Keep individual LED brightness steady when other LEDs are added or removed

Question

I'd like to use several LEDs on a project, with brightness control (it's a scale model of a street with houses, with LEDs of different brightness in each house, which I've been working on with my son). However, I'd want the brightness of the LEDs not to change when I turn some of them on and off (or equivalently, if I add or remove LEDs).

I know I can control dimness of LEDs either by directly varying the current applied to them as in this project from someone else or by using PWM as suggested here -- but in both cases (I implemented the two projects listed above as a test), the current source is the same so their brightness will vary depending on the number of connected LEDs (which make sense, since the full LED set is plugged on a single transistor's collector).

What would be the simplest way to achieve what I'd want in this case? I'd rather avoid using, for example, an Arduino (would be too physically large and would involve programming -- I'd like this to be as simple, small, cheap as possible, without microcontrollers). Is there some simple way to decouple each LED from the others, and still have central control of their brightness?

Connecting the LEDs in serial would not be good either, since I would not be able to remove those in the middle (and this would require a too high voltage to work, because of the accumulated voltage drop of the LED string).

Thank you!

Answer 1

Update: Op has noted his issue is with using a NPN transistor as a high-side controller. It would not be working in a saturated mode and as the load changes it will have odd voltage and current properties. The proper simple solution is to use a PNP transistor suitable for the load. Or switch to a NPN low side setup, noting that the PWM period will be inverted.

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I'm looking at the PWM circuit, a simple high-side transistor + 555 timer setup. I'm not seeing why this would change the brightness if you remove a series string from it. Unless you are removing a single led from a series string.

^{simulate this circuit – Schematic created using CircuitLab}

Here, all the leds and resistors are the same. So the voltage between Node1 and Node2 are the same, as is the voltage and current going through all the strings.

If you turn off string 2 (R2, D3, D4 via SW1), the other strings should not see any change in voltage, and thus, current and brightness. If you turn off D6 in string 3, via SW2, then the voltage across D5 goes up, and thus current and brightness (if it doesn't burn out right away). But String 1 and String 2 are still the same.

If you add leds to any of the strings, you will see a brightness change as the voltage is divided among them. You would have to adjust the resistor value to make sure the same amount of current still goes through.

The only problem you may face is if your voltage source is a constant current source instead. Or if your voltage source is very load sensitive. If the source has a high equivalent series resistance, then as your load increases in current, a higher voltage drop is seen, affecting everything. Use a high current capacity and/or quality power supply. Also keep in mind that your transistor needs to be sized for your load. A 2n3906 will only handle 200mA, so if your load is more than that you will see problems.

Answer 2

Put all of the LEDs in series. Drive the series string of LEDs with a current source (or sink). All of the LEDs will light.

Put a short across any LED (s) that you want to turn off.

Because the string of LEDs is driven with a constant current, the brightness of the remaining LEDs will remain the same.

Answer 3

The starting point should be a common anode LED matrix and just on and off. Once you have the on off working, add dimming.

And if desired in the future add a micro.

This is the matrix used in signs.

Typically driven with a micro-controller, but could be done with out one.

Rather than a micro-controler build a state machine with a PLD. The PLD would also have registers holding the on off values

State machine logic is a good way to learn how a micro controller works. After all a micro is just a large state machine.

To "program" the on off use two 8 position switches, one for address and one for data.

Once the on off is working add another dimension to the on off registers and up the frequency of the on off matrix and tweaking the state machine logic. You will need a set of on off registers for each level of dimming. Registers come cheap in PLCs.

When, if, you want, remove the two sets of switches and replace them with a micro controller.

When you are done, if there is not a chip already out there doing this the state machine PLC could be a salable product.

I do not know how well you will understand this. In the 1980s I was designing data networking ICs for IBM (token ring 802.5) and GE (token bus 802.4). This project as described is how ICs are designed. You build your model with off the shelf logic then migrate it to custom silicon.