Home GuitarPCB Forum General DIY Pedal Discussion reading and understanding the schematics

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  • #19497
    Miah
    Participant

    okay, next question for all you genius types on here…

    Looking over circuit schematics trying to understand what is happening with the flow of electrons here, (I’m not a schematic reading guru) I see a lot of them have these separated portions of them, like the Tone TwEQ for example:

    Now the bottom portion I think I mostly understand (the part between IN and OUT), what I’m not getting is the other separated portion (the top left part with the +9v, a bunch of grounds, a VB and what I assume has to do with wiring the switch because it says the S4, 5, and 6) but I don’t follow how the rest of it connects to the overall circuit, or does it not connect? can someone please help me understand how this works?

    #19499
    wilkie1
    Moderator

    The upper portion of that schematic is the power supply wiring.  The lines marked 4 and 8 indicate the pins on the IC that receive both negative (ground) and positive power.  These terminals are not always shown on the main circuit diagram.  S4, S5 and S6 are the switch terminals.

    Good questions to help understand schematics.

    #19503
    Miah
    Participant

    Thank you!

    #19504
    Miah
    Participant

    Wait, so those 4 and 8 lines, those connect to each IC?

    #19507
    wilkie1
    Moderator

    Correct.

    #19508
    Miah
    Participant

    So, if you were building a circuit for a dual build, each would need their own power circuit coming off the DC in, that then feeds that individual effect? Am I right? Or would they all share the same power circuit off the DC in?

    #19512
    Miah
    Participant

    I clearly have a lot more reading to do in terms of understanding the design of an effects circuit

    #19521
    Cybercow
    Moderator
    #19523
    fig
    Participant

    Excellent guide Cybercow. I’ve bookmarked it.

    #19528
    Billy
    Moderator

    With schematics anything that has a flag or label like VB are connected

    As Wilkie says that part is the power section usually separate from the main circuit on schematics and very useful for giving you an idea of what voltages to expect at certain parts of the circuit

    Most are more or less the same with filtering capacitors in this case C8 and 9 which stop any voltage ripple creating unwanted noise by filtering those frequencies to ground and not into the audio circuit and a reverse polarity protection diode D1 where you’ll notice the cathode side is connected to the +9v rail so if you plugged in an incorrect polarity adaptor (centre positive)  it would conduct through D1 to ground and not into the circuit stopping any potential damage, if you used a correct polarity adaptor (centre negative) D1 would not conduct and power would go to the circuit as it should

    If you look at the power section you’ll see +9v goes along to power pin 8 of the IC as Wilkie said, then onto R11 and 12 which form a voltage divider, because they are the same value 10K they half the power exactly so you’d expect all VB points to be around 4.5v I say around because depending on your meter impedance it may load down readings and you will get some voltage drop through components

    VB is voltage bias, sometimes labelled VREF (voltage reference) or VR etc and would connect to anything requiring a bias voltage to ensure it operates correctly in this case IC1

    All schematic flags or labels of the same type VB, VCC, ground etc should all be connected

    You can see VB flags at the non inverting + inputs of IC1 at pins 3 and 5 so you’d know what voltage to expect at each input and output pin

    To calculate what voltage you should get at the junction of R11 and 12 if for example they are not the same value resistance you’d use the voltage divider calculation

    Which is Voltage supply x R2 divided by R1 + R2,  ( R1 being the resistor connected to +9v (R11) and R2 the resistor connected to ground (R12)

    So 9 x 10 = 90 / 20 = 4.5

    You only need to use this calculation if the resistors are different values if they are the same it always halves the power supply

    To start understanding schematics you should check a completed pcb and compare it with the schematic

    Check that all identical flags are indeed connected and you get the same voltage on them etc, it does break up the monotony of staring at a piece of paper, putting the knowledge you gain to practical use with continuity and voltage checks

    Another tip for understanding parts of a schematic circuit is to look at the datasheet for example if you had a 7660S IC in the power supply and weren’t sure of its function looking at datasheet circuit examples and comparing them to your schematic would let you know if it’s being used as a voltage inverter or multiplier etc again letting you know what voltages to expect

    Anyway this is obviously simplified, I  hope! and hopefully gives you a little more info specifically on the schematic power supply section

     

    #19530
    Miah
    Participant

    Thank you so much Billy for taking the time to explain that! And cybercow thank you too! I will definitely take some time and play around with my multimeter and some of the other diy kits I built. I’m sure I’ll be back with more questions but thank you all for now!

    #19539
    fig
    Participant

    Great circuit analysis and advice (as always) Billy!

    #19542
    Billy
    Moderator

    You’re giving me way too much credit Fig

    Everything I know or think I know I learnt mainly on this forum

    Regardless of what they say the guys that really know what they’re talking about are Wilkie and Cybercow and of course the Cap’n – Barry and Bruce

    Tonmann was the king sadly no longer frequenting these parts I’d love to know how he’s getting on and hope he’s doing well

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