Multiple Relays(up to 16) in a project. Feasible?

Hi there, I'm in the middle of a project that requires me to control up to 16 solenoid valves at any given time, and I'm going to be doing it through a set of two 8-channel relays (something like http://www.amazon.com/gp/product/B0057OC5WK/ref=s9_simh_gw_p21_d0_g21_i4?pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-2&pf_rd_r=0DTV7RA1K72SKKS5QHMK&pf_rd_t=101&pf_rd_p=470938631&pf_rd_i=507846.

These relays are Active Low, apparently, so they actuate when LOW is applied to them. My question is, since each one apparently draws 15-20 mA when actuated, how does this affect the arduino? Is it constantly pushing out 15-20 mA on each output pin I have set to HIGH? Or does it only draw the 15-20 when I set it to low? Also, it is apparently possible to power the relay board externally. If I do this, am I safe in assuming I could actuate every single relay at once and have no overcurrent on the arduino? I found this page http://arduino-info.wikispaces.com/ArduinoPower that goes into relay boards, and they make it seem as though almost no power at all is drawn from the arduino.

This quote in particular has me thinking

"+5 volts flow through 10,000 ohm resistor R1 to the Base of transistor Q1, and a current of about .0005 amps (500 microamps) flows and turns the transistor ON.".

Does this mean that just setting the pin to HIGH or LOW only draws 0.005 amps? I'm confused as it said that when the relay is actuated it draws 15-20mA.

Any clarification would be great.

PS:Hope I got the right subforum

It only draws the 35 when I set to low when powered from 5V per the info below.

Hard to say more with the published info.

Note that ATMega has limit on total current thru the power/ground pins, and also thru any particular part. If you turn on 7 relays at 35mA you could damage something.

Power Supply: 5V DC / 400mA (relay all ON) Input control signal voltage: 0V - 0.5V Low stage (relay is ON) 2.5V -5V High state (relay is OFF). Input control signal LOW state current: 2.5V: 0.1mA. 3.3V: 0.18mA. 5V: 0.35mA. JD-VCC or VCC: Power supply input, 5V DC. (JD-VCC RELAY POWER VCC:SYSTEM VCC) GND: Power supply ground and control signal ground. CH_x: Control signal input, Low: relay ON, High: relay OFF. COM / NO / NC: (C1=COM1, C2=COM2) Control signal state low, the relay ON, COM - NO disconnected, COM - NC connected. Control signal stage high, the relay OFF, COM - NO connected, COM - NC disconnected

Perfect, thanks much. Looks like I'll only be controlling a few solenoids at a time then :)

Even if I power it externally it still draws the current from the pin, oh well! Thanks!

Just add a buffer in between - two ULN2803 chips (datasheet www.ti.com), let them take of the current buffering for the arduino. Or two shift registers lie TPIC6B595 with high current outputs (also ti.com, 83 cents at avnet.com)