Nano circuit runs on battery but won't start

Hi, arduino noob here
I’m building a small wooden box that opens via fingerprint sensor.
I have an Arduino nano with a small servo and a fingerprint reader attached. Everything is fine when powered from USB. I want to power it from batteries, as I want to keep the circuit inside the box.

I have 4 AA batteries connected to the VIN pin, which seems to boot the Nano, but then nothing happens. The servo is supposed to move to its start position, and the fingerprint sensor is supposed to light up. As I said this works fine when powered from USB.

However, if I start everything from USB, then connect the batteries and disconnect the USB, everything keeps running fine. Fingerprint sensor works, servo works.

My guess is that the batteries can’t provide necessary power to start the fingerprint sensor, servo and arduino nano at the same time.

Could I insert a delay somewhere in the starting process, to keep everything from wanting power at the same time?

Fingerprint sensor looks like this. Current max <140 mA.

Datasheet for servo here
Current max <600 mA.

Rest of the circuit is one switch to connect/disconnect batteries, and one button. No LEDs.

Post a pic of your actual wiring.

Are all GNDs connected?

Servo's can take over 1AMP when under load...

Are your 4 AA batteries fresh?

Hi, thanks for replying!

I tried to draw the circuit, at least + and - wiring. Not included in drawing is two wires from Fingerprint Sensor to D2 and D3 on Nano. One wire from Servo to D7 on Nano. One simple pushbutton connected to GND and D9.

Batteries, 4 x AA, fresh.

Servo is small, weighs 11g. A small screw is attached to servo arm (part of the simple lock mechanism).

circuit_box.pdf (18.4 KB)

I tried to power up my nano in the exact same way, and got the same result. I don;t know if my nano is damaged or not, but just try to hook up the batteries' ground to one of the nano's ground. try each one of the nano's ground seperatly until it works. I don't really know a lot about powering the arduino so I can't rally help futher then that.

Hope it will help ;D

You must ofcourse power the servo straight from the battery (switched).
Powering a servo, that could draw 1Amp every time it starts moving, could damage the 5volt regulator on the Nano.

If you’re sure that the batteries are <5.5volt, you should connect the battery to the 5volt pin.
Not sure if the FS needs a well regulated 5volt though.

Hi Arad, thanks for your suggestion. Tried to connect the battery ground directly to Nano GND, both of them, but no luck unfortunately.

When I power from USB, the onboard LED blinks once, and onboard power led goes green. After a few seconds, onboard TX blinks twice, then servo moves into start position and fingerprint sensor lights up.

When I power from batteries, it stops after onboard TX light blinks twice.

Hi Wawa, thanks for replying!
Maybe I'm daft :slight_smile: but I don't follow completely. I should connect batteries straight to servo? How do I connect power to Nano and FS?

The servo draws max 600 mA according to datasheet.

Yes, the red wire from the battery switch should go straight to the servo (assuming you use four AA batteries).
So that high servo current doesn't travel over the tiny 5volt regulator of the Nano.

Powering the Nano with four AA batteries is a bit of a problem.
If you power the Nano on the V-in pin, then it should at least be 6-7volt for the onboard 5volt regulator to make a stable 5volt.

A workaround is to power the Nano directly on the 5volt pin.
You just need to make sure it never exceeds ~5.5volt.

The (for me) intriguing part is that if I boot the circuit with USB power, then switch to batteries (4xAA) the circuit continues to work as it should. It's just that it wont start on battery power.

Is it plausible to believe that it would work with a lipoly battery such as this (Adafruit 3.7V, 2500mAh) together with this boost converter (Adafruit, output 5.2V, connect to +5V on Nano)?
More expensive that I had hoped, but if works, it's worth it to me.

Did you use good quality and fresh AA batteries.
If the batteries are not able to provide the startup currents...

You could power the Nano with a 9volt battery on V-in (raw), and the servo with the AA batteries.
Must use a dubble-pole switch for this.

Thank you Wawa, I'll try with a DPDT switch today.