Electronics > Projects, Designs, and Technical Stuff
Allegedly a TTL clock (build log)
Cliff Matthews:
I'd add granularity with a 9-bit R2R off U7 and a LM3914 in dot-mode. Adjust input trimmer so the dot moves each ~36 sec (some may protest, but IMO it's permissible to toss-in another vintage era chip).
harnon:
Thanks for the replies! The schematic in the first post is a bit out of date, I haven't uploaded a new one yet.
--- Quote from: David Hess on April 25, 2019, 09:57:44 pm ---It is missing the connection from the gate output to the two resistors. 32kHz crystals have special drive requirements which may not be suitable for TTL.
--- End quote ---
Yes, I'm still a bit nervous about the clock signal and its one of the things I need to test. The circuit changed a little from the schematic above following the earlier feedback. As a fallback I'm thinking about just throwing on an RTC with a 32kHz output pin (i.e. DS3231?) if things don't go well, although I haven't read any datasheets yet so I don't know how workable this is. I've seen another post on here talking about PCB considerations so it might need some adjustments to the PCB if I go this route.
--- Quote from: David Hess on April 25, 2019, 09:57:44 pm ---battery power / diodes
--- End quote ---
I've given up on the idea of running from battery but my feeling is (as I'm not using particularly high power LEDs) that the 74HCs will draw far more current than the LEDs. I can't do any current measurements at the minute due to a faulty multimeter - I'm waiting on a new one to ship. Similarly I decided not to use diodes and most of these type logic gates I'm using 74HC chips. I did breadboard up some diode AND gates but found it wasn't very reliable.
--- Quote from: David Hess on April 25, 2019, 09:57:44 pm ---If I wanted to make a cool clock and limit the power, I would use injection logic using discrete transistors.
--- End quote ---
--- Quote from: Cliff Matthews on April 26, 2019, 12:16:53 am ---I'd try some granularity with R2R and a LM3914 in dot mode (some may protest, but IMO it's OK to add-in another vintage chip)
--- End quote ---
Interesting ideas and some new things to read up on :) I'm fairly set on the TTL approach for this project but I'll definitely add these to the list of things to try in future projects :)
David Hess:
--- Quote from: harnon on April 26, 2019, 01:35:57 am ---
--- Quote from: David Hess on April 25, 2019, 09:57:44 pm ---It is missing the connection from the gate output to the two resistors. 32kHz crystals have special drive requirements which may not be suitable for TTL.
--- End quote ---
Yes, I'm still a bit nervous about the clock signal and its one of the things I need to test. The circuit changed a little from the schematic above following the earlier feedback. As a fallback I'm thinking about just throwing on an RTC with a 32kHz output pin (i.e. DS3231?) if things don't go well, although I haven't read any datasheets yet so I don't know how workable this is. I've seen another post on here talking about PCB considerations so it might need some adjustments to the PCB if I go this route.
--- End quote ---
74L (low power) gates were commonly used for crystal oscillators but 32 kHz is a different matter. A single transistor oscillator is feasible. HC logic can be used although HCU (unbuffered) may be necessary or more reliable. 32 kHz crystals are kind of weird.
--- Quote ---I've given up on the idea of running from battery but my feeling is (as I'm not using particularly high power LEDs) that the 74HCs will draw far more current than the LEDs.
--- End quote ---
HC (HCMOS) will draw very little power because of the low operating frequencies involved. HCMOS or CMOS would be suitable for a battery powered design.
Note that HCMOS (HC) does not have TTL compatible inputs. There is a variation with asymmetrical gate voltages called HCT or HCMOS TTL compatible. It is only required when interfacing to TTL levels but otherwise performs identically to HC.
harnon:
Thanks again. Yeah I got corrected earlier on re HC not being TTL, hence why its "allegedly" a TTL clock ;D I'm learning (slowly).
Here is an early layout of the timber base that my Dad is building up for me. (EDIT: Its upside down)
Sapper:
Maybe I'm missing something or my observation has been rectified in the latest schematic but on the second 4040 producing the 5min output the text states bits 2,3,5 & 8 but the schematic shows Q1,Q2,Q5 & Q8 outputs going to the 74LS21 and gate. To me this lloks like you are feeding bits 1 & 2 rather than 2 & 3.
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