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1  Using Arduino / General Electronics / Re: Moisture sensors reading very low values on: August 27, 2014, 07:51:02 am
Or, as many implementations of this kind has used... move to using an AC signal.

This post may be useful...,22250.0.html
2  Using Arduino / General Electronics / Re: Auto-Power on/off circuit on: August 27, 2014, 07:00:46 am

You simply buy one of these:

It allows you to use a pin to "shutdown" on command.

This is exactly how I created an auto-shut-off option on my LC meter.

If you are still feeling creative and want to build your own... check this out:
3  Using Arduino / General Electronics / Re: I need help selecting a transistor on: August 19, 2014, 03:30:58 pm
And.... sadly, the schematic is sort of wrong.  You do not put the current limiting resistors on the common display pins, you place the resistors on the individual  display 'segment" pins.  If you don't do this, when you have only 2 segments lit to display a "1", each led gets gets 1/2 the current from R and if you display an "8", all segments... each only gets an 8th of the current.  So, with 330 ohms, and displaying a "1" each led gets ~5mA..  With an '8'  on the LED each gets ~1mA.

And then you will be coming here and saying... why are my LED's so dim?
4  Using Arduino / General Electronics / Re: Will this work correctly? on: August 16, 2014, 07:06:15 am
5  Using Arduino / General Electronics / Re: Will this work correctly? on: August 16, 2014, 05:23:00 am
First point:  Resistors have a purpose.  Please try to begin to understand why they exist and why they get used in designs.  And when doing a drawing... if you used them, don't omit them.

If your optocoupler still works (the LEDS are not fried) they may be hope in getting this to work.

Problem #1:

The photosensor (based on my understanding of them) is basically an an open collector NPN output.  It can only pull to GND or be open-circuit.   So, you correctly have the collector from the sensor going to cathode pf the internal LED.  What is missing is a 220 Ohm resistor between the anode side 5V supply.

Problem #2:

The sensor is open collector (meaning very versatile) and this means that I fail to see why you needed an optocoupler there at all.  If you have placed a 10K resistor from the sensor NPN output to +5v you would have a TTL Logic (and therefore arduino pin) compatible level change... You would see logic state changes correctly

Problem #3:

If you want to use an opto-coupler, they usually have open collector npn outputs.  (just like your sensor)  The same solution applies... Tie the collector (output) pin to 5V through a 10K pullup resistor... then attach the collector to a digital pin for state sensing.  When the internal LED is not lit, the mega will see +5V or logic "1" , when the internal LED is lit, the mega will see a logic "0".

So, please explain why you have an opto coupler here because I feel some important information has been skipped.
6  Using Arduino / General Electronics / Re: Edging your DIY PCBs on: August 11, 2014, 07:27:22 pm
I do 1  or 2 passes on a stone wheel grinder.
7  Using Arduino / General Electronics / Re: Protecting diy pcb boards.... on: August 11, 2014, 05:43:46 am
I use the rub-on Silver coating "Cool-Amp" in combination with a clear coat lacquer sprayed "before" soldering.  The Lacquer melts  away while soldering and you end up with a protected board that won't look ugly due to tarnish.  Personal experience is that it takes a long time for the Cool-Amp to actually show tarnish even when not painted over with clear coat.

The product for silver plating copper may seem expensive, but I have had mine for nearly 10 years and have only used about 10% of what is on the jar.
8  Using Arduino / General Electronics / Re: Will these two circuits work for me? on: August 10, 2014, 07:10:59 pm
I explained to him hours ago in IRC that the AD0 pins of the Gy-88's needed to be opposite to be unique on the bus...   one high, one low.  I can now see our time was wasted.
9  Using Arduino / General Electronics / Re: Voltage Source Accelerometer Resistor Value on: August 08, 2014, 07:28:33 am
This is why you want an oscilloscope...
10  Using Arduino / General Electronics / Re: Multiple I2C devices on: August 07, 2014, 08:00:14 pm
Should not be.

I2C is a well established, well published bus architecture.  It remains in NXP's control though, it's not a public or committee based standard.  Manufacturer of an IC should be f no concern.
11  Using Arduino / General Electronics / Re: Relay Question on: August 07, 2014, 07:33:50 pm
Also, most of the boards on the market is directly dangerous, and is clearly designed by people that know nothing about electronics.

I tend to agree with you.

And recommending that an opto-Isolator be used in 120V switching situation in a solution that includes a mechanical relay is damn close to creating a Rube Goldberg device.

You do not need to do ANY optical isolation if you have a mechanical relay in a solution.

 This image attached is *all* you need in a circuit that wants to eventually control a 120V AC lamp.  You just need to make sure that your RELAY CONTACTS are rated for more than your nominal AC load.   IE; use a relay with switch contacts rated for  3-5 amps at 250V and you are done. Your AC voltage is completely isolated from the low voltage side.

If you want to start talking about optical Isolation and 120V AC circuits, the logical solution is  to use a Solid State Relay.  A commonly available, and sometimes quite affordable pre-assembled solution for switching AC that appears to the low voltage (arduino side) as a simple LED.

...and if you are confused by AC circuits in general, best to avoid them altogether until you gain that knowledge.

12  Using Arduino / General Electronics / Re: Multiple I2C devices on: August 05, 2014, 06:49:35 am
I2C. by definition is a "bus" architecture...  to you attach devices by "tapping" into the bus pairs (sda, sdc) with the matching pins from your devices.  I2C is based on Active Low (the TALKER pulls the bus lines LOW to talk) so you need to "pull" the pins of the bus "high" by attaching a 4.7K resistor between both SDA and SDC to V+ power source.  (aka 5V)  Being a BUS, each device mush be addressed uniquely... make sure you have enough addresses available on your IC's.
13  Using Arduino / General Electronics / Re: Etched copper PCB question on: July 11, 2014, 08:35:04 pm
FYI: Harbor freight now carries a package of small carbide drills now.

14  Using Arduino / General Electronics / Re: basic relay circuit on: July 01, 2014, 08:23:31 pm
I personally would not use an opto isolator in thar circuit, so I can't really recommended where it would go or how it would be wired.  Redundant parts are illogical to me.
15  Using Arduino / General Electronics / Re: basic relay circuit on: July 01, 2014, 06:53:42 am

Diode: neeeded, yes... or optional if you don't mind buying replacement parts in the future.  Diodes tend to be cheaper than transistors so adding one to protect your transistor is cheap insurance against the voltage spike you see when the coil is de-energized, for example.

An opto-isolator is not needed since the relay provides more thn adequate isolation from ac lines.
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