2D Schematic Diagram

[VijayHegde]

Hello I am Vijay.

I had received an interview question. The interview question was as follows

Product Design Electronics(ULN Driver)

Objective:
Design a complete electronic circuit that utilizes an ATmega328P micro controller to
control and power a 12V LED strip using a ULN2003 Darling-ton transistor array
driver IC, with the primary input power sourced from a 48V battery.

Expected Deliverables:
1.   Circuit Diagram:
■ Complete schematic presented in A4 sheet format.
■ The circuit should clearly demonstrate all connections between
components, including power supply regulation, micro controller I/O,
driver IC inputs/outputs, and LED load interface.
2.   Power Management:
■ Proper voltage step-down mechanism from 48V to 12V (for the LED
strip).
■ Additional regulation to provide 5V supply to the ATmega328P and
other logic-level components.
3.   Component Labeling and Notation:
■ Use standard electronics naming conventions (e.g., U1, R1, C1, D1).
■ Label all connections (e.g., VCC, GND, INx, OUTx, COM).
■ Indicate signal direction where applicable.
4.   Design Considerations:
■ Ensure all power grounds are properly referenced (common ground).
■ Include protection elements where necessary (e.g., fuses, bypass
capacitors, fly-back protection).
■ Clearly show the interfacing between ATmega328P, ULN2003, and the
LED strip.

Expected Output
● Schematics.

I have few doubts reagarding the assignment.
1.   How to convert 48V DC to 12V DC? Do I need to use a readymade buck converter like the LM5118 or LM2596HVS or do I need to redraw a new circuit for the buck converter?
2.   Will it be correct to connect the 12V output of buck converter to ULN2003 driver and to 12V LED strip. Won't there be any voltage drop for the 12V LED strip?
3.   I couldn't get the footprint for 12V led strip. Can I connect 9 LEDs (Each 1.5V) in an array and represent it as 12V LED strip?

Unfortunately, I have been rejected in the interview assignment. I don't need any kind of 3D PCB design for it. My only aim is to show proper 2D Schematic connection. It should also work if implemented using hardware components. I have uploaded the schematic diagram. Can anyone tell me what are the mistakes I have made in the schematic diagram?

[themadhippy]

Will it be correct to connect the 12V output of buck converter to ULN2003 driver and to 12V LED strip. Won't there be any voltage drop for the 12V LED strip?

suggest you read up on how to use the ULN2003,especially if you dont want to invert the micros output

  I couldn't get the footprint for 12V led strip.

Id put a connector in,clearly marked "output to led strip or similar" and the polarity

[squadchannel]

label wiring and box layouts irritate us.

[PGPG]

Can anyone tell me what are the mistakes I have made in the schematic diagram?

My opinions.
1. Drawn as separate boxes schematics are easy readable for their author and hard readable for anyone else. For example I spend several minutes trying to locate where ULNOUT signal is going and failed. I still don't know if I am just missing it or it is not there.
2. Why you drive U1 IN1..IN7 inputs while you use only one output (if that output is connected to something I can't find)?
3. At the beginning I was sure you have no quartz there but found it. That symbol suggests some IC and not simple quartz element.
4. I think you miss understand LED strip. Typical 12V LED strip has 3 LED segments with resistor setting the current. All 3 LED segments are connected paralel. But your circuit probably needed only output socket (terminal block) to connect it.
5. I have never heard of LEDs working with 1.5V voltage drop (except infrared used internally in transoptors).
6. For me ready made buck converter means some kind of ready device while LM symbols (didn't searched for them) are rather ICs.
7. It would be good to connect 12V output directly to 12V LED strip, but connecting it to protecting diodes of ULN2003 I would not name as connection 12V output to ULN2003 (even it is connected). Saying that you connect 12V to ULN2003 suggests that then ULN2003 provides this 12V somewhere else while it is not. ULN2003 provides connections to GND.
8. Practically it is very hard (or may be impossible) to protect 48V input against surges to limit it to be less than 57V while LM2596HV max input voltage is 57V.
9. In point 4 you were asked to include protection elements. Have you used any?
10. How many pushes your reset pin you expect to withstand (directly discharging 100uF can be hard experience to it).
11. I know it can work in many cases, but I never use L with smaller saturation current than DCDC IC current limit to be sure saturation can't happen ever. I'm just not sure if during start up the 1.1A L limit will not be exceeded. It need be very carefully analyzed.
12. I suppose uC is expected to be only LED controlling tool and true control should be provided for user so there should be at least some input buttons to allow user any influence on circuit.

[zapta]

The specification of "LED strip" is not clear. What is the interface of the LED strip?  Is the circuit supposed to control 1 channel or 7 channels?

I hope that this was an assignment at home and not a tight limited exam.