# What batteries should I use to power this circuit?

Joined Nov 5, 2015
7
I'm trying to power a circuit that is composed of a string of LEDs (30 LEDs in parallel, 2.1-2.2 forward voltage, each with a 47 ohm resistor) and a 3v mini DC motor. I can either do this on two separate circuits connected by a DPST switch or on one circuit with a SPST switch. What size battery should I use for either option, and which would be the better choice?
I'd prefer the SPST just because I want to use a momentary push button switch and I already have some (I'd have to order a DPST online), but I want max brightness for the LEDs and sound/vibrations from the motor. Either way the circuit(s) will only be closed for a few seconds at a time with the momentary switch.

#### MikeML

Joined Oct 2, 2009
5,444
Get a data sheet for AA and AAA batteries. With 21mA per LED times 30, plus another 100mA?? for the motor, (ie 3/4A) you will likely need the AAs for the peak current. They won't last long.

Joined Nov 5, 2015
7
How many AAs would I need to run it as one circuit as opposed to two circuits?

#### crutschow

Joined Mar 14, 2008
30,122
It depends upon how long you need the batteries to last(?).

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#### ScottWang

Joined Aug 23, 2012
7,244
Assuming that the led is 2.2V/20mA/3mm and using 80% as 16 mA, I = 16 mA*30 = 480 mA.
So maybe you need to use an adapter or buy a 18650/3.7V/3100 mAH chargeable battery.

#### Bernard

Joined Aug 7, 2008
5,788
Assuming a load of 700 mA @ 20% duty cycle, would use 3 AA Ni-Mh of 2000 mAh giving about 14 h.
Could add a I A diode in series to reduce V closer to 3 V.

Joined Nov 5, 2015
7
The motor is 380mA and each LED is 20mA... The circuit will only be closed for a few seconds at a time whenever the button is pressed (it's a model, it's for show and not going to be used very often or for very long). Could I use a 9v battery with a different motor and resistors?

Joined Nov 5, 2015
7
Similar project but I'm making it with 30 individual 5mm LEDs and twist the leads between two parallel wires... I've made this before with two AA batteries and a SPST switch but the LEDs aren't that bright and the motor isn't that loud. I don't have all that much technical knowledge.

#### ScottWang

Joined Aug 23, 2012
7,244
Alkaline batteries from 1700 mAh to 3000 mAh
Assuming that you choosing two 1700 mAh AA batteries.
You can calculate the resistors for each LED.

I_total = I_led + I_Motor = 480mA + 380mA = 860 mAh
Assuming that you press 10 sec then
I = 860 mAh(10 sec/3600sec )
= 860 mAh*0.0028
= 2.4 mAh
Times = 1700 mAh/2.4 mAh = 708
So you can press 708 times for 10 seconds.

Joined Nov 5, 2015
7
Should I run the LEDs and the motor on two separate circuits with two separate sets of batteries?

#### blocco a spirale

Joined Jun 18, 2008
1,546
One circuit for everything is fine, nothing to be gained by splitting it up.