I am currently working on a diy 4 bit computer but I have ran In to a problem. In order to create bytes that can be changed by my processor I need to have some kind of transistor that latches to true as soon as it is given a signal (imagine a piston or actuator hitting a latching button any time a signal is sent). I don't own any relays and I would prefer to have this component in a IC form (if it exists) and I wouldn't mind buying them, but I'm just wondering if there is a way to make my own without relays.
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1\$\begingroup\$ is an scr what you are looking for? en.wikipedia.org/wiki/Silicon_controlled_rectifier Although the double transistor equivalent circuit may be more useful when correctly configured. \$\endgroup\$– jippieCommented Nov 18, 2015 at 17:10
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1\$\begingroup\$ What about "unlatching"? How do you envision that? \$\endgroup\$– jippieCommented Nov 18, 2015 at 17:16
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\$\begingroup\$ Cross coupled transistors can be latched or unlatched .Remember the RS latch taught bin most colledges. \$\endgroup\$– AutisticCommented Nov 18, 2015 at 19:16
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2 Answers
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What you're looking for is called an SR latch. It's not a transistor - though it can be constructed from discrete transistors - and it's available in IC form, for example the CD4043/CD4044.
The latch has two input pins, (S)et and (R)eset. A high level on S turns the latch on, and a high level on R turns the latch off again.
More generally, if you're working on building your own processor, it's worth spending some more time reading about different types of latch and how they behave. D latches and registers in particular are likely to prove incredibly useful to you.
Latches are also frequently called Flipflops.
answered Nov 18, 2015 at 17:16
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\$\begingroup\$ SR and D latches are also known as "flip-flops", which might be a better search term. \$\endgroup\$ Commented Nov 18, 2015 at 17:47
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\$\begingroup\$ Yes, flip-flops indeed!! And they come in configurations of 1, 2, 4, 6, 8, 16 per IC package -- sometimes even many many more. You can get them where they work individually or in parallel in the package. There are also a plethora of other combinations available as well including serial access, special interface access such as I2C or SPI, and connected at will such as inside FPGAs where some have 1000's of flip-flops. \$\endgroup\$ Commented Nov 18, 2015 at 18:20
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2\$\begingroup\$ @PeteBecker, but OP should also be aware that a D-latch and a D-flip-flop are not the same thing. \$\endgroup\$ Commented Nov 18, 2015 at 18:25
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\$\begingroup\$ @ThePhoton - thanks. I didn't know that a D-latch and a D-flip-flop were different. \$\endgroup\$ Commented Nov 18, 2015 at 20:10
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\$\begingroup\$ @PeteBecker: Flip-flops synchronize to a clock \$\endgroup\$ Commented Nov 18, 2015 at 22:19
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Consider something like a 74HC74 flip-flop.
For this purpose you can ground the clock (CP) input and D input or use it for reset purposes.
The FF powers up in an indeterminate state. It can be reset to Q low by bringing RD low momentarily, or a low-to-high transition on CP with D held low.
Bringing SD low will cause Q to go high and it will stay high after SD goes high again.
If you prefer active-high inputs, there are 4000-series CMOS parts that are similar but with inverted inputs.
answered Nov 18, 2015 at 18:41
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\$\begingroup\$ There used to be some SR latches in the 74HC series (like 74HC279), but thery're not made anymore, so indeed this seem a necessary substitution for high-speed CMOS. Actually Toshiba still makes those on special order it seems. \$\endgroup\$ Commented Nov 18, 2015 at 19:55