SparkFun Forums 

Where electronics enthusiasts find answers.

For the discussion of Arduino related topics.
By InoIno
Problem: the current circuit design have a servo motor connected directly to a 9V battery via a voltage regulator for power and a 5V signal drawing from an ARDUINO UNO digital pin, but the servo is draining the battery in about a few days which is no good.

I was thinking about incorporating a relay in the design in order to control the flow of current after the servo has completed its movement. The relay would then deactivate and open the circuit de-energizing the servo and wait for the next signal. I’m very new to the whole electronics scene, so I’m sorry if this seems like a novice question?! The coil voltage would need to operate on 3.8V 50mA and control a 5V circuit, but all the relays I’ve found are for 100+ volts. Can those be use for 5V loads? I’m not sure what baring the max Load Voltage has on the min load voltage.

There is likely a code solution that I’m not aware of and that would be best, but a relay would work.
By Valen
Sure you can use relays with 100+ volt ratings in 5 volt circuits. It should not be used in circuits that go above (or near) 100 volt. Be mindfull though that digital pins cannot supply much current. Less than the 50ma that you state. Also, they need to be protected from the inductive kickback of the coil when you switch them off. Check this video as a starter:

But don't you think the relay coil current would drain the battery also? Mosfets do not require any continous operating current on the gate to switch (off) a load. Investigate how to implement those. (the above video shows it a bit) 9 volt batteries have notoriously low capacity. Better use a bunch of AA batteries (6) in series to go above 7+ volts. They are bigger as a pack but will last longer.

Using a 9 volt battery and voltage regulator to power an UNO and servo is far from optimal. With or without a relay. Much energy is wasted. With an UNO you won't get much saving with sleep modes (the additional parts on the board consume too much for the savings to matter). That board could be a significant battery drain itself. A minimal circuit can be made, but could be a level up for a beginner. Better is to use battery with voltages that are a closer match to what the arduino needs to function (no regulation) and use a seperate battery for the servo itself. The latter being shut off with a (bipolar)transistor/mosfet.