A Discrete Transistor 7400 NAND GATE

If we did not have ICs then this is what a 7400 NAND gate would look like!

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The 7400 TTL (and LS-TTL) quad NAND gate is one of the most common integrated circuits (IC) around. Here is a discrete transistor version. To show that it works it is used as an LED flasher.

An LED Flasher driven by Discrete Transistor 7400 NAND Gates

Here is my LED Flasher based on a discrete transistor 7400 (note the built in power plug):

The basic transistor NAND gate is well known on the Internet. Here is my version:

The reason two gates in series is to make sure it the input and outputs are properly loaded for the simulation. Unfortunately it is also well known on the Internet that the simulation of these circuits is unreliable.

Like the real TTL gates, the supply voltage has to be 5 volts (or the design voltage) for best performance.

The 1 nF "speed up" capacitor is not absolutely necessary but does result in an approximately 50% speed improvement.

How fast is the above circuit? A real 5 stage ring oscillator gave an average gate propagation of 65 ns (about 10 times slower than a real 7400 gate).

The collector resistor is not critical but should not be very much less than the 680 ohm specified.

If a 5k1 ohm bias supply resistor (Rs) is selected (to approximately the input impedance of a real 7400 gate) then the recommended base drain resistor (Rb) is:


=1k2 ohm

The selected base resistor should not be very much less than calculated value.

(The above formula was the result of many many simulations and works well in practice.)

The LED Flasher

The LED flasher is very standard:

Here is the simulation:

So here is the discrete version:

And the discrete simulation:

The main problem is the large 50 uF capacitor. To get around this two 100 uF electrolytic capacitors were used back to back with protective diodes.

Finally, the value of the 10k resistors had to be reduced to 4.7k each for oscillation:

The simulation estimates a flashing frequency of about 1.7 Hz, the actual frequency was 1.2 Hz. So the simulation was pretty reasonable.

The discrete transistor 7400 Board layout

I used DIYLC for the board layout. My days of pencil, graph paper and an eraser are gone (it just takes too long). DIYLC needs some patience with the mouse as the software is slow but it is quite easy to use and very flexible. I tried a few board layout tools and this was the best. Best of all it is free.

I used Protostack's prototype board ( if you live in Australia). Its perfect for this application. Great quality and low cost. Unfortunately DIYLC does not have a prototype for it, maybe my next task is to set one up. I just edited a strip-board design in MSPaint to Match the prototype board:

Now finally the finished project:

Regards AlanX

  • BC548B 5 Stage Ring Oscillator

    agp.cooper08/02/2016 at 03:10 2 comments

    BC548B 5 Stage Ring Oscillator

    I dug up my old BC548B 5 stage ring oscillator just to compare with the 2N3904 version:

    As you can see the propagation delay is about 88 ns (where did I get the 64 ns from, now I remember, I bread boarded a set of BC547Bs recently to check the performace with and without the speed up capcitor) and a nicely formed wave form.

    The 2N3904 is faster but there is something else is going on?

    Thinking about this, the propagation delay of the 2N3904 transistors is most definitely much lower than the BC548B transistors. Note the distinct on and off periods of the BC548B and their practical absence for the 2N3904. The main problem here is the slew rate (the rise and fall times) is about the same for both transistors.

    I decided to increased the base drain resistor from 1k2 ohm to 1k5 ohm. The wave form is now both more symmetrical and faster (3 MHz or 33 ns propagation delay). Now increasing the base drain resistor turns on the transistor a little more when off so the output high voltage is now only about 4 volts (previously about 4.8 volts). This tells me I have pushed this configuration as far as I can without changing the bias feed resistor, the collector resistor or the supply voltage (to stay TTL compatible I don't want to change them):

    How good is this simple circuit? A 7400 (TTL) has a nominal propagation delay of 10 ns.


  • PCB Isolation and D-Latch/Flip-Flop performance

    agp.cooper08/01/2016 at 13:35 0 comments

    Well I am going to throw away those cheap Chinese PCB boards away. One mistake and the tracks lift. Horrible! But I did get the board finished and tested:

    A respectable 42 ns propagation delay but not that much better than a BC548B (64 ns). It falls short of my expectation of 15 ns (as reported by others).

    I think the base drain resistor probably needs adjustment. However, I don't think the board will survive a couple of rounds of resistor swaps.

    I think I would be better off soldering the link wires (the rows of holes above the pin header) to convert the board into a D-Latch/Flip-Flop and testing that and then make a new board out of a better quality PCB.

    In case your wondering how this fits in the with TTL: the above board is currently equivalent to two 74LS10. After the link wires are added it is 1/2 a 74LS74.


  • After 35 years of service

    agp.cooper07/31/2016 at 06:58 0 comments

    Well I was complaining about my Weller soldering iron so it spat the dummy.

    Not really, but I went to plug it in and I got crackle and pop. Unplugging it and the fault was a broken power cable:

    My Weller cost $80 in 1981 so in today's money that would be in the order of $400 dollars. I remember that inflation was over 10%, you could get 13% interest at the credit union and a home loan was 17%!

    Now my Weller used to have a power switch (long gone), two metal trays (one for stuff and the other for a wet sponge) again, long gone.

    It sports a cable burn (hidden behind the power cable) and the power cable cover (PVC) has shrunk exposing the power wires (safe - no exposed copper).

    The case is somewhat aged, but the black bakelite iron holder and silicon flexible cable are as good as new.

    The soldering tip was replaced (once) after about a month due to using the wet sponge, now I just poke the tip through some paper to clean it and it has lasted 35 years (go figure?).

    So I will have to replace the plug and bring it back to life.

    The Weller will still need an offsider for the fine work.

    In the mean time you will have to wait for the speed test and D-Latch/Flipflop build (assuming you are interested in this thing).

    One guy has messaged me to said that the six transistor (DTL) version of the D-Latch works and I note Johnny Lovqvist has gone with the six transistor design for his Discrete Transistor 8080 CPU. So it must work!


  • Testing Propagation Delay

    agp.cooper07/31/2016 at 01:32 4 comments

    Testing Propagation Delay

    I have previously mentioned propagation delay tests using a 5 stage ring oscillator.

    So what is that?

    A 5 stage ring oscillator (

    Here is my implementation:

    Note: I have not made provision for speed up capacitors in this design but you can work it out.

    So to measure propagation delay measure the frequency and then divide the frequency period by 10.

    Now you can easily use bread board here if you don't like strip-board.

    I used a ring oscillator to check if my base biasing formula worked (and it did):


    Where Rb is the base drain resistor (1k3 ohms in the above design) and Rs is bias supply resistor (5k1 ohms in the above design).

    Nowadays I would use 1k2 ohms and 4k7 ohms as these are easy to get in bulk on Ebay.

    If you use a one diode design (2 input NAND gate) then use Rb=0.72*Rs/(Vcc-1*0.62-0.72):

    If your interested in a really fast and minimal design consider Johnny Lovqvist's design:

    The only down side with this design is the reduced noise margin (which is a worry if used to build complex circuits).


  • A PCB Isolation PCB for a D-Latch/Flipflop

    agp.cooper07/30/2016 at 03:50 0 comments

    PCB Isolation Failure

    Well I had a few hour too myself (my partner is working a double shift tonight - best not to run my CNC machine wile she is at home) so I found the files and knocked up the gCode file for the D-Latch/Flip-Flop PCB. Double the design up for two boards. Just after I started I realised I had not offset the engraver tool (my macro traces the exact outside of the PCB image). So the tracks and pads will be smaller than planned. Too small to be practical and I scrubbed one off while cleaning the board:

    I had some other problems that I have not quite figured out. How to mount the board without it bowing in the middle.

    I also need a new soldering iron with a smaller tip. My old Weller is 30 years old, electronics has shrunk in that time but not the old Weller.

    I am not going to waste my time with these boards.

    Another Attempt

    Here is the updated design. This time I added a clamp area left and right, and the isolation is properly offset (~0.1mm):

    Well it is done. Found a few more set up issues that took some time to resolve:

    This cheap Chinese PCB (about $0.50 per 3" x 4") is a bit fragile. May be better to pay a bit more money for better quality.


  • Discrete Dual Quad NAND using PCB Isolation

    agp.cooper07/30/2016 at 03:42 0 comments

    PCB Isolation

    I recently revisited the design using a PCB rather than a prototype board.

    Here is board after milling:

    Here is the complete board with eight NAND gates:

    Want to build a Discrete Transistor CPU?

    If you have ever contemplate building a transistor CPU, one approach is to design simple building block.
    Here is a six 3 input nand gates that can be configured as a D-Latch or a flipflop.
    You will need about 150 of these as a minimum (about 800 for an Intel 8080).

    A 6 gate D-Latch

    Hers is my board design (the components values are as discussed in this page):

    If you have not realised the top red links are actually wire links (the board is actually single sided).

    After cruising the Internet another "transistor logic" guy has build one of these (I can't re-find the webpage) and says that it does not work and recommends the 9 gate version. That suggests I will have to build a couple to check.

    Another "transistor logic" guy says the 2N3904 is much faster (x4) than the BC548. So why not go for a 2N3904 version (its a common garden variety).

    Just waiting now on delivery of a resistors/capacitors and transistors.

    Just have to check the 2N3904 pin-outs before finalising the board.

    Regards AlanX

View all 6 project logs

Enjoy this project?



Dr. Cockroach wrote 12/24/2016 at 01:30 point

I like what I see here. I am fairly new to discrete transistor gates but learning a lot with my own build and lots of help from others.

  Are you sure? yes | no

agp.cooper wrote 12/25/2016 at 05:46 point

Hi Mark,

If you are interested I have a PCB at EasyEDA ( for a 9 x 3 input NAND gate.

Also a D-Latch FlipFlop (

You can get 30 PCBs for US$21 and about $19 postage (from EasyEDA - I don't get anything).

These two PCBs make up the basic building blocks for a DTL computer.

One day I will get around to finishing the design and building it.

Regards AlanX

  Are you sure? yes | no

Andrew Starr wrote 07/29/2016 at 04:05 point

The 6-gate flip-flop works fine, the 7474 uses this arrangement. I'm also using it in my discrete transistor project.

  Are you sure? yes | no

agp.cooper wrote 07/29/2016 at 06:22 point

I did read one guy had problems with his DTL version. But I did suspect the problem may have been somewhere else. I have all my components now so I should make an effort to knock two of them up to see if my version works. Basically this is a building block for a discrete CPU, but the prospect of 150 of these scares me!
Cannot afford to make too many mistakes.

  Are you sure? yes | no

Andrew Starr wrote 07/29/2016 at 07:27 point

Looks fine to me - just bear in mind that the BC548 and 2N3904 have opposite collector/emitter pins

  Are you sure? yes | no

agp.cooper wrote 07/29/2016 at 02:32 point

Ah yes, I have seen your work before!

  Are you sure? yes | no

agp.cooper wrote 07/29/2016 at 02:10 point

Yes correct, the transistors are limited by slew rate. I was targeting LS-TTL and TTL compatibility. Others have found the 2N3904 at least 4x faster than the BC548B I was using. I have yet to check. AlanX

  Are you sure? yes | no

agp.cooper wrote 07/29/2016 at 02:00 point

Yes, transistors are limited by slew.

  Are you sure? yes | no

matseng wrote 07/28/2016 at 15:50 point

I got considerably better performance in my tests when I lowered both the supply voltage to 2.4-2.5 volts and also had the pull-ups much lower than yours.  A reasonably value for the collector pullup is about 120 ohms and proportionally lower for the input pullup as well.

It will for sure be drawing a lot more power - but *that* much more since the supply voltage is halved...

  Are you sure? yes | no

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