting false "off" readings caused by PWM dimming.
4  Using Arduino / General Electronics / Re: help with an LED dim / switch circuit, please on: September 25, 2014, 03:27:05 pm
You mean Q5? This inverts the signal so the Arduino pin is high when the light is on. I could have done this in the sketch but I'm going to connect some other relays with state output (similar to these but for mains and without dimming) and they report high when switched on. So the code would have been a bit more messy if I only need to invert some.
5  Using Arduino / General Electronics / Re: help with an LED dim / switch circuit, please on: September 25, 2014, 01:37:24 pm
I've done it like this now. Looks faaar to complicated for what it does but works as expected this way.
6  Using Arduino / General Electronics / Re: help with an LED dim / switch circuit, please on: September 22, 2014, 06:15:38 am
I still have a few 4N25 opto couplers lying around, so I could just use one of these instead. Though it does bug me that I can't explain that unexpected behaviour.
7  Using Arduino / General Electronics / Re: help with an LED dim / switch circuit, please on: September 21, 2014, 06:50:11 pm
Could it have something to do with how the MOSFET is used? When the 2 switches result in open, I'm basically pulling the source of the MOSFET to 12V through the 47k resistor (red).
8  Using Arduino / General Electronics / Re: help with an LED dim / switch circuit, please on: September 21, 2014, 04:33:47 pm
This is actually how I've done it first. This works BUT when the LEDs are off, I get about 7V on the "state" line that goes to A3 on the Arduino. I was not able to explain that either.
Then I added that transistor for that reason (and for the added bonus of inverting the State signal).
9  Using Arduino / General Electronics / help with an LED dim / switch circuit, please on: September 21, 2014, 02:22:18 pm
I'm having a bit of a brain blockage and could use a fresh pair of eyes, please. ;-)

I've built a circuit that isn't quite behaving as expected. I have attached 2 versions of the schematic, one normal and one with the separate parts of the circuit highlighted with different colours to make explaining easier.

The circuit is part of my home automation project and it's meant to switch a 12V LED strip on and off and also dim it. The catch is that the switching must also work in case the Arduino fails. I'm using a bi-stable relay and a wall light switch (like a 2 switch light circuit). The dimming is done after the switching with a MOSFET. The gate of the MOSFET is pulled high using the LED supply voltage so that the MOSFET is closed even if there is no signal from the Arduino.

In the highlighted schematics the
yellow parts are the relay control,
green is the dimming parts and
blue is the LED power parts. This all works fine.

The problem is the red parts. This is the feedback back to the Arduino that tells me whether the LEDs are on or off. Since there is a manual switch involved, I don't know whether that is on or off. So I need to detect the state.

The problem is that when I build it like in the circuit the LEDs never go off (but they do dim ever so slightly). I just can't get my head around why that is happening. When I cut the wire where the purple X is, then the circuit works fine. But of course then I can't read the state any more.

Any ideas what I'm doing wrong?
10  Using Arduino / Installation & Troubleshooting / Re: [SOLVED] Deek/Dswy Robot Mini Pro - can't upload a sketch & bootloader on: April 03, 2014, 01:29:28 pm
Hi,
thanks so much for posting the solution!! A life saver!
I bought 5 of these Arduinos, already installed some of them in my project and then wasn't able to upload any sketches. The Nano as ISP method worked perfectly! smiley-wink All 5 Minis saved!

Chris
11  Using Arduino / General Electronics / Re: does a "normally closed" transistor or similar exist? on: October 23, 2013, 12:00:38 pm
The Arduino built-in PWM is around 800Hz. So that would require external PWM components, which would cancel out the advantages of the solid state relay.
Thanks for the suggestion though... smiley-wink
12  Using Arduino / General Electronics / Re: does a "normally closed" transistor or similar exist? on: October 23, 2013, 11:52:29 am
I've not really looked at these solid state relays in detail but at first glance they don't seem to be fast enough for PWM dimming.
13  Using Arduino / General Electronics / Re: does a "normally closed" transistor or similar exist? on: October 23, 2013, 11:50:34 am
So would this work?
14  Using Arduino / General Electronics / Re: does a "normally closed" transistor or similar exist? on: October 23, 2013, 11:36:28 am
Thanks for all the replies.
I think for now it's good to know that depletion mode MOSFETs exists, but if the same can be done with a "normal" MOSFET and a few components, then I'll go for that as I already have some normal MOSFETs that I can use.

I'll update my circuit diagram to add the small transistor on the gate of the MOSFET and change some resistor values (according to Pelleplutt's post) and will post it again.
15  Using Arduino / General Electronics / Re: does a "normally closed" transistor or similar exist? on: October 22, 2013, 06:27:04 pm
Thanks for that. I was unaware of these.
But hmmm... They need a negative voltage. I would have to generate that just for the MOSFET.

I've come up with the attached circuit in the meantime. Looks like that might work.

This is going to be for a home automation project. I want to be able to control a lamp (LED) from both a wall switch and an Arduino. The wall switch must always work as if it was a normal switch (for the wife smiley-wink )
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