Thank you Michael. I still have my uProf-B which I bought in 1981 or thereabout. Will you be able to sell me two of your new 7seg pc boards? I can buy them empty from you and then I will solder on the components. If you can sell then please let me know how we can pay you but please include shipping to me in Cape Town - South Africa. Thanks, Jose
Is there a schematic for your extra board? You talk a lot about the original circuit and how it works, but nothing about the add-on. The problem, such as it is, with the 75491 chips is the resistors between the four C pins (that's C for collector, these aren't enables) and Vcc. That's R5 in the case of U12 and the unlabelled resistor in the case of U15. These resistors, along with the unlabelled 10 Ohm resistors feeding the four C pins are what's limiting the current to the LED segments Reduce these for your blue LEDs and it will work. You say that there's only 2.3 Volts (I think) on the LED anodes, so what's the voltage on the C pins?
It has nothing to do with the resistors. Even if you replace these with 0 Ohm networks, these chips aren't able to drive blue LEDs. I tried - that's why I put in a socket for the resistor network in the first place ;) I tried this first thing (and I think I also mentioned that in the video?) - don't you think I would have done this first thing before designing an "active" PCB with the 74LS244? ;-) So no, just playing with the resistor values doesn't do it.
PS "Schematics" and Gerbers are here (well, I didn't put a lot of effort in the schematics, but it gives an idea). hackaday.io/project/183618-exploring-the-microprofessor/log/229422-the-worlds-first-mpf-1-with-a-blue-led-display
@@michaelwessel4953 Sorry, I'm not following. Your resistor network (RN1 on your schematic) is on the pull down (emitter) side of the 75491 output transistors. I was talking about the pull up (collector) resistors on the original PCB.
@Brian_Of_Melbourne forum.classic-computing.de/forum/index.php?thread/32631-bastelhilfe-gesucht-blaue-7segment-anzeigen-im-microprofessor-mpf-1/ Know that R5 is 0 Ohms already in the original design and on the board, forum.classic-computing.de/forum/index.php?thread/32631-bastelhilfe-gesucht-blaue-7segment-anzeigen-im-microprofessor-mpf-1/ and that I replaced RA4 with a 0 Ohm network as well. To no avail. So I think what you are suggesting doesn't work, and that's why I had to add the 24LS244 as an LED driver. No matter how you tune the resistors, the original circuit is not able to bring the voltages above 2.3 V or so. Insufficient for blue LEDs..
That's quite neat little piggy back solution, now you can try even different sizes and types of 7 seg displays.
MPF-1!!!
Blast from the pasts!!!
I remember that thing fondly back in my High School days.
Pimpin' the MPF-I... :) It came out quite nice!
... viel Aufwand für ein paar LEDs. Aber wieder eine tolle Arbeit von Dir. Allzeit interessant.
Thank you Michael. I still have my uProf-B which I bought in 1981 or thereabout. Will you be able to sell me two of your new 7seg pc boards? I can buy them empty from you and then I will solder on the components. If you can sell then please let me know how we can pay you but please include shipping to me in Cape Town - South Africa. Thanks, Jose
Please send me an email to lambdamikel@gmail.com - I'll be in touch. You can have one for 1 $, but postage will be a bit more I suppose.
Is there a schematic for your extra board? You talk a lot about the original circuit and how it works, but nothing about the add-on.
The problem, such as it is, with the 75491 chips is the resistors between the four C pins (that's C for collector, these aren't enables) and Vcc. That's R5 in the case of U12 and the unlabelled resistor in the case of U15. These resistors, along with the unlabelled 10 Ohm resistors feeding the four C pins are what's limiting the current to the LED segments Reduce these for your blue LEDs and it will work. You say that there's only 2.3 Volts (I think) on the LED anodes, so what's the voltage on the C pins?
It has nothing to do with the resistors. Even if you replace these with 0 Ohm networks, these chips aren't able to drive blue LEDs. I tried - that's why I put in a socket for the resistor network in the first place ;) I tried this first thing (and I think I also mentioned that in the video?) - don't you think I would have done this first thing before designing an "active" PCB with the 74LS244? ;-) So no, just playing with the resistor values doesn't do it.
PS "Schematics" and Gerbers are here (well, I didn't put a lot of effort in the schematics, but it gives an idea). hackaday.io/project/183618-exploring-the-microprofessor/log/229422-the-worlds-first-mpf-1-with-a-blue-led-display
PS2 The resistor array I currently have installed is actually a 0 Ohms one! You can see from the video that this doesn't do it.
@@michaelwessel4953 Sorry, I'm not following. Your resistor network (RN1 on your schematic) is on the pull down (emitter) side of the 75491 output transistors. I was talking about the pull up (collector) resistors on the original PCB.
@Brian_Of_Melbourne forum.classic-computing.de/forum/index.php?thread/32631-bastelhilfe-gesucht-blaue-7segment-anzeigen-im-microprofessor-mpf-1/
Know that R5 is 0 Ohms already in the original design and on the board,
forum.classic-computing.de/forum/index.php?thread/32631-bastelhilfe-gesucht-blaue-7segment-anzeigen-im-microprofessor-mpf-1/
and that I replaced RA4 with a 0 Ohm network as well. To no avail. So I think what you are suggesting doesn't work, and that's why I had to add the 24LS244 as an LED driver.
No matter how you tune the resistors, the original circuit is not able to bring the voltages above 2.3 V or so. Insufficient for blue LEDs..