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schematic

simulate this circuit – Schematic created using CircuitLab

I am trying to make a BCD to 7 segment decoder, but I can't make it work. I have a 7 segment with a common anode and I have connected a 1 kΩ resistor to each one of its segments.

I am using a 9V battery to power it.

  • How should I connect the DIP switch in the circuit?

Also, in the circuit, when I try to test my ICs (74LS32 and 74LS08), by connecting the ground to the inputs and output to a LED light, but it's not working properly.
All of my 7408 ICs are only producing a high output when both inputs from switches are off, and when one or two are on, it will have a low output, while my 7432 ICs are acting like AND gate (only high output when both inputs are high).
I have tested the same ICs on a digital trainer and it was working fine, but when I try to put it in my circuit, it's back to not working properly.
What could have I possibly done wrong?

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closed as unclear what you're asking by Marcus Müller, Edgar Brown, Sparky256, Elliot Alderson, Finbarr Jan 31 at 14:49

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ where does the DIP switch suddenly come from? Why 1kΩ? You're using ICs, but you're not telling us how you've connected them. So, I'm sorry, you'll have learn to use the schematic editor that is built-in to this very site in the question editor. We can't help you on a basis of "I can't show you what is connected, how it's connected, but it's not working, help!". \$\endgroup\$ – Marcus Müller Jan 28 at 14:38
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    \$\begingroup\$ We also need to know what you mean by "but it's not working properly". What did you do to the switches, what result did you expect, what result did you observe? \$\endgroup\$ – brhans Jan 28 at 15:11
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    \$\begingroup\$ I'm guessing that lack of pullup resistors is the problem. \$\endgroup\$ – Hearth Jan 28 at 15:13
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    \$\begingroup\$ Please add that information to your question; generally, add as much info as you can! \$\endgroup\$ – Marcus Müller Jan 28 at 15:17
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    \$\begingroup\$ Please specify: are 7408 and 7432 the exact markings on the chips, or do they say 74LS08/74LS32 or 74HC08/74HC32 or something else? \$\endgroup\$ – Hearth Jan 28 at 16:08
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From your schematic, it appears that you think that a TTL input that is not connected high automatically goes low. This is incorrect. A TTL input should never be left floating. For your circuit to work correctly, each input that is floating when its switch is open needs a resistor to ground. For standard TTL, the resistor must be 470 ohms or smaller. Common values are 220 ohms and 330 ohms.

Your schematic does notshow power and ground connections to the logic chips. Do you have the Vcc and GND pins connected?

AND

The TTL parts you mention cannot run on 9 V. They are strictly limited to 5 V supply voltage.

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  • \$\begingroup\$ Yes, my logic chips have their Vcc and GND connected. Thank you. I will change the the supply voltage to 5V and add resistors to the switches. \$\endgroup\$ – user75283 Jan 28 at 15:48
  • \$\begingroup\$ @user75283 Note that this does mean that your chips are fried since you connected them to 9V power, and you need to get new ones. \$\endgroup\$ – Hearth Jan 28 at 15:48
  • \$\begingroup\$ @Hearth is it possible for them to still work? I have tried them on a 5V power supply after connecting to the 9V and they are still working fine \$\endgroup\$ – user75283 Jan 28 at 15:57
  • \$\begingroup\$ @user75283 It's possible, yeah. I'm surprised they still work, though. If they're not actually the TTL version (74C series for example, but not 74 or 74LS or even 74HC) it's possible they could work on higher voltages and be fine. \$\endgroup\$ – Hearth Jan 28 at 16:07
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    \$\begingroup\$ The OP's schematic shows the switches connected to Ground/negative. If he is using 74LS parts, no resistors are required on the inputs, as an unconnected TTL input will appear as a High (but pull-up resistors of 5 K or so are still a Good Thing) \$\endgroup\$ – Peter Bennett Jan 28 at 16:40
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Like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Serial In line resistor packs exist to make this easy.

Note TTL logic 7432 and 7408 needs 5V and will be killed by 9 volts, but you already know that from the answer to your question about why your chips didn't work.

(If you're using real TTL and not modern HC CMOS versions you can omit the pull-up resistors R1-R4)

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  • \$\begingroup\$ R1 appears to be useless. \$\endgroup\$ – Hearth Jan 28 at 15:37
  • \$\begingroup\$ How do I know if the ICs that I am using are TTL or HC CMOS versions? \$\endgroup\$ – user75283 Jan 28 at 15:39
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    \$\begingroup\$ @Hearth it's exactly as useless as SW1, but I'm not aware of people making 4 pin, SIL 3 resistor packs. \$\endgroup\$ – james Jan 28 at 15:54
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    \$\begingroup\$ @james Fair point. For the sake of completeness (and because I went to look out of curiosity), they do actually make those, but they're certainly not a part you'd encounter very often, and probably no cheaper than having the extra resistor anyway. \$\endgroup\$ – Hearth Jan 28 at 16:13
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    \$\begingroup\$ Inputs on bipolar TTL parts (74xx, 74LS, 74AS, and others without a "C" in the middle) will appear as High with no connection, so pull-up resistors aren't usually required (but still Good Practice). CMOS parts (74C, 74HC, others with a "C") have very high impedance inputs, and REQUIRE a pull-up resistor to ensure the inputs appear as High with no other connection. It is important to specify the exact part you are using. Also, as mentioned, bipolar TTL has an absolute maximum voltage rating of 7 volts. Some CMOS versions can accept higher voltages. \$\endgroup\$ – Peter Bennett Jan 28 at 23:44
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Since you're having a bit of trouble putting your logic chips to work, I should point out that there's a good source of documentation for these guys: datasheets.

Google Search online for the codes written on top of the component and add the word "datasheet". Look for a PDF from a reputable IC manufacturer (preferably the one who produced your component).

For example, "74LS32 datasheet" leads me to a PDF from Texas Instruments that says:

absolute maximum ratings (...)
Supply voltage, VCC (see Note1) ... 7 V

And later on:

recommended operating conditions
Vcc Supply voltage ... 4.5 ... 5 ... 5.5 (min nom max)

That says that, if your IC has 74LS32 written on it, a 9 V power supply isn't a good thing.

And no, I'm not telling you to RTFM, it's just that datasheets are your friends. =)

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