Help with designing an LED circuit

Question

I am working on an electrical project and I need some advice on resistors and batteries. I have little electrical engineering education, I learned a little bit about circuits in high school a few years ago. The LED lights I’m going to be using are 3.0-3.2V, 20mA current, and 0.06 wattage. My plan is to use 8 AA batteries to power 88 of these LED lights. I am wondering what resistor I am going to need. If you have any better suggestions for the circuit I am all ears. Also, how would I make it so the lights blink. I’m pretty sure I need a transistor and a capacitor but I’m not entirely sure how to get it all working.

Edit: So I decided to switch to C batteries as they have a much higher mAh (8,000) so I think battery life will be fine now. I found a holder that can support 4 batteries so the new voltage will be 6V. What resistor/other components do I need?

Edit: So would I need to put every LED on it's own resistor with a parallel circuit or even every 2 LEDs?

Answer 1

The first thing I would do if I were you is consider using a different power supply. 8xAA batteries is alright, but your LEDs are going to drain them sure quick. You mention the battery capacity is 2650mAh. This means it can power a circuit of 2650mA for 1 hour, or 1325mA for 2 hours etc etc. Your circuit is 1.75A which means assuming perfect batteries and perfect conditions with no loss of power during discharge, you still only get 1.5h of battery life. This seems like a complete waste of 8 batteries. Hence, I would say change your power supply.

If you insist on using these, consider lowering your current. Get some low current LEDs or limit these ones to around 5 or 10mA (if that is bright enough) to lengthen the time your circuit works. You will also be needing a voltage regulator. Your batteries are going to drain, and as the voltage gets lower, your LEDs will dim. You will need a regulator to stabilise the voltage.

Now to tackle making them blink. If you want to use the transistor/capacitor way of doing things, google astable multivibrator circuit. Read up on how they work and understand it before building it.

Answer 2

You can use ohms law (V=IR) to calculate resistance required with known voltages. When powering the LEDs, assuming they will all be in parallel, your total current is going to be 88 X 20ma= 1.76A, which is quite a lot for 3 AA batteries to handle, if even possible. As for the resistor, assuming you have sorted out the power supply issue, you simply use ohms law to figure out the resistance required. R=V/I, where V = supply voltage - led voltage drop and I= desired/specified current through LED. In your case 3 AA batteries giving 4.5V with 20ma leds at 3 v. R=(4.5-3)/0.02 R= 75 so a 75 ohm resistor would be required for 1 LED.

Answer 3

Considerring that you are using now a new type of batteries I suggest you to calculate againg the necesary R. As MCG suggested it's not a good idea to make them work at the limit, it's better to work with lover Current. (6-3)/0.01 = R.

For blinking you can do in many ways. MCG suggested a great sollution. Also you can use a manual switch a transistor and generate a Square signal with NE555 and feed it to transistor.

PS: If you use the regular batteries I suggest you to connect them not all 8 in serie but 4 in serie together with another 4 in paralel, this way instead of having 12 V and 2650mAh you get 6V and 5300mAh.

Answer 4

I would recommend three things:

1. Instead of having a resistor on each and every LED, split them into small groups of 2-4. Then, for every group, give it its own resistor. If you are using RGB LED's then remember the R LED tends to require 2.0-2.2v (check your datasheet), so they will require their own resistor. Use Ohm's law to determine resistance.
2. Making them blink would by far be the best with a board as small as an arduino mini. You might need a bigger size board depending on the number of inputs/outputs you have.
3. Use a voltage regulator (3.3v is probably best for you).

Answer 5

The new C size alkaline cells are 1.6V when new and soon drop to 1V each. Their capacity is 8000mAh when the current is low and is less than 1500mAh with the very high current of 88 LEDs. The battery will be 4V after about 45 minutes.

Then you calculate each resistor with 20mA, a 3.0V LED and a 6.4V battery which is (6.4V - 3.0V)/20mA= 170 ohms which is not a standard resistance, use 160 ohms for each LED. The power in each resistor will be 0.072W. The LEDs that are 3.2V will be a little dimmed. All the LEDs will dim more and more as the battery voltage quickly runs down.