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EEVblog 1493 - The MacGyver Project - Part 2

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Peabody:
I certainly had my doubts too in the beginning, but the more Dave reveals about how this circuit works, the more impressed I am with the engineering.  After all, you eliminate all 40 segment resistors, but still have no excess current issues or segment brightness issues, and you don't have to bother with multiplexing.  And you can adjust the brightness by PWMming the CC transistor.

On the downside, you do have to produce the 2V supply, and the clock and data lines also have to be 2V.  We don't yet know how that was done.  And of course you have five HC164 chips.  If you multiplex by digit, you only need one, but then the brightness would be much lower.

I think today you can find efficient 7-segment displays that are bright enough when multiplexed by digit.  And even at 5V you can eliminate most of the resistors if you multiplex by segment.  But controlling the current by reducing the power supply voltage is still an interesting idea - one I hadn't thought of.

NivagSwerdna:

--- Quote from: Peabody on August 12, 2022, 02:37:42 pm ---...and the clock and data lines also have to be 2V.

--- End quote ---
According to the Nexperia version of the datasheet you can have the input lines at above VCC as long as current limiting resistors are used.

Kleinstein:
For the data signal a series resistor is no porblem. For the clock signal one has to be a bit careful, as many shift registers want a minimum slew rate to work reliable. A large resistor may slow down things quite a bit, and the clock signal goes to all chips and thus sees quite some capacitance. One may have to plan with a capacitor in parallel to the resistor for speed up.
Many modern µCs / FPGAs have no problem with a low supply and with some types even 2 V is OK.

Peabody:
I believe Dave's video showed both the clock and data lines were actually 2V.  Maybe they didn't have to be, but they were.

thm_w:

--- Quote from: Peabody on August 12, 2022, 12:42:39 am ---But these displays are tested by the manufacturer and binned according to the luminance they produce with a given current.  In the video it shows these are marked as H group parts, which is a middling category.  So if they always order H parts, the performance should be pretty consistent.

--- End quote ---

Given current and color bins, not given voltage.
But yes, it should be quite consistent.

Chance of biting you in the ass if this LED display went out of production and you needed to put in an alternate part.

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