Discrete transistor NOR logic clock

[TomS_]

Hi everyone,

Recently I saw a post about a game that could be played on an oscilloscope, and although I am not one for self promotion, it did inspire me enough to share a project of my own that I worked on - I guess because it is also "hand built" to a similar degree.

As the title suggests, it is a clock made out of nothing but NOR logic (ok, and inverters), and fully discrete - not a single IC to be seen (unless the linear regulator can be considered an IC, but maybe not in the "traditional" sense like logic gates).

As I was building it I was also blogging about it at https://ornotblog.blogspot.com, and if you dare try build one yourself, I have also uploaded all of my schematics to github: https://github.com/tomstorey/discrete-nor-logic-clock

I havent done a full and proper count yet, and there is still one more module that I want to build for it, but some fun facts:

* Over 1500 transistors (2N7000 MOSFETS specifically)
* 660 odd NOR gates/inverters
* Nearly 100m of kynar insulated wire
* 5 months construction time (as I said, if you dare)
* Handles leap years, but it doesnt count years so you have to tell it when the current year is a leap year
* Clocked from AC mains frequency

Its fully complete now, and working great, but still feel free to share any insights in to how I could have done things better/differently - Im sure there are plenty of goofs in there - it would still be interesting to see if there is anything I can learn.

[rstofer]

That's a magnificent project!

[David Hess]

I agree with rstofer; it is magnificent.

Bipolar RTL logic would have required fewer and less expensive transistors (and more resistors) but the simple 2N7000 NOR gates are probably more reliable do to higher noise margin.  I might have tried a discrete version of bipolar IIL (integrated injection logic) with added emitter degeneration resistors to make up for Vbe mismatch in discrete parts and perhaps other improvement but it would be more complicated yet.  Being current driven, IIL could operate without a regulated supply and with some cleverness, the current of all gates can be adjusted for minimum power at a given performance.

The integrated voltage regulator could be replaced with a discrete version made up entirely of 2N7000s including matched pairs of 2N7000s for differential amplifiers.  High voltage bias for the n-channel MOSFETs could be provided by a 2N7000 (or 2N7000 NOR gate) astable multivibrator driven charge pump.

[james_s]

That's awesome, I love this sort of absolute madness, duplicating a $10 digital clock with hundreds of dollars worth of discrete components that takes countless hours to build   

I sometimes wish I had the time and patience to do something like that myself.

[David Hess]

Unlike when I was selecting parts, 2N7000s are a reasonable price now at $82 per 1000.  But bipolar transistors are less than half that price.

[floobydust]

Nuts and Volts Magazine Transistor Clock
194 transistors, 566 diodes, 400 resistors, and 87 capacitors. Flip-flops instead.

[amyk]

How many of the MOSFETs did you have to rework due to ESD damage?

If I were you, I'd definitely use bipolar parts.

It's always interesting to see the discrete equivalent to what would fit in a < 1 mm^2 IC, to fully realise how big the difference in scale is.

[TomS_]

Thanks for the comments so far, and to address some:

Thanks! It was a fun project, although I was glad to see the end of it. It sucked up probably way too much of my life, so Ive got it to a place where I can use it, and the last thing I need to build for it is something to enable the display to be dimmed between certain hours. Im slowly mulling over ideas of how to implement this. 

I bought a reel of 2000 qty 2N7000's from RS, I think it cost about £83. That was by far the biggest cost of the project. The LED displays would have likely been the second biggest cost, followed by the remaining passives.

I am aware of the Transistor Clock, I believe I actually link to it somewhere towards the start of my blog and explain why I didnt want to build that one in particular, although it is very impressive and I do like the look of it and kind of do want one . Besides that reason, I wanted to design and build something myself, from scratch. :-)

No MOSFETS were harmed by ESD during the construction of this clock. I used an anti-static wrist strap while handling them - didnt want to have to go fishing one out.

[Cyberdragon]

How are you getting the seconds for the clock from the UK 50Hz mains?

EDIT: Reworded to make more sense.

[David Hess]

How are you getting the extra 10Hz for the clock from the UK 50Hz mains?

The line frequency prescaler divides by 50 instead of 60 that would be used for a US clock.

https://ornotblog.blogspot.com/2017/05/clock-build-log-1-analogue-board.html

[TomS_]

How are you getting the extra 10Hz for the clock from the UK 50Hz mains?

The line frequency prescaler divides by 50 instead of 60 that would be used for a US clock.

https://ornotblog.blogspot.com/2017/05/clock-build-log-1-analogue-board.html

Yup. There are some notes in the schematic that explain how to adjust the circuit to work with 60hz. 

[Cyberdragon]

Surprised you actually answered my brain fart question where I couldn't fathom a clock having non-identical 6 and 10 registers for seconds and minutes.

Although, I can't seem to find where it says how to convert it to 60Hz on the schematic. (well, the pdf/github schematic)

EDIT: I assume it would be in the time module?

[TomS_]

Surprised you actually answered my brain fart question where I couldn't fathom a clock having non-identical 6 and 10 registers for seconds and minutes.

Although, I can't seem to find where it says how to convert it to 60Hz on the schematic. (well, the pdf/github schematic)

EDIT: I assume it would be in the time module?

You'll actually find the prescaler on the "analogue board". It's on the second page of the schematic.

It was a bit too much logic to include in the time module itself, no pun intended.

[floobydust]

Does it have battery backup of any sorts?
The 70's digital clocks I have use a 9V backup battery and one-transistor reference oscillator. The battery keeps things up running, but display is off. No hassle of setting the thing after a power failure.

I like the project, it must have been fun
Enjoyed the K-maps and their explanation, it was a good read on your blog.

I could not find the LED display part numbers, if you could add to the schematics.

[texaspyro]

I bought a reel of 2000 qty 2N7000's from RS, I think it cost about £83. That was by far the biggest cost of the project. The LED displays would have likely been the second biggest cost, followed by the remaining passives.

I got 30,000 of them for $10 off of Ebay.  I thought I was buying a reel of 3000,  didn't read the fine print and got 10 reels.  I hate it when that happens...   

[TomS_]

I got 30,000 of them for $10 off of Ebay.  I thought I was buying a reel of 3000,  didn't read the fine print and got 10 reels.  I hate it when that happens...   

Lifetime supply.

[TomS_]

Does it have battery backup of any sorts?
The 70's digital clocks I have use a 9V backup battery and one-transistor reference oscillator. The battery keeps things up running, but display is off. No hassle of setting the thing after a power failure.

I like the project, it must have been fun
Enjoyed the K-maps and their explanation, it was a good read on your blog.

I could not find the LED display part numbers, if you could add to the schematics.

Thanks. 

No battery back up. The power here in London has been rock solid for the 4-5 years I have been living here, I think there has maybe been one blackout in that time. So I didnt bother with any kind of backup, and if the power goes out I will just re-set the time on it.

I posted another entry to my blog today going over the goods/bads etc of the project, along with a BOM which has all of the parts listed: https://ornotblog.blogspot.co.uk/2017/10/clock-project-debrief.html

The part numbers may be buried in a blog post somewhere, but to save going through them they are SA08-21CGKWA for 7 seg and PSA08-12CGKWA for 16 seg. I'll add them to the schematics as well.

[David Hess]

The backup battery would be unreasonably large to support the power hungry open drain logic using 2N7000s.

That is one reason I would consider bipolar injection logic which can be tuned for minimum power but it would require more than four times as many resistors and more than twice as many transistors albeit less expensive ones.

[james_s]

The clocks usually keep terrible time running from the battery anyway, and the battery is usually dead by the time a power outage occurs. I modified my digital clocks with a resistor to trickle charge a NiCd 9V and that saves the alarm settings and is always charged, but I still usually have to adjust the time a bit after an outage lasting more than an hour or so. Easier to just make the setting interface easy to use.