Electronics > Projects, Designs, and Technical Stuff
Allegedly a TTL clock (build log)
harnon:
Hi all,
I'm building a TTL clock as a gift. The basic idea is to have LEDs to represent the "hands" of the clock. One set of LEDs will show 5 minute intervals and another the hours. Aesthetics is quite important so I'm putting a lot of thought into that as well.
At this stage I'm trying to get the circuit right and would appreciate any feedback as this old school TTL stuff is fun, but a bit overwhelming for a newbie - lots of traps for young players.
The basic idea is to divide down a 32768 Hz oscillator to count out 5 minute and one hour intervals using using 74HC4040 and 74HC161 chips. Later I have 74HC238s in 3-to-8 configuration to drive the LEDs (via transistors which are on one external boards connected via J1 to J12). There are debounced momentary switches to skip forward 1 hour or 5 minutes per press.
Lots of questions, sorry...
1) I don't even know what I'm doing with the crystal, is this even a circuit? I copied the circuit from another TTL clock design I saw, but is there anywhere that explains how to properly wire one of these up?
2) I was thinking replacing some logic gates with diode logic gates (for aesthetic reasons) are there any advantages/disadvantages vs the TTL chips?
3) I was thinking battery powered, but read in a few places that these TTL chips can be quite power hungry. I'll check when I breadboard, but is this kind of thing likely to require a wall wart?
4) Do I need diodes before the momentary switches, i.e. to prevent current flowing back into U6A / U1E from SW1 when it is pressed?
The next step for me once I'm happy with the schematic is to buy some components and bread board it up. Thanks!
mc6800:
I'm not convinced that the clock will oscillate. :-\
AIUI, the idea with CMOS oscillators is that you bias an inverter into its "linear" region with a feedback element such as a resistor. The crystal + capacitors then "nudges" the circuit to oscillate at the desired clock frequency. At one point Intel had an excellent App note explaining this, but as I can't locate it, here is one from Maxim:
https://www.maximintegrated.com/en/app-notes/index.mvp/id/2154
1) It looks like the feedback from the output pin is missing.
<pedant on> 74HC isn't TTL, it is CMOS. TTL is plain old 74/74LS etc. 74HCT is also CMOS, but with I/O levels better suited to interfacing with "old" TTL<pedant off>
2) Diode gates work, but require one diode per input, plus one resistor. You need to choose a suitable output resistor for the speed of input/output, based on the capacitive loading of the output. You also lose 0.3-0.6V of noise immunity. For example say the input to a diode "or" is 2.9V, the output might only pull up to 2.3-2.6V (as the diode will have a voltage loss) - meaning that the next input is more prone to responding to "glitches". Given that you can get single gates in ICs these days, I'm not sure if diode gates are as useful as once they were.
3) TTL is relatively power hungry (although 74LS is less so than 74 for example), CMOS isn't (except at high speeds - 32KHz isn't). The power hungry items are the displays. For example, I have a fancy wall clock with 12 x 7-segment displays for time etc. It draws about 20mA with no display, and 120mA when the displays are lit
If you haven't already, I would suggest reading a copy of "The Art of Electronics".
mc6800:
Found a link to the Intel App note on oscillators (155):
https://www.eevblog.com/forum/beginners/microcontroller-oscillator/
harnon:
Hey thanks for the reply. Funnily enough I have a copy of the Art of Electronics about 30cm from my hand. Just didn't think to look up something in a book :palm: I'll read through those app notes. Also, I did say "allegedly" a TTL clock so I think I'm covered CMOS wise :)
I'd be using the diode gates purely for the aesthetics of a bunch of them lined up in rows but as I'm planning a pretty weirdly routed board, noise immunity might be a thing. The whole clock is going to be mounted on a 300 year old oak base so I don't want to waste the timber with a shoddy looking PCB!
My display will literally be 3 LEDs at a time so hopefully power consumption will be ok. I guess I'll breadboard it and see before I commit to a power supply. Debugging it is going to be interesting as I only have a multimeter lol. Might have to re-purpose an Arduino Due to record the signals.
klunkerbus:
I only glanced at the schematic, but I happened to notice that the output of the spare AND gate at U2B is tied to ground. There shouldn't be any connection made to that spare output. With all the inputs tied high, that gate will be attempting to drive the output high.
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