(moved from the project description page)
Since I want to program both 74187 and 74188, I need 8 bits of input and 8 bits of output. I started with my favourite parts: a couple of hexadecimal encoding wheels and a couple of TIL311 for the address bus.
The data bus is a bit unusual : one of the data bits is shorted to 0V, so I use a 11-positions rotary switch (leftover from the #Clockwork germanium discrete clock project) to select the desired bit. For the lower nibble (in the 74187 case) I might add a TIL311 for inspection of the output but LEDs should be enough otherwise. A 7-segments module is used as well to check the output.
The fuse is blown when the power supply is raised from 5V to 10.5V. The different voltage sources are
isolated by diodes so approximately 5.6V and 11.2V are required. Add to this that the TIL311 are undervolted at 3.6V... That's a few diode drops, but too close to the margins to use LM317s.
Starting from 12V, two diode drops provide about 10.6V but this is not accurately regulated and might affect the programming. A 14 or 15V input source would allow the use of a LM317 so the power supply would be a 12V unregulated "wall wart" (let's see what I have in stock).
From 10.6V down to 5.6V, another LM317 drops 5V. This is probably the one that dissipates the most power.
Then from 5.6V to 3.6V, two volts are dropped but the TIL311 power rail does
not need a tight regulation so a string of 3 silicon diodes would do.
The TIL311's current is measured in the 125mA ballpark (around 60mA per module) and this is the most consuming circuit. The power drops are: 1/4W for the 2V drop and 0.7W for the 5V drop. This is significant but still in the range of the circuits made in the 80s... The good side is : this current keeps the LM317 in the regulation range. In the end, the wall wart must provide at least 200mA (probably rated at 500mA) and most of it will be turned into heat.
I found several LT1086, in fixed and adjustable versions, with are low dropout versions.
Minimum load current less than 5mA, input voltage is high enough (25V max) and the dropout does not exceed 1.5V at max current. That's just enough margin for the input regulator (11V) and the TIL311 rail (5.6V to 3.6V) without weird hacks.
Let's start with the 3.6V rail : I got the regulator and I just have to add a couple of resistors to set the voltage. I find that the ratio of resistors is 2.88 so if I set one resistor to 1K Ohms, all I have to do is find a 2.88K-1K=1.9K resistor. Fortunately this is close to the standard 1.8K value, and the voltage does not need high precision. There is also a need of capacitors but that's not a critical consideration... 100uF/25V will do, I have a significant number of them in stock. Ahem.
The 1.8K and 1K resistors give about 3.59V which is pretty good :-)
This works quite well when you don't mistake a fixed with an adjustable version. And it also made me devise a little trick to calculate the resistors : just convert the volts in kiloohms :-) With a 1.2K resistor for the adjust pin, if I want 3.6V I need 3600-1200=2400 ohms. This is less reliable because of the disturbance of the adjust pin's current but we'll add a trimpot later.
For the 5.6V part I'll use a different trick that the datasheet suggests:
If we consider that 1K provides a 5V swing, 1K/5=200 ohms will provide the required 1V swing :-) That's possible though if I haven't burned the first LT1086-5 that I mistook for an adjustable version for my first attempt...
(Now I realise I could have used this trick because I have 3.3V versions and that would have saved one resistor)
With a 330 Ohms trimpot, I can vary the output from 5V to 6.8V :-) This amounts to about 180 ohms per Volt so a 1K resistor will increase the output by about 5.4V, resulting in 10.4V.
Using a 990 Ohms and the same type of 330 Ohm trimpot, I get a range of 9.6V to 11.2V, which is perfect. The 10.5V output is obtained with the trimmer at approximately half position.
For the PSU, I have a 12V 500mA AC block...
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