# How much voltage and mA do i need?

I have a laserpointer, it runs on 3x 1.5V cell batteries(suncom lr44) so that would make 4.5 volt right. The batteries are to empty to use the laserpointer so i don't know if that gives a problem to figure out how much it needs.

When i test voltage atm there around 1.2V per batterie. When i test current there around 70mA per batterie. If i connect 2 batteries then it adds up to around 100mA, if i connect 3 batteries then i get around 130mA. 210mA would have been more logic to me (3x 70).

Also by the summary info from here: http://arduino.cc/en/Main/ArduinoBoardUno stands: DC Current per I/O Pin 40 mA DC Current for 3.3V Pin 50 mA

When i have a wire to GND and on to: 3.3v, then i meassure -0.21A which is 210mA? 5v, then i meassure -0.29A which is 290mA?

If i test pin 7:

``````void setup() {
pinMode(7, OUTPUT);
}

void loop(){
digitalWrite(7, HIGH);
}
``````

then i meassure 75mA.

So is my arduino ok (or my digi meter)?

And can i connect my laserpoiner to the arduino with the batteries in them? Cause this would be easier to connect since i can't get the element out of the huls.

You need a switch to turn the laser pointer on/off. The arduino cannot do it directly.

Look in the playground/forum for e.g. how to drive a big magnet / relay with a transistor (NPN) or a mosfet (N-channel).

"When i test current there around 70mA per batterie..."

How is it you are testing the current?

clankill3r: If i test pin 7:

``````void setup() {
pinMode(7, OUTPUT);
}
``````

void loop(){   digitalWrite(7, HIGH);  }

``````

then i meassure 75mA.
``````

Don't do that. 40ma is the limit for the amount of current you should pull from any pin. Pulling more risks burning out the pin or chip.

You need a switch to turn the laser pointer on/off. The arduino cannot do it directly.

Isn't it possible with a few resistors? And then use HIGH and LOW from a pin to set it on/off?

How is it you are testing the current?

Red probe on + side of battery and black probe on the - side. To test 2 batteries i put on on top of the other

Don't do that. 40mA is the limit for the amount of current you should pull from any pin. Pulling more risks burning out the pin or chip.

So running a wire from any pin to the GND is a danger since without a resistor it pulls always to much?

clankill3r: Isn't it possible with a few resistors? And then use HIGH and LOW from a pin to set it on/off?

No, you need to use a transistor.

clankill3r: Red probe on + side of battery and black probe on the - side. To test 2 batteries i put on on top of the other

That isn't how you measure current. In order to measure current, you must break the circuit placing the meter in the path the current will flow. Using just a multimeter, you cannot measure the current capacity of a battery.

Isn't it possible with a few resistors? And then use HIGH and LOW from a pin to set it on/off?

Not likely, however first you have to actually know how much battery current the Laser pointer draws. So far you haven't done that properly.

Red probe on + side of battery and black probe on the - side. To test 2 batteries i put on on top of the other

That just measures the 'short circuit' current capacity of the battery, not what the pointer draws when operating. You need to take a current measurement by being in series with one of the power wires and the module, turn on the pointer and see what the current consumption is.

So running a wire from any pin to the GND is a danger since without a resistor it pulls always to much?

Configuring a digital pin to output mode and setting it high and then wiring the pin to ground sets up a short circuit path that will then destroy the output pin and possibly the whole chip.

Lefty

I would strongly recommend reading this before you play with your laser:- http://www.repairfaq.org/sam/laserdps.htm

So running a wire from any pin to the GND is a danger since without a resistor it pulls always to much?

Configuring a digital pin to output mode and setting it high and then wiring the pin to ground sets up a short circuit path that will then destroy the output pin and possibly the whole chip.

So i'm lucky my board is still working, those getting started books sucks, they don't warn for those things.

For the battery testing, i first need to buy fresh batteries then, and wire the laserpointer.

I would strongly recommend reading this before you play with your laser:- http://www.repairfaq.org/sam/laserdps.htm

i will read it someday, it's a lot of text, i want to print it first.

Best read it before you destroy anything.

, it’s a lot of text

There’s a lot to know.

clankill3r:

How is it you are testing the current?

Red probe on + side of battery and black probe on the - side. To test 2 batteries i put on on top of the other

I'm actually surprised you haven't burned your meter out (or at least blown the fuse) - I'm not sure I would trust a meter to give correct readings after being "abused" in this manner...

Grumpy_Mike:

Best read it before you destroy anything.

i was planning on that, the laser didn't work anyway, it's really old and the spring in it has gone green. Don't know how to call it in english, maybe it works when i remove the green stuff put i can't reach it.

I'm actually surprised you haven't burned your meter out (or at least blown the fuse) - I'm not sure I would trust a meter to give correct readings after being "abused" in this manner...

You are shorting out a voltage source. Using Ohm's law, we get, say, 4.5V / 0 Ohm = infinite current. Luckily, batteries are not ideal voltage sources and your ammeter is not an ideal short-circuit, so you are left with a limited current that left both your batteries and your multimeter intact. The current is usually not a property of the battery (unless something goes wrong and you short it out), but a function of the load attached.

those getting started books sucks, they don’t warn for those things

I just read the manual for my new car.
Nowhere in it does it say that I shouldn’t use a cigarette lighter flame to provide light to check the level of fuel in the filler pipe.

Some things are left to common sense.

Seriously - you're not supposed to read battery voltage by simply connecting the plus and the minus with the multimeter? Wow - i've been doing this for years... Gives me a good indication of how full the batteries are...

Your not supposed to do it with the meter set to amps!!!
If you do you now nothing about electricity and should stick to knitting.

Its ok to measure battery voltage in that manner, as battery voltage only varies slightly from the load drawn, and voltmeters have a very high impedance, because they can be placed in parallel with a circuit. Ammeters are meant to be placed in series with a circuit, to accurately measure current flow with minimal disruption to the circuit.

Seriously - you're not supposed to read battery voltage by simply connecting the plus and the minus with the multimeter? Wow - i've been doing this for years... Gives me a good indication of how full the batteries are...

yeah i learned that from my dad, i think Grumpy_mike will hate my dad...

Seriously - you're not supposed to read battery voltage by simply connecting the plus and the minus with the multimeter? Wow - i've been doing this for years... Gives me a good indication of how full the batteries are...

There is nothing wrong with measuring the battery voltage like this. However, if you take the trouble to actually read the rest of the thread the poster was measuring the CURRENT from a battery by simply connecting the plus and minus of a multimeter across the battery.

If your dad actually taught you that trick then he was presumably in the business of selling either batteries or multimeters.