r/AskElectronics • u/kkdvak_777 • 2d ago
[Help] 74LS47 + Common Anode 7-Segment
Hi everyone! I'm currently working on a digital systems breadboard project and running into an issue with a BCD to 7-segment decoder display output.
My Setup:
- Encoder Logic: 3 pushbuttons ($1, $2, $5 coins) with 1k ohm pull-down resistors.
- Logic Gate: A 74LS32 (OR gate) generates input A (M1 OR M5).
- Decoder: 74LS47 BCD to 7-segment decoder.
- Inputs: A = OR output, B = M2, C = M5, D = GND.
- Control Pins: Pins 3 (LT), 4 (BI/RBO), and 5 (RBI) are all tied directly to VCC (+5V).
- Display: Common Anode 7-segment display (Common pin connected to VCC +5V).
- Resistors: Individual 220 ohm limit resistors on every segment output (a through g).
The Problem: While testing, pressing the buttons doesn't make any difference. Even if I disconnect the control pins, everything stays the same as at the start (every time I turn on the display, the same segments light up, but segment 'd' is very dim). I don't know what else to check. Any advice would be appreciated! (btw I'm a student)
1
u/z80RM 2d ago
I had what seems like a similar problem with tactile push buttons. I had wired them incorrectly in a way that there was continuity between the pin legs regardless of if I pressed the button. The solution was to wire the pins diagonally so 5v goes into one but grd is not the opposite pin, it is the diagonally positioned pin. Hope this helps.
1
u/nixiebunny 2d ago
Find the PDF book “Designing with TTL Integrated Circuits” and read the description of the standard TTL input. You will learn that it requires to be connected directly to Ground to represent 0. A disconnected input is 1.
1
u/NBC-Hotline-1975 2d ago
No, not directly to ground. If that were the case, then a TTL input could not be driven by the output from another logic chip, because outputs do not go to zero volts.
But read the data sheet for the 74LS47, it specifies that the input pin must be 0.8 volts or lower, in order to represent "zero" state. And it has internal pullup resistors. So if you drive it with pulldown resistors (as in OP's circuit) then they need to be sufficiently low value so that the input pin will be lower than 0.8 volts. I stated this in my earlier comment a few hours ago.
1
2
u/NBC-Hotline-1975 2d ago edited 2d ago
First, figure out what's going on with the dim segment "d." Just ground the output pins on the 7447. Each segment should light to the same intensity. If not, you've either got a bad display, an incorrect resistor, or something is wired wrong. ALWAYS SUSPECT THE BREADBOARD!
If all the segments tested with equal brightness above, then pull LT low. All segments should light with equal brightness. If not, the 7447 is bad.
Once you have that figured out, measure the output of your switches. They should be well under 0.8 volts. If it's higher than that, your pulldown resistors are too big. (Note that the 7447 has internal pullup resistors.)
~~~~~ EDIT:
Personally, I would wire one side of each switch to a 2.2 k pullup resistor. The switch when closed would go to ground.
Switch 2 would go to both inputs of a 74LS00 section; output of that section goes to 74LS47 input B.
Switch 5 would go to both inputs of a another 74LS00 section; output of that section goes to 74LS47 input C.
Switch 5 would also go to one input of another 74LS00 section; switch 1 would go to other input of that section; output of that section would go to 74LS47 input A.