What batteries should I use to power this circuit?

Discussion in 'The Projects Forum' started by bigapplehades, Nov 5, 2015.

Nov 5, 2015
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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.

2. MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
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.

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

4. crutschow Expert

Mar 14, 2008
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3,386
It depends upon how long you need the batteries to last(?).

Last edited: Nov 6, 2015
5. ScottWang Moderator

Aug 23, 2012
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777
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.

6. Bernard AAC Fanatic!

Aug 7, 2008
4,247
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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.

Nov 5, 2015
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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?

8. blocco a spirale AAC Fanatic!

Jun 18, 2008
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Nov 5, 2015
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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.

10. ScottWang Moderator

Aug 23, 2012
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777
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.