# Display circuit - readjusting for a single supply voltage?

#### metermannd

Joined Oct 25, 2020
83
Looking at the display circuit, and am wondering if it would be a good idea to tweak the circuit to run on 5V only?

The original design used these now obsolete parts: U14 (MC14547), U13 (UDN2981), and U1-U10 were MAN74A.
I'm also considering a minor re-spin to use the smaller LSHD-7803 display.

Thoughts?

#### dl324

Joined Mar 30, 2015
12,255
The label on the power pin for the anode driver makes me think that it needs to be operated from a 9-12V supply. If that's the case, you'll also need to change the anode drivers.

With the Darlington cathode drivers, the current limiting resistors are going to be significantly smaller.

Those considerations may be offset if you use a higher brightness display and operate at a lower peak current.

#### metermannd

Joined Oct 25, 2020
83
All things considered, I'll probably leave the overall design intact, other than changing the LED pairs to bi-color LEDs to reduce parts count.

Particularly now that I can see how I can tap one existing board for a reliable 12V source (and get a more direct 5V feed than using ribbon cable).

#### kirkmans

Joined Mar 13, 2011
1
UDN2981 is now available as MIC2981 from DigiKey, and probably others.
74LS47 is available from DigiKey

#### metermannd

Joined Oct 25, 2020
83
kirkman: the MIC2981 is almost $3 per, while a replacement recommended on another thread here, the TBD62783A, is just a hair over$1 each. I do thank you for the suggestion, though.

#### metermannd

Joined Oct 25, 2020
83
The label on the power pin for the anode driver makes me think that it needs to be operated from a 9-12V supply. If that's the case, you'll also need to change the anode drivers.
The data sheet says it can do -0.5V to 30V DC, so I should be able to just swap the VCC connections from 12 to 5, then.

I'll compare the current requirements for the original UDN2981 and MAN74 display and compare it to the TBD62783A / LSHD-7803 combo.

Edit: Moot. Study of the power supply board reveals 12V was likely chosen as the 5V supply and its traces are relatively inaccessible for modification.

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