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I've got an interview for my first job out of university coming up, and I'm preparing a presentation on a project I did a while ago. I'm currently translating my hand-drawn schematic to a computer-aided one and I'd like to know how I can improve upon it. I read through this post and I'm fairly certain I haven't broken any of those general rules, but I'd still like to ask for a critique just in case.

One point I know I should change is to add my decoupling capacitors to the op-amp power supplies, but I'm not certain how I should format that. I read elsewhere I should add a separate block to reference for cleanliness -- would anyone happen to have a schematic I can look at that does so?

In case it's relevant, this is meant to detect a 2kHz IR signal and output high/low to be read on a microcontroller. I'm not really looking for critiques on the circuit itself, but if you happen to notice anything that really grinds your gears please tell me!

Thank you!

Circuit schematic for a 2kHz IR light sensor

ahzired
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  • If you don't want to grind the interviewer's gears, it would be better to spell the frequency as 2 kHz. R8. It's the only place you have a 4-way dot. Don't. Split it into two 3-way dots. R8? Is it really 220, and not 220k? Oh, and R19 has a 4-way. Otherwise, very neat and tidy. – Neil_UK Jun 24 '23 at 19:43
  • Is the U2D connection safe with just a cap connected to positive input? – Ralph Jun 24 '23 at 21:11
  • You should add a part number for D1 and D2. And it's good design practice to equalize impedance of op-amp inputs. See U2A - add a resistor for feedback. – PStechPaul Jun 25 '23 at 00:14
  • Is C8 supposed to be a resistor? That just looks wrong. – Dave Tweed Jun 25 '23 at 12:16

2 Answers2

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This looks pretty good - a few adjustments/nit picks:

  1. V1 is just a generic power symbol. For a schematic, you should provide an indication as to what you are using to bring the power in. Is it a pin header, a plated through hole for solder... etc? Same idea for the digital_io_pin signal - where does it go?
  2. Polarity markers on capacitors - just check, are you sure that the 1 uF capacitors, e.g. C3, are polarised?
  3. Be consistent with your spacing of components with wires. For example, in the R20/D3 pair, there is a short wire (green) between the R20 ends. D3 has its anode connected to 3V3 with no wire.
  4. For pleasant viewing, try to balance the distance between top and bottom of a net. For example, C1 is much closer to the ground side - try putting it in the middle.
  5. Power symbols should be reserved for actual power nets. You have used a power symbol for the 1.65 V bias voltage; this should just be a named net.
  6. Test points. There are none. You should add some or be willing to discuss why you didn't put any in.
  7. Power symbols should always point UP for positive and down for ground/negative. For example, at the non-inverting input of U2D the 1.65 V power symbol (see point 5) is pointing down.

For the opamp decoupling, when you add the opamp part in KiCad, there is an additional sub-symbol with the V+/V- pair. Here's an example from one of my schematics:

opamp power decoupling in KiCad

But, nothing shocking - good job, just clear that it's new to you. That's not intended as a criticism, this is your first job where you will learn these things :)

awjlogan
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Most of the opamps are biased at 0V then they rectify the signal producing a positive-going signal. Then D2 is not needed.

Opamp U2D is missing a DC bias voltage on pin 12.

The total gain is over 50 thousand times so the output will be full of noise.

Audioguru
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