Newbie trying to make a simple LED circuit

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I do have some experience but I’ve never built a circuit, always just theory in first year electrical. I want to add lighting to a Lego Sydney opera house that is on display, i want to light the 3 large shades and was hoping to run some sort of a led string running underneath the “lamp posts” running around the outside (I’d like these ones to be duller).

As i said I haven’t done much practical work but I was hoping to run the system off a 6v source of 4xAA batteries. I haven’t worked much with mcd as a measure of brightness so i still don’t know the comparative brightness levels either. What i was thinking was 2 LED’s per large shade (one at each end) and one smaller one for the smaller shade, or one bright one per shade if it does the same job. I was looking at White Cool LEDS with a Vf of 3.2v but obviously 6v doesn’t equal 6.4v. I’m not sure of the difference between a cool white and a yellow led but i was considering a yellow led with a Vf of 2v. However this still left me wondering about the light string i wanted to run around the base of the construction to light the “lamps” (they’re hollow lego posts with a clear bit on the top and a small hole drilled through the base plate).

What sort of mcd should i use for this project and am I able to power the whole project from one 6v source? Any and all help or suggestion to turn this practical will be much appreciated :slight_smile:

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White Cool LEDS with a Vf of 3.2v but obviously 6v doesn’t equal 6.4v

With LEDs, it is extremely important that you control the current to a safe and useful level. A few milliamperes is usually bright enough, and much more can destroy the LED. The voltage is less of a concern, but of course the power supply must have voltage higher than required.

Always use suitable resistor in series with the LED. There are several pages on the web that will help you select the right values. Be sure to wire the LED the right way around.

As for brightness, if the model looks right, it is right.

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jremington:

White Cool LEDS with a Vf of 3.2v but obviously 6v doesn’t equal 6.4v

With LEDs, it is extremely important that you control the current to a safe and useful level. A few milliamperes is usually bright enough, and much more can destroy the LED. The voltage is less of a concern, but of course the power supply must have voltage higher than required.

Always use suitable resistor in series with the LED. There are several pages on the web that will help you select the right values. Be sure to wire the LED the right way around.

As for brightness, if the model looks right, it is right.

When purchasing the LED Howe do I decide what mcd rating to use if my brightness is dictated by current?

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You need to think of it the other way round.

Buy a LED then try different series resistors until you get the brightness you want.

The spec for the LED will tell you a maximum forward current and a forward drop at one or maybe a range of forward currents.

The resistor value is roughly:

Rseries = (Vsupply - Vfwd_@_desired_Ifwd)/desired_Ifwd

where Vsupply is the supply from which you are driving the LED + resistor series circuit.

and desired_Ifwd is the current you want to try out through the LED.

The current won’t be exactly right but it will be close enough for your purposes as the recommended or typical LED current is usually about 1/2 to 2/3 of the absolute max (damage it) current rating.

Make sure that you have a large enough (Vsupply - Vfwd_@_desired_Ifwd) value (the drop across Rseries) that any changes in the battery voltage over the life of the battery, Vfwd vs actual Ifwd and due to temperature changes (not sure about white leds but somewhere around -2mV/degC) plus the device to device variation are negligible compared to the voltage drop across Rseries. Somewhere around 10 times the worst case total variation is good.

Another way to think about that is that you don’t want to set a LED current with a flat battery and then find that the LED current with a fresh battery is so high that it blows the LED.

The choice of Rseries is illustrated here:

https://easyeda.com/andyfierman/LEDs_mu … -OoGYgCK2k

(To actually run the simulations you have to sign up for a free, unlimited account but this account lets you do schematic capture, simulaton and PCB design with Gerber download all for free with a no-ties option to buy PCBs, parts and even assenbled boards. It even allows commercial use!)

With 4*AA batteries at something around the 10mA to 20mA for most low current LEDs, the voltage variations should not be too much of a problem.

For interior lighting of a model like yours, you may not want a very bright light source (think of how bright the windows in building look form the outside at dusk/night/dawn).

Wide beam angle and/or diffused LEDs may be better for interior lighting whilst if you want to light the exterior then you could look at narrow beam angle parts, maybe with little shrouds.