I need some practical advice on how to combine different component types.

I designed a circuit with at the heart an up/down counter (74f269) The outputs of the counter are connect to via 330 ohm resistors and leds to ground level. They also need to drive a relay-driver. Furthermore these outputs are connected to a 74ALS11N (3 triple OR-gate) and 74HC4075 (3 triple AND gate). The OR gate yield Q0 v Q1 v Q2 v Q3 v Q4 v Q5. The And Gate Q0 & Q1 & Q2 & Q3 & Q4 & Q5. The outputs of the OR's and AND's are each fed to 74HC00 (NAND).

The NAND gates do an AND operation with the value and that of an NE555 that is used as debounce for a switch. All components are fed from a 5V voltage source.

I read about combing different logic families and am a bit confused about when to use resistors and how many:

  • I read connecting TTL to CMOS I need a pull up resistor. The 74ALS11N output connected through a load to ground (pull down). Can I still connect the AND and OR gates to the outputs of this counter?
  • Am I correct to assume that I do not need a resistor when an output of a gate is connected to an input of a gate of the same logic family, both for gates in the same chip and in another chip. Nor do I need resistor when CMOS of one type is connected to CMOS of another.
  • One NAND drives both U/D of the counter an input of an XOR-Gate (HEF4075BP) do I need resisors and if so pull up or down? Also: can the 74HC00 drive both the input of the counter and XOR gate?
  • Do I need a resistor for the output of the NE555 to the NAND-gate?


As requested in the comments below, I added the schema of the circuit. It's a DIY project so bear with me if the schema is not up to par with industry standards. The strange looking GND and 5V connections on each chip and the bridge were introduced, because Eagle could not route a single sided PCB. I'll place the soldering pads around the circuits an then connect the GND and 5V of the chips with wires. The functional purpose of the circuit is to count down and up (6 bits) upon button pushes and have underflow and overflow protection. schema

  • \$\begingroup\$ If you use the data sheet specs , can you not figure out the answers to figure out the analog margin to logic levels? Then figure out Zout ~ Vol/Iol for nominal with tolerances and Zin (TTL)=Vil/Iil \$\endgroup\$ Feb 19, 2021 at 21:13
  • \$\begingroup\$ It would be useful to post a schematic. \$\endgroup\$ Feb 20, 2021 at 0:04
  • \$\begingroup\$ These discrete logic chip can typically safely driving 10 input. No resistor is needed and don't worry. \$\endgroup\$
    – Willis Lin
    Jun 29, 2023 at 16:51
  • \$\begingroup\$ What is the point of the exercise: Why recreate the terminal count for both 000000 and 111111? Loading 0, you can use the '269's internal TC logic&output. \$\endgroup\$
    – greybeard
    Jun 30, 2023 at 7:09

1 Answer 1


You do not need pull up resistors when driving CMOS inputs with TTL outputs unless the TTL outputs are specifically open collector. (check the datasheet)

It should not be a problem to connect an output to multiple inputs as long as the input currents don't add up to more than the output can handle. (check the datasheet) In most cases, the input current will be under 50uA for HIGH and under 1mA for low. You do have to be careful if you are adding external pull-ups to not make the outputs sink too much current in the low state.

You do not need resistors when connecting TTL to CMOS or the converse as long as the Vcc is the same on all devices. The reason that you might want to have a resistor is when you are driving some of your CMOS at higher voltages than the 5V limit of TTL.

The NE555 has a TTL compatible output which can sink or source 20mA. There is not problem with driving multiple gate inputs directly. (check the datasheet since there are some low power versions of the 555 that may not comply with this statement)

The 74F269 will function better driving LEDs as a sink rather than a source. If you look at the IO Loading Table you'll see that the outputs are only rated to 1mA in their HIGH state, but they can sink 20mA in their low state. In the current sink configuration you would connect the cathode of the LED to the output and the anode to your current limiting resistor and the resistor to Vcc. This will allow you to run the LEDs at a much higher brightness than in the source configuration. The only issue here is that the LEDs are active low.

Make sure you don't mix up the AND (74LS11) and OR (74HC4075) in you circuit like you did in your question.

One problem that can be encountered when mixing TTL and CMOS is when running circuits at high speed, the threshold value differences can cause timing errors which may result in transient invalid outputs.

  • \$\begingroup\$ Thanks quick for the reassuring answer. Also thank you for pointing out the error regarding the AND and OR gate in my question. Luckily it's wrong in my description, and not in my schema. I had a test setup with the counter and the leds, and they were bright enough to be used as indicators in a panel. This makes it simpler because I would not have liked to add NOT-gates as I want the leds to be on when the relay is. \$\endgroup\$
    – dre
    Feb 20, 2021 at 11:21
  • \$\begingroup\$ Are you planning to drive LEDs and relays from the counter outputs? What is the input current on the relays? It's typically at least 3mA for SSRs. You can also drive the relay as active low exactly like the LEDs. As long as your LED and relay inputs combined don't pull more than 13mA, you should be okay. That timer chip shouldn't source or sink more than 100mA (datasheet says max sink is 135 but recommends 113 but it's good to leave a little headroom). One more note, I don't see any bypass caps in your schematic. Those are important so you don't get switching glitches in you circuits. \$\endgroup\$ Feb 20, 2021 at 21:03
  • \$\begingroup\$ I planned to run them through a ULN2004A. They need to be driven from a 12V source, and zero up 6 are closed at the same time. Thanks for the instruction to add the bypass capacitors. I'll add them \$\endgroup\$
    – dre
    Feb 20, 2021 at 21:10
  • \$\begingroup\$ Make sure you look at the peak current vs duty cycle on those ULN2004s. They boast 500mA per channel, but that's not for 100% duty cycle. If any relay in on for more than 1-2 seconds, I'd consider that 100%. In this case your maximum current per channel is going to be considerably less than 500mA (for the ST SO-16 package, it's under 100mA for all 7 channels running at the same time). \$\endgroup\$ Feb 20, 2021 at 21:27

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