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Replacing TTL logic with discrete components

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neazoi

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page 4 of this article shows the operation of IC3.
I would like to replace this IC with discrete components.
I have tried my gates **broken link removed** but they do not work as they rely on DC input.
Can you please help me what to do?
Thanks
 

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  • Article-6digitFreqCounter-SCaug01.pdf
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I don't see any reference to discrete design of ST-NAND gate or generally ST gate in the link. What do you mean?

The basic 74132 function used in the circuit is an inverting ST gate. I would rather refer to a generic ST circuit than specifically discrete TTL modelling. Are you really out of reach of any ST gate IC, e.g. 74HC14?
 
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    neazoi

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A logic gate can be seen as a half-bridge which responds with high gain. Therefore you can use an NPN & PNP. (Place the PNP at top so you get greatest sensitivity.) Or use a sziklai pair, or darlington arrangement.

Often you can substitute a class A amplifier. Since it inverts the signal it makes a convenient 'NOT' function.

IC3 contains the gates which create the sequence of Count/Latch/Display signals. RC networks create small delays. If you find that an RC network yields sufficient voltage swing, then you do not absolutely need a logic gate to square up the waveform, but can use the class A amplifier instead.

This gating arrangement became a bottleneck when I built my 6-digit frequency counter. I could not get it working properly. I had to set the project aside until I got an oscilloscope. Then I found out where I had made wiring mistakes. Then I was glad to finish the project.
 
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    neazoi

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I would do something like this. And perhaps add a stage with another transistor to invert the signal at the end. (I posted an application (flash swf) that calculates the resistors in a website designed by me, but I don't know if I can post the link here. If you know I can, let me know)

**broken link removed**
 
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    neazoi

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I would do something like this. And perhaps add a stage with another transistor to invert the signal at the end. (I posted an application (flash swf) that calculates the resistors in a website designed by me, but I don't know if I can post the link here. If you know I can, let me know)

**broken link removed**
It does not open...

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A logic gate can be seen as a half-bridge which responds with high gain. Therefore you can use an NPN & PNP. (Place the PNP at top so you get greatest sensitivity.) Or use a sziklai pair, or darlington arrangement.

Often you can substitute a class A amplifier. Since it inverts the signal it makes a convenient 'NOT' function.

IC3 contains the gates which create the sequence of Count/Latch/Display signals. RC networks create small delays. If you find that an RC network yields sufficient voltage swing, then you do not absolutely need a logic gate to square up the waveform, but can use the class A amplifier instead.

This gating arrangement became a bottleneck when I built my 6-digit frequency counter. I could not get it working properly. I had to set the project aside until I got an oscilloscope. Then I found out where I had made wiring mistakes. Then I was glad to finish the project.

Is this YOUR design???

There are some errors in the circuit (pin numbers on ICs and PNPs) but I corrected them. The counter works ok and I did not need the extra prescaler chip. But the NANDs could be replaced by discrete logic, so one IC minus, for a total of 6 ICs. Not bad for a MCU-less counter.

I wonder why not just use a transistor amplifier? It is an inverter if the signal is taken from the collector of an NPN. I am confused a bit with the capacitor pulse thing why is a pulse needed instead of a square wave.
 

You don't yet pay attention to the fact that IC3 is a schmitt-trigger gate, which is required to generate proper pulses. Using simple transistor circuits without internal positive feedback may result in malfunction.
 
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    neazoi

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You don't yet pay attention to the fact that IC3 is a schmitt-trigger gate, which is required to generate proper pulses. Using simple transistor circuits without internal positive feedback may result in malfunction.
Yes I noticed that but I thought it would not be that important in this application. With a single transistor inverter I have managed to make it count but what happens is that the display is updated as the counter counts. Then it stops and shows the right frequency. This cycle repeats after 2 seconds.

The difference from the chip is that with the chip, the count process is not displayed and the count value is kept on screen until it is updated.

But I am pretty sure it can be done with discrete components, it does not seem that difficult in theory.....

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Hm... I see from the waveforms in the article that the gates outputs are not pulsed but they are square, whilst they are driven by pulses. Am I right?
By pulses, I mean the capacitor output spikes.

Then maybe this discrete monostable multivibrator trigered by pulses is the answer https://www.electronics-tutorials.ws/waveforms/monostable.html

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But in the case of the IC, how is the output pulse time determined?

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Hm... now that I am thinking of it, there is no timing in the case of the IC. It is the schmitt-trigger that takes care of the output pulse widths.
I wonder if a TTL levels-compatible schmitt-trigger can be made
 

The pulse width is obviously set by the RC circuits.

I didn't check in detail if the circuit operation depends on having proper square waves ("pulses") for the both mono flop outputs, but malfunction must be expected. By datasheet specification, logic ICs need digital input signals with a certain maximum rise time.

Redesigning the circuit with different components requires much deeper comprehension than just reproducing it. The objective of using discrete transistors for a small part of the CMOS IC counter design hasn't been yet revealed, by the way.
 

Is this YOUR design???

No, I followed a similar schematic in Forrest Mims Engineer's Notebook. (I posted a link in your other thread.) Purchased at Radio Shack in 1980 ($1.99). Schematics for dozens of popular IC's. An education in itself. Copies are on Ebay at 20 times original cost.

My counter updates every second. For some reason your article talks about every 2 seconds, which seems unreasonable.

There are some errors in the circuit (pin numbers on ICs and PNPs) but I corrected them.

Then you are experienced enough that you should have an easy time completing your project. I too had to puzzle over similar errors in the book.

The aim of the logic gates & RC networks, is to produce the timing signals as shown in the article. It's similar in the Forrest Mims book. They need to be long enough to activate Latch/Count/Display, but they also need to be brief because they are part of the update process which can miss a few cycles if you're counting a very high frequency. To get a close look at the pulses, set up your oscilloscope to trigger a sweep at the first pulse, and you should see the second pulse after a few millionths of a second (I think). Etc.
 
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