Counter Idea

[MistaT]

True, but with a lot of wall warts, I have seen voltage deviations as high as 50% from their printed voltage rating. That's why I don't trust their specified voltage, and always do what David King recommends.

[tonedeciple]

You can put a dummy load across your supply voltage and see what the actual voltage is under load with a meter and then add in a resister in series to get you closer to the rated input voltage for the device.

This sounds like a good idea given how sketchy wall warts are. One other question: The specs for the counter I'm going to use state the input as 3-30VDC. If the wall wart puts out 12VDC (or less under load), do I need a resistor? I know I will for the LED emitter.

[Berman]

Hi
You will get the problem of too high voltage with every not stabilized power suppy.
There is one effect (especially) with small cheap transformers without a minimum load will the output voltage of the transformer will raise up to 1.5 of the nominal voltage. Then there is the effect of smoothing capacitor, which raises the voltage without load by the factor of 1.4
So a cheap 12V wall wart may have a output voltage up to 25 volts.
With te right load it will have 12 Volts.
So you can do tree things:
Buy a cheap powersupply with the right current (30 to 50 mA)
Buy a regulated (or switching type) wall wart, it will work at (nearly) any low currents.
Tage the powersuppy you can get and make a paralleled load.
For example: a 12 volt 5 watts 'gooseneck' lamp as used in cars of by deejays needs about 420 mA , so a 12V 500mA wall wart will work.
If you dont want a lamp, you can use a resistor with the right watts to 'burn' the current (not so ecological)

I hope it helps

Hermann

@ tonedeciple
You need the resistor on the input of the counter to give the electronics a defined voltage for logical 'HIGH'

[tonedeciple]

This is the winder I'm thinking of building...

...just kidding of course.

[tonedeciple]

Okay, so I hooked everything up using a 12VDC, 100mA wall wart and Elepro's schematic. At first, it worked great. Counts up to speeds faster than I want to wind. Then, when I plugged it in a second time, nothing. I know the counter is working, but I think the emitter burned out. I'm attaching an image of how I wired it up. Any Ideas?

[-Elepro-]

it is very strange.... with 510ohm at 12V it works at 20mA..... try to measure wall wart output with voltmeter

you can test emitter with a digital camera.... if it works the display of a digital camera will show it as a white light led....

bye

[tonedeciple]

it is very strange.... with 510ohm at 12V it works at 20mA..... try to measure wall wart output with voltmeter

you can test emitter with a digital camera.... if it works the display of a digital camera will show it as a white light led....

bye

I have power going into the resistor, but not coming out the the other side unless I disconnect the wire to pin 1 of the emitter. Then the power flows through. I'm guessing the led is burned out. I'll measure the voltage of the wall wart as I think it's probably way too high. Would a wall wart with regulated power be the solution? This one claims to be regulated. SparkFun Electronics - Wall Adapter Power Supply - 12VDC 600mA

[-Elepro-]

first of all test emitter viewing it with a digital camera....

[tonedeciple]

first of all test emitter viewing it with a digital camera....

Here is what I did. Photo 1 shows the emitter from the front. Nothing. Photo 2 shows a simple led tester touching the input side of the 560 Ohm resistor. The red led lights. Photo 3 shows the tester touching the output side of the 560 Ohm resistor. The red led does not light up. However, if I pull the red wire off pin 1 of the emitter, power flows through the resistor and lights the red led. This tells me the emitter bulb is burned out and can't draw power through when grounded.

Next I tested the voltage of my wall wart. With my meter set to 200 ACV, it reads 34.2. If it's outputting 34.2 volts, that's got to be the problem, right?

[-Elepro-]

Next I tested the voltage of my wall wart. With my meter set to 200 ACV...

why ACV?? you have to use DCV scale...

[tonedeciple]

why ACV?? you have to use DCV scale...

It's too early in the morning!! 16.5 volts. Still high, though.

[MistaT]

Hmmm.. something fishy here.. First of all, if you are reading 35 vac, then I wonder if all they did in the wall wart to create DC was to use just a diode. Which is sorta DC only because its rectified AC, and is missing the "negative" half of the wave. (yet another reason not to use wall warts..)

Off the top of my head, a way to test this out is to get an electrolytic capacitor of pretty much any value, say 100 uF or greater (with say a 150 vdc voltage rating), and hook it across the wall wart. Be careful and make sure you hook neg. of the cap. to the neg. of the wal wart.

Electrolytic caps. want to blow up if they are in the circuit backwards. Then measure the voltage again on the AC scale. If it drops a lot, like down to 0.5 volt or something, then I would say what you have is a DC wall wart w/o AC filtering.

Its my guess that the current rating of the sensor was exceeded and has blown. ESP. if you have un-rectified AC of 35 volts.

I would either make absolutely sure that you have good DC (and no AC) voltage coming from that wall wart, and that it is in the 12 VDC range.

One final thing. If you do in fact do the electrolytic capacitor check, after you power off the circuit, make ABSOLUTELY SURE that it is completely discharged. Do this by jumpering across the cap. Be ready, as it might (probably) give a BIG spark!

[tonedeciple]

Hmmm.. something fishy here.. First of all, if you are reading 35 vac, then I wonder if all they did in the wall wart to create DC was to use just a diode.

I made a mistake when I tested the voltage earlier. It's actually 16.6VDC, which I think may have been way to high for the emitter even with a 560 Ohm resistor.

I'm thinking of dropping down to a 5VDC regulated wall wart. I know how to calculate the resistor for the emitter, but what is the formula for calculating the sensor side resistor? Elepro's schematic for 12VDC shows a 4K7 resistor, but what would it be for a 5VDC power supply? Of course, I'll test the voltage from the 5VDC wall wart first before I calculate the values.

[MistaT]

Well the magic formula in all cases is good old Ohms law:

E = I * R.

So in calculating the desired resistance you would change the above formula to this:

R = E / I

Where:

R = Resistance
E = Voltage
I - Current

Now the question is, what current to pick? Something less than the max. current of the sensor (IIRC 50 ma?), and something greater than what is required to trigger the counter. (IIRC 600 uA? which equals 0.06 mA)

So for me, I would go for something in the 20 ma range.

So if you were to use a 5 volt supply, then

R = 5 / 0.020 = 250 ohms. So a standard value of resistor would then be 270 ohm resistor.

Hope this makes sense.

[tonedeciple]

Hope this makes sense.

You'll have to forgive me in this area. I mostly carve wood and inlay, but I've decided to take more control of my product by winding my own pickups.

This all makes sense, except for one thing; how did Elepro come up with the 4k7 R2? I need to know this since I want to drop the power source to 5VDC.