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Finally getting back to my LED-lit house number sign project. Here's a block diagram of the system:

Block Diagram

I keep flip-flopping between which voltage to power the electronics inside the sign: 3.3VDC or 24VDC. (Actually there's a third option as well: route 16VAC to the box, rectify & do two DC-DC conversions to get both supplies. Unless people say I should consider this option, I'm not going to because I want to minimize the heat buildup inside the box and keep things simpler (see below for more on this).

I decided to do a Pugh Matrix so I can make a decision and move on. I'm asking people to review the following criteria and see if I missed some important ones, or any trade-offs that I may need to address.

I'm going to review some tradeoffs I've considered to help you understand the criteria below.

First of all,

In the basement I have 16VAC from the doorbell transformer (it also illuminates the current house number sign). I don't want to run any other wires, so I only have the existing two wires to work with. I would only consider running more wires if I hit a dead end, which I don't expect will happen.

The electronics in the house number sign need to be waterproof, so they will be encapsulated. Thus, heat buildup is a concern (not as much now that I've done some research & posting a question). Sending 3.3V to the Sign has the nice option of allowing the microcontroller to disable the 24V boost converter when the LEDs aren't on. The I2R losses should be minimal as I estimate the distance to be ~ 15 ft. (Before I make a final decision on 3.3V, I'll connect a power supply and measure voltage drop, to be absolutely sure.)

I don't need to worry about heat buildup or space in the basement, so I can be more relaxed on the converter there. It would be nice to buy a module and not have to design & build one. The only issue is I did a quick look at 3.3V ones and they don't tell you if they can handle near zero load without the output rising. That's where 24V might be nice: if I buy an AC-24VDC converter that's meant for LED strips (there's a LOT to choose from), it stands a chance of being able to handle low currents well. In the end, if I have to, I'll consider a linear power supply for the basement.

I didn't include cost in the criteria because with the exception of something silly (like outsourcing everything), I don't care about cost--it's a "one off" and I'm doing this for the fun of it.

My criteria:

  • Heat buildup inside the House Number Sign
  • I2R losses from the basement to the Sign
  • Low-current stability of the DC-DC conversions
  • Availability/large selection of supplies
  • Energy efficiency (trying to be "green")
  • Simplicity/ease/speed of design & build
  • Voltage drop caused by pressing of doorbell button
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  • \$\begingroup\$ I'm a microcontroller (MCU) guy but if you drop the MCU your design would be way simpler. Why do you need the MCU? \$\endgroup\$
    – Rodo
    Nov 17, 2021 at 18:03
  • \$\begingroup\$ Could automatic snow removal be a criterion? Or making it too warm for insects to build a nest behind it? \$\endgroup\$ Nov 17, 2021 at 18:09
  • \$\begingroup\$ Rodo: I'm a software engineer and it's easier for me to do things with a microcontroller such as dimming the LEDs, adjusting the color temperature and deciding when it's dark enough to turn on the LEDs, along with a thermistor in case I need to compensate ir photodetector response with winter/summer temperatures. It will have Bluetooth which will make it easier for me to adjust LED intensity, color temperature and thresholds and it provides a potential ability to send a message to something in the house so I can tell when mail arrives. \$\endgroup\$ Nov 17, 2021 at 18:37
  • \$\begingroup\$ I'd better explain the last part of my last comment. I'm now thinking about removing the number sign and putting the numbers on my mailbox (along with LEDs). I'll want to dim the LEDs when the mailman opens the lid (don't want to annoy him), thus I can tell when mail arrives. \$\endgroup\$ Nov 17, 2021 at 19:08
  • \$\begingroup\$ feed it the voltage the LEDs need, use a buck converter to run the MCU \$\endgroup\$
    – dandavis
    Nov 18, 2021 at 2:21

2 Answers 2

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Without getting into why there is microcontroller in a number sign ... What are the currents at 3.3 V and 24 V?

For absolute lowest heat in the sign, run three wires (24 V, 3.3 V, RTN). I mention this for completeness only, as I think having only one rail out to the sign will be way more reliable long-term.

In fact, I'd add a bridge rectifier inside the box, so the incoming DC can be connected "backwards" without problems.

My vote is to create LED power (constant voltage, constant current, whatever) inside the house, run that out to the box, and pick off some of it for the uC (assuming the uC isn't stupid-hungry).

Oops, just noticed you asked another question (about currents). I don't have exact numbers yet, but they will be low. Assume max of 20 mA for 3.3V (when adjusting thresholds via Bluetooth) and 25 mA for LEDs.

I might be missing information from some previous thread, but - Those current levels are so low that:

  1. Don't worry about I^2R losses in the wiring. With #18 lamp cord, the voltage drop will be trivial.

  2. At those currents, I would go with all linear regulators. With 24 V in and 3.3 V out, the power dissipation in the regulator will be about 0.4 W. An LM317 in a TO-220 package can handle that without a heatsink, although 2 square inches of aluminum wouldn't hurt. No electrical noise, a 5 mA minimum load, no high-frequency decoupling, no electrical noise, no long-wire source impedance issues, almost zero design time, and no noise.

  3. If that is 25 mA total for all LEDs, then again, linear. Either a single resistor, or a constant current source. For the latter, and parts commonality, an LM317 does a terrific job.

I whipped up a red/green status indicator for each of my garage doors. Because of differences in the number of LEDs and differences in the Vf of the two colors, I could not get a good-enough brightness match with one resistor. Enter the LM317 current source schematic on the datasheet. I can switch in any number of LEDs from 1 to 10, and the current sits there like a stone.

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  • \$\begingroup\$ Thanks--you bring up a point--I didn't mention that I want to reuse the existing wiring out to the current sign (two wires). I will edit my question. My knowledge of electronics is only what I had way back in school. I don't understand your comment about a bridge rectifier and reversing the DC. \$\endgroup\$ Nov 17, 2021 at 18:40
  • \$\begingroup\$ Could you elaborate on why you'd do 24V? It's not obvious to me which is better and is why I keep going back & forth on this (and want to do a Pugh Matrix). \$\endgroup\$ Nov 17, 2021 at 18:50
  • \$\begingroup\$ Oops, just noticed you asked another question (about currents). I don't have exact numbers yet, but they will be low. Assume max of 20 mA for 3.3V (when adjusting thresholds via Bluetooth) and 25 mA for LEDs. \$\endgroup\$ Nov 17, 2021 at 19:00
  • \$\begingroup\$ See answer update. \$\endgroup\$
    – AnalogKid
    Nov 17, 2021 at 20:34
  • \$\begingroup\$ "Could you elaborate on why you'd do 24V?" My read of the original question was that only power options are 3.3 V and 24 V. At any current level, it is way easier to regulate 24 V down to 3.3 V than boosting 3.3 V up to 24 V. Please post a link to the LEDs and a link to the preceding project. \$\endgroup\$
    – AnalogKid
    Nov 17, 2021 at 21:34
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I don't understand your comment about a bridge rectifier and reversing the DC.

With a DC output power supply in the house, and a DC type load such as an LED, polarity matters. If you connect the + output to the cathode end of an LED string and the - output to the anode end, the LEDs will not light. So the interconnect between the supply and the load is polarity-sensitive.

A bridge rectifier can fix this at a small cost. It usually is marked with a + output, a - output, and two AC inputs (sometimes a small sinewave symbol). The two inputs are interchangeable. No matter which way you connect your two wires, the + output always will be more positive than the - output. There will be a voltage drop of around 0.8 V to 1.2 V, depending on the voltage rating of the diodes and the current through them. So you can hook up the two wires from the house 24 V supply with no regard for their polarity, and get about 22 V out the right way all the time.

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  • \$\begingroup\$ Interesting. Thanks. \$\endgroup\$ Nov 17, 2021 at 22:13

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