How to power a 7.4v servo

Hello,

I’m relatively new to Arduino and wanting to set up a circuit for an art project I’m working on.

I’m looking at setting up a circuit that will consist of:

A PIR motion sensor that when activated triggers a servo to rotate and then return to its starting position.

It needs to be powered by wall mains, as it will be left on for around 12 hour stints and be triggered when someone enters the gallery.

I am looking at getting this PIR sensor, and believe it should run off of the Arduino 5V.

With regard to the servo, I have read a lot on the forum about the 5V pin not supplying enough power to a servo, and as the 5V pin is already being used to power the PIR sensor, I will need to power the servo externally, so to speak.

The project calls for a relatively powerful servo, and as I want to increase the rotating angle of the servo I will use a digital servo

To test the project while building I will buy a similar servo that runs on the same voltage and is digital, but cheaper: servo

I am stuck at how to set up powering the servo. The Arduino will run from the wall mains and power the sensor. The servo is 7.4V. What is the most efficient way of powering a 7.4V servo?

Any ideas would be greatly appreciated.

Tom

Photo-Tom:
and as the 5V pin is already being used to power the PIR sensor,

That is of course absolutely BS... If you want to plug your laptop and phone charger into the wall and you only have one socket, do you go off and buy a generator? No, you just split it.

Same here. And if it's an istallation that's at least semi permanent you should start to solder stuff anyway.

But you are right about the 5V pin not being able to supply enough. Just get yourself a DC-DC converter for the servo.

Although I have no idea if you can use 5V signals to control that servo when it's powered with 7,4V...

Thank you for your reply, it’s much appreciated.

I have written to a stockist of the servo and they confirmed that the servos pulse amplitude is 3-5v. I also found a detailed datasheet for it here.

I found a dc-dc power converter to use.

Would the most efficient way to split the power be to use a 2.1mm 2 way splitter, that is connected to the 12v power supply

Then getting a 2.1mm cable and cutting the female end off the cable of and soldering the pos and neg to the inputs on the dc–dc converter?

I will draw up a schematic of the circuit.

Alright, then it should work. Bit pricey but alright.

No need to mess with a splitter, just cut the cable and screw it into the terminal connectors of the converter. The converter even has two input terminals so you don't even need to mess with two wires into one whole.

But that power supply is wayyyyyyyyyy under powered. Grab at least 2A for that. A servo is a power hungry beast, even a small hobby servo pulls 1A+ with ease.

Did you notice the fact your cheaper servo is also OUT_OF_STOCK?

Paul

Thanks again for the help.

I was going to have the other end of the splitter going to power the Arduino board. Should I not set it up like that?

I found a new 2a power supply.

I’m having a bit of trouble understanding the converter. On the diagram on the website I understand the ‘voltage out interface’, where I can chose one of the three different interfaces to run to the servo and GND to Arduino, for example the screw in method of interface 3-4.

Then there is the ‘original voltage input’ and ‘original voltage output’. I believe I would only be using the ‘original voltage input’ by connecting the wires from the other end of the splitter of the 12v supply.

But in the Schematic supplied by the manufacturer, it has 5 Pins:

1 GND: Ground Pin. Care must be taken in layout. This pin should be placed outside of the Schottky Diode to output capacitor ground path to prevent switching current spikes from inducing voltage noise into GS2678.

2 FB: Feedback Pin (FB). Through an external resistor divider network, FB senses the output voltage and regulates it. The feedback threshold voltage is 0.8V.

3 SW: Power Switch Output Pin (SW). SW is the switch node that supplies power to the output.

4 EN: Enable Pin. Drive EN pin high to turn on the device, drive it low to turn it off. Floating is default high.

5 VIN: Supply Voltage Input Pin. GS2678 operates from a 3.6V to 25V DC voltage. Bypass Vin to GND with a suitably large capacitor to eliminate noise on the input.

I understand them as:
5 VIN is the 12v input supply.
3 SW is the converted 7.4v supply to the servo
1 GND. Im not sure why it says GND only has one GND pin, as both the input and output have GND.

And im a little confused about Pin 2 and 4.

Am I right in thinking output GND from the converter (7.4v) goes to the arduino ground as does the servo GND and PIR GND?

I have uploaded my schematic:

Photo-Tom:
I was going to have the other end of the splitter going to power the Arduino board. Should I not set it up like that?

Like I said, why mess with splitters and adapters? Just plug the connector from the power supply into the Arduino but just cut the cord and also wire in the converter :slight_smile:

Photo-Tom:
Then there is the ‘original voltage input’ and ‘original voltage output’. I believe I would only be using the ‘original voltage input’ by connecting the wires from the other end of the splitter of the 12v supply.

Those two connections are 100% the same :wink: But you can use them as intended. Cut the cord and connect the PSU to "original voltage input" and the cut of plug to "original voltage output". And from the output of the module wire to the GND and Vcc of the servo.

Photo-Tom:
But in the Schematic supplied by the manufacturer, it has 5 Pins:

They are NOT talking about the same. The website talks about the module which USES that chip on board. So don't worry about them :wink:

Photo-Tom:
Am I right in thinking output GND from the converter (7.4v) goes to the arduino ground as does the servo GND and PIR GND?

GND passes right through the module, so GND on input and output are the same. So it doesn't really matter where you connect it but I would NOT run the GND of the servo through the Arduino. Because then the Arduino needs to carry all the servo current. Just connect the servo GND and Vcc to the output of the converter. And only connect the signal line to an Arduino pin.

Photo-Tom:
I have uploaded my schematic:

That is correct only the connections to the converter don't make sense because now you use the pins of the chip on the module, not the module. :wink:

Thanks so much for clearing that up with regard to the converter.

I think I have got my head around the power supply. In the updated schematic I have uploaded, I have the 12v supply going into the “Original Voltage In” on the converter, then I have connected the arduino uno to the “original Voltage Out” of the converter, so it will receive 12v of power via the 2.1mm jack.

I want to run the project for about 10 hours a day. It will be activated at random intervals when people enter a space and trigger the PIR. Is there anything else you would do to he circuit?

Many thanks.

Updated Schematic:

That seems just fine :slight_smile: Only don’t connect the GND of the servo to the Arduino, just connect it to the GND on the output of the converter.

The project calls for a relatively powerful servo, and as I want to increase the rotating angle of the servo I will use a digital servo

How much power do you need ?
It is not necessary to use a digital servo to achieve a greater rotation angle.

Thanks Septillion. In the schematic i updated it so the servo GND goes to the converter. its only the pulse now going to the Arudino board.

Thank you for all the support i really appreciate it, and it has helped a huge amount.

UKhelibob, i need quite a bit of power, plus the advantages of the digital servo giving more speed and torque in the small command inputs.