I have been doing a lot of researching ever since Christmas when I got the Arduino Uno. Anyways, I want to build a 4 leg spider robot using 2 Arduino's. I had a lot of problems on coming up with how to do it, and I think I got it. What I want to be answered is this question, is my circuit good to go and will not fry anything? Anyways, I needed 12 servos. Only 6 pins on 1 Arduino has PWM. So I thought to use 2 Arduino's to communicate through the HC-06 blue-tooth. Power was another problem. I discovered that double AA batteries have an impressive 7 amps on my multi-meter. However, to much for the Arduino. That is why it is only supplying servos and not going directly to the Arduino. A 9v will do that. Anyways, I can't give you guys the schematic as it is to big of a file. However, I will explain the circuit in words. One Arduino is master and the other is slave. These steps go to both master and slave unless stated.

  1. 6 servos on master and 6 on slave. Power is going to 4 double AA batteries in series. This supply is not connected to the board except the ground wire. This is a battery holder with switch already on it. All PWM pin's are in use for servos.

  2. 9 volt battery going to the Vin pin. I will incorporate a switch.

  3. Fan with positive and negative only. Positive goes to the 9 volt battery. Ground goes to ground pin on Arduino.

  4. Bluetooth module with VCC going to 3.3v pin. Ground to ground pin. TXD going to the RX pin. RXD going to TX pin.

  5. For master Uno only. Distance sensor with Trig going to A5 pin and echo to A4 pin. Ground going to ground pin. VCC going to 5v pin.

  6. 2 led's in circuit with 220 ohm resistor. I will use pin 4 and 2. Ground goes to ground pin.

That is the setup. I am in college where soldering is banned. So I am including a fan, like you saw, to cool down the breadboard. Let me know if you think that will work. I could not find a good schematic online for this robot, so I made one. Please let me know if you have any questions. Anyways, please tell me if there are any flaws and thank you so much for your time and effort.

Edit 1: @gilhad I did not know about I2C, if that is easier than blue-tooth, then I will get that. Thanks for that input. This way I don't have to use 2 blue-tooth modules. Most of the robot is running in series, not parrel.

Also, I said 4 AA batteries and not 2. If I had 2 in series, then that is 3v only, so that is why I am going for 4 in series for a total of 6v, with amps being the same as with one battery. This will supply 7 Amps. I tested it with my multi-meter. The servo motors are going to need 1 amp of power at max. I have 7. I will probably be using 4 servos at one time. So that is 4 amps. It will run for about an hour. That is fine by me. This 4 AA battery is not connected to the arduino. There are 2 power sources per arduino. The other one is a single 9v battery, that one is going directly to the arduino, the Vin pin to be clear. If it is going to reset, could I do something in the programming to account for that like a time delay that is short?

I do not want to mess with my breadboards, that is why I am using a fan to cool them. Will that work?

The power source will not be moving all the servos at once, sorry that I did not include that.

Edit 2:

@killer rabbit

I guess I did not want to get a mega because I have 3 Uno's already, but I see they're pretty cheap online. I will ponder this idea of yours. I looked at my schematic and determine this would eliminate the 2 blue-tooth modules, 1 power source, 1 fan, and 2 leds along with their resistor. I think it would also eliminate an extra switch. Programming would be easier to because I don't have to worry about blue-tooth communication. I had no idea what to do for that one. So maybe I will get a Arduino mega.

The reason I am not using RC battery is because they're kinda heavy and SG90 servos are kinda small. I was worried about torque. Li-ion is not to heavy, but I was worried about power. The ones you gave me though, look like that might work. There is 2 problems though. First, I can't use 2 of these batteries in series because that is 7.4 volts when the servos I am using is at max 7.2 volts. I would have to reduce voltage somehow, I have no idea how to wire that into the circuit. Problem 2 is that if I wire is parrel, that is 6 amps, which is fine, but only 3.7 volts. The motors can run on that, but that is on the low end. Will torque be less?

About the third paragraph, there is limits on the amount of current an Arduino can take. I looked them up. It is about 2 amps for the Vin pin I think or that might be the DC jack, one of the two.

I am using 2 Arduino, so that is why the blue-tooth was necessary. However, if I go with the mega, it will eliminate both of them. I forgot to mention for the distance sensor, I am using ultra sonic one. That is my fault for not mentioning that.

The reason I need 12 servos is because I am using 4 servos at the base, one for each leg to control direction of leg for steering. Then I have 4 legs, each using 2 servo motors to make the leg kick out and in. So 4 + 2(4)=12.

Yes, I would be using, if 2 Arduino, wireless communications, that is why the blue-tooth module. About the schematics, the picture is too big, so that is why I can't upload it. So sorry about that. I did try, believe me.

2 Answers 2


There are multiple points where you should reconsider your plans: First of all, do not use multiple Arduino Unos, it's just a pain in the arse to make them work synchronously, instead use (for example) a single Arduino Mega 2560 which has 14 PWM outputs and still cheap as dirt.

I'm not sure about the life length of the AA batteries, I might consider using Li-Po (something from an RC shop, or something) or Li-ion (18650 is very popular and cheap) batteries. The Arduinos can take basically anything from 9 to 6 volts (and 5 from the USB), so you have a bit of freedom there.

A bit of electrical stuff: High available current is not a problem for the Arduino, it takes as much as it needs, so as long as you are in the voltage range of the board, you won't have any problems. Less amperage however is a problem, so be sure your battery can provide enough current!

You probably won't need cooling, there are no devices in your spider that would require more than a heatsink, if at all. But soldering is very recommended, I urge you to reconsider about that.

The Bluetooth module becomes unnecessary by going with a single Arduino. For distance sensor I recommend an ultrasound one, those are really (ridiculously) cheap and simple to use. (HC-SR04 is very popular for example and there are tons of tutorials for it)

LED's are okay, most people don't think about the resistors, but those are important!

A question: Why do you need 12 servos?

That's all I got in the first round, feel free to share more info and I'll try to help you with your planing. I highly recommend you to share the schematics with us so we can help you to improve it!

Edit: I almost forgot: do you intend to use some kind of connection to the spider, like Bluetooth or Wi-Fi?

As far as batteries go, I'd recommend at least 6 volts, so you can use the Arduino's barrel connector. You may use 2 serial Li-ion batteries and a buck converter, but I believe you'll do fine without it, your servos will handle the extra 0.2 volts.

About the current limits: Your Arduino can supply around maximum 500 mA from all it's pins, and 40mA from a single pin, BUT it won't be a problem, because you do not connect the servos' VCC (+) and GND (-) pins to the Arduino, instead you hook them up to the battery directly. You only connect the SIGNAL pins to your Arduino which draw less then 10 mA.

For the communication with the spider I'd recommend a Wi-Fi module and UDP, as the protocol for the communication.


  • I commented to your answer at edit number 2.
    – Sean.D
    Commented Feb 18, 2017 at 18:15
  • Got it, I'll edit my answer accordingly. Commented Feb 18, 2017 at 19:27
  • I can't tell if you edited or not? If you edited, can you just add some like edit 1, so I know? Thanks.
    – Sean.D
    Commented Feb 18, 2017 at 19:58
  • I finished it now, you'll see a line in my answer. Commented Feb 18, 2017 at 20:16
  • I saw the edit. Let us assume I am going to use the Arduino mega for now. I ordered one as it was only $13 with free shipping. If I put the Li-ion batteries in series, it will have 3 amps, I might need 4 amps at a time though. This concerns me. What can I do about that part? I won't need communication anymore because of the using Mega. Power is still a concern. In series, 3 amp is low, minimum of about 4 amp at a time will be used. In parrel, will 3.7 Volt decrease torque because it is on the low end for voltage supplied to the servo? I think I am close to a finish design now.
    – Sean.D
    Commented Feb 18, 2017 at 20:25

For connecting 2 Arduinos use some wires - I2C works good for me, but Serial is also good. Anyway you probabely will came to problem with interrupts and many tasks running in paralel, or the spider be slow - you choose not so easy target (but reacheble).

As for bateries - Arduino need constant power, than it takes as much current as it needs (if it is aviable), so no problem in 2 AA for Arduino.

You would need a lot of servos - they are, what takes energy - even if 2AA can give a lot for short time, this will not last long. Also giving high current means voltage drops (as for cable and mainly internal batery resistence), so if you feed arduino from the same source, it can reset every time, when too much servos try to move/stang agains gravity.

Different source for Arduino is good idea, as the CPU will last longer, then those actuators and could shutdown properly (not let move just one leg or so). Also the electrical interference from servos would be much less this way. (There are other ways, more complicated to do that).

Breadbourd can be stripped the bottom plastic and have it exchange for more thinner double-size tape, then be put on large alluminium (or other heat-conducting) plate for better cooling too.

Arduino cannot move all those servos from its source, think about alimitation currrent per pin and per arduino itself - you may need some other IC/transistors to power it properly on each servo and the servo power should go direct from bateries, not via Arduino. Connect just grounds.

  • I commented at your answer at edit number 1.
    – Sean.D
    Commented Feb 18, 2017 at 18:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.