I have an Arduino Uno with 48 photocell sensors (yes, 48). The current draw is approximately 2.5 amps, which i know is too high for the Arduino. So, i need to power them from my external supply (that is also supplying my Arduino). The question i have is, where do i ground the arduino? I imagine that i maintain all sensors on the external supply, and just stick in my sensor reading cables to the arudino (that has a Mux shield). The problem is that the arduino needs ground to read the sensors.
Where is the link to the mux shield? If you power the mux shield you may also power the Arduino. In any case if the Arduino gets it's power from the same power supply you should already have a common ground.
I am referring to power of the sensors only, not the arduino or the MUX shield. These runoff the onbaord 5V. The sensors need an seperate power supply and i'm not sure how i ground the arduino to take the sensor readings. Any help??
With 48 sensors and a total 2.5 amp load, that averages to over 50 mA per sensor. If they're powering them at 5V, and it sounds like they are, that corresponds to a resistance of less than 100 ohms, and a power dissipation of over a quarter of a watt per device. It sounds like you might have these things connected straight across the 5V supply.
Please tell more:
How do you have these things hooked up? Please be very specific, or post a schematic.
What are you trying to accomplish? Please be specific about what your project will do - don't just say something like, "I'm trying to detect light in 48 places." [Edit: Fix spelling and formatting]
The sensors need an seperate power supply and i'm not sure how i ground the arduino to take the sensor readings.
In many cases you can just connect the GND signal of both sides. But I don't suggest this because you haven't told us more details about your setup nor have you posted a schematic.
Yes your correct, "I'm trying to detect light in 48 places"..
I have 48 photocells connected as attached (only showing two). They are all in parallel sharing a common ground. The pins all go out to the MUX Shield, not to the Arduino analog pins (the MUX shield uses three pins and allows me to connect 16 sensors to each analog pin). The mux shield is the only thing connected to the Arduino and a jumper cable to Vref so i can measure the voltage.
I have a power supply that will connect to LED light strips. This same power supply i can use to power the 48 sensors, but the ground (as in the schematic) should go to the arduino or the power supply ground.
In the schematic, it states 5V, i did think about regulating it, but can i run it straight from my 9 v external supply?
Here's how I interpret the schematic and accompanying text:
Each photoresistor is connected to a 10K resistor, and each series combination of the photoresistor and 10K resistor is connected between 5V and ground.
Have I got that right?
If so, please do the math. A photoresistor's resistance can be no less than zero. The resistance of the series combination of a photoresistor and a 10K resistor can be no less than 10K. The parallel combination of 48 10K resistors is 10,000/48 =~ 208.3 ohms. Ohm's law says 208.3 Ohms will draw 5/208.3 = 0.024 amps from a 5V supply - 24 mA. You seem to be measuring over 100 times that much, at 2.5A.
Something's wrong. Either you meant to say something other than 2.5A in the original post, or something's not hooked up the way you think it is.
I took the power dissipation of the photo resistors from the spec sheet (see link in my first post). This i converted to amps and multiplied by 48. This gave me the power draw of 2.5 amps. Perhaps i have done this wrong.
TMD3: To me that draw seems far too low. 24 milliamps for 48 sensors? Can someone confirm this? If so i can just wire everything to my Arduino and problem solved.
burt46:
I took the power dissipation of the photo resistors from the spec sheet (see link in my first post). This i converted to amps and multiplied by 48. This gave me the power draw of 2.5 amps. Perhaps i have done this wrong.
This is like saying that if your water meter is capable of measuring one hundred litres a minute, you are expected to consume water at that rate.
The power dissipation is the absolute maximum permissible power that you might permit the LDRs to dissipate. Without looking at the datasheet I can tell you that it is classified under "Absolute Maximum". It is entirely irrelevant to your application.
burt46:
TMD3: To me that draw seems far too low. 24 milliamps for 48 sensors? Can someone confirm this? If so i can just wire everything to my Arduino and problem solved.
Hi, check out tmd3 maths in his reply.
The 250mW is the MAXIMUM rated dissipation, you have put the LDR in potential divider configuration, which is good, just your understanding of current and voltage appears a bit mislead.
The current consumption of the LDR will depend on the circuit, the series resistor and the applied voltage.
OHMS law then needs to be applied as tmd3 has shown.
burt46:
I took the power dissipation of the photo resistors from the spec sheet ... converted to amps and multiplied ...
As has been noted already, the power dissipation you're talking about is from the listing, "Absolute Ratings Maximum," in the datasheet. That's not how to use the absolute maximum ratings. They don't describe normal operating conditions for the device. Instead, they describe the limits at which the device won't be damaged. I think you intuitively know this: you saw that the absolute maximum DC voltage is 250V, but you didn't consider operating the sensors at that voltage.
The onus is on the user to design his circuit so that the absolute maximum ratings aren't exceeded; indeed, the onus is to see that the normally more restrictive "recommended operating conditions" are maintained, but this datasheet doesn't provide that listing.
If you're still concerned about the power requirement of the sensor array, hook up one sensor and resistor and measure the current through them with an ammeter. You should probably do that test even if you're already convinced. I wouldn't recommend hooking up a bunch of stuff, against your intuition, on just the say-so of some anonymous guys in an Internet forum. Please post your results.
I recommend that you bone up on the theory involved in this circuit. Look up Ohm's law in the Wikipedia. Google "series resistors" and look for something that explains things understandably. If you have specific questions, come back to this forum under "General Electronics."
Finally, asking again:
tmd3:
What are you trying to accomplish? Please be specific about what your project will do - don't just say something like, "I'm trying to detect light in 48 places."
