5 ms·
I hadn't realised people would be so interested ! I'll try and answer some of the questions that have been asked. I just use a single type of MOSFET, the 2N700
by JamesNewman 11y ago
I hadn't realised people would be so interested ! I'll try and answer some of the questions that have been asked.
I just use a single type of MOSFET, the 2N7000.
I did look at BJTs (in fact the whole thing started because I wanted to teach myself about "proper" transistors) but working out what resistor values to use to cope with varying fanouts/ins etc was too scary for me. One of the fundamental drivers when making design choices was that I really really want it to work when its built, it'll be heartbreaking if/when it doesn't, so I try to favour robustness.
The single MOSFET is also, I'm currently guessing, the cause of the slow speed. If you look at how the professionals (chip makers) do gates they tend to have two versions of the logic, one driving the positive case and the other driving the negative using both P and N MOSFETS. So the output is actively driven either way. My circuits only actively drive in one direction and rely on a pullup resistor (the 10ks) to generate a 1 output. Halves the number of transistors I need, but cripples the speed. I need to do some experiments to prove/disprove that.
I am planning on putting the design files on the website.
And I'm in Cambridge UK as opposed to Cambridge MA.
- p1esk 11y agoHow much power will it consume?
- JamesNewman 11y agoExtrapolating from the three frames so far it's looking about 48A at 5V, so 250W. Nearly all of that is for the LEDs.
- p1esk 11y agoDo you have an estimate for power consumption without any LEDs?
- JamesNewman 11y agoNot at the moment, I'll try and remember to have a look at that next time I test a module. Currently everything is shuffled into the corners so I can start the construction of the decoder frame.
- IanDrake 11y agoWill this be placed in some sort of public display? It would be cool to see something like this in a science museum.
- x0x0 11y agoThis would be perfect for the computer history museum [1]. For a long time (perhaps they still do?) they had a working version of the babbage engine, a circa 1850s six ton mechanical calculator built to solve polynomials [2]. I can't overstate how cool it is; if you're in the neighborhood, they used to do daily demonstrations. Here's one such video [3] A small hand-built computer on breadboards you can walk into would seem to be right up their alley. [1] http://www.computerhistory.org/ http://www.computerhistory.org/ [2] https://en.wikipedia.org/wiki/Difference_engine https://en.wikipedia.org/wiki/Difference_engine [3] https://www.youtube.com/watch?v=BlbQsKpq3Ak https://www.youtube.com/watch?v=BlbQsKpq3Ak edit: s/1950/1850/
- JamesNewman 11y agoI saw the Babbage machine when it was first built at the Science Museum. It is truly beautiful.
- barbs 11y agoCambridge has their own computer history museum :) http://www.computinghistory.org.uk/ http://www.computinghistory.org.uk/.
- JamesNewman 11y agoI'd be delighted if there was enough interest and a way for it to be on public display. But there's a way to go yet.
- twic 11y agoMount it on a unicycle, ride to a beer festival, and be the most Cambridge thing ever.
- p1esk 11y agoWhat is the total cost of components (assuming 256B of memory)? What is the rough cost breakdown?
- JamesNewman 11y agoThe cost is a rather embarrassing £20k. The biggest cost is the circuit boards about 7 or 8k now I think. The simple hardware such as the aluminium, nuts and bolts etc was a few thousand. Cables and connectors were thousands. I've had to get 300m of ribbon cable so far and its not going to be enough ! There's about a thousand connections. The proper electronic components transistors/resistors/LEDs not as dominant as you might expect. There seemed to be just an endless amount of stuff that was needed.
- p1esk 11y agoDid you try to simulate anything with SPICE before building it? Do you see any benefit in doing so?
- JamesNewman 11y agoI did do a lot of simulation of the logic. I built models of the hardware in software (C++). But I didn't do any analog simulation, which was a mistake. I was aware of SPICE and had a look around but the tools I found seemed expensive. It was only after I'd started that someone pointed me at some free versions, LTSPICE in particular.
- p1esk 11y agoWhy is analog simulation needed? Is it mostly for timing issues? Did you have to use any passive components in your design (like resistors or capacitors)?
- JamesNewman 11y agoA couple of reasons. Firstly its dynamic behaviour, If I'd known about the 1us gate propagation time earlier I may have tweaked the componentry a bit. The second reason is that it may have picked up some of my mistakes such as transposing the outputs or missing off some pullup resistors.
- p1esk 11y agoWhy did you decide to use transistors and not relays? What are the cons/pros of each?
- orthecreedence 11y agoI think relays are a lot slower in general. They are more binary as well, being either ON or OFF, where a transistor is actually amplifying a signal based on its input (so it's possible to have a transistor output a range of voltages, between the current of its main source and pullup resistor being used to drag the off state to 0). Not sure how that affects in total, but I think using transistors lets you vary the voltage needed to drive the system without switching out all the transistors.
- JamesNewman 11y agoI never really thought of using relays, just not into them. There's a chap called Harry Porter someone mentions below who has built a processor out of relays which looks very impressive. My starting point was that I was trying to learn about transistors and was looking for ideas of something to build.