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I'm making a space-sensitive circuit for a friend that uses an SR Latch to trigger an LED to go on (and then stay on) once a laser hits an IR receiver, with a button to reset. My only problem is, I can't find any 8-pin latch chips in DIP format (So I can put it on a breadboard/veroboard) anywhere! I had the idea of getting a dual NOR gate 8 pin chip and wiring the latch myself, however I cannot find any of these anywhere either. As the enclosure I wish to put this in is very small, I don't believe it would fit a 14-pin quad NOR chip.

Does anyone know how I can combat this?

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  • \$\begingroup\$ make an SR flip flop with a bjt and p-chan mosfet, smaller than dip and handles LED currents \$\endgroup\$ – dandavis Jun 9 '17 at 14:07
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    \$\begingroup\$ Why use a chip at all? There are plenty of two-transistor circuits that could do what you want. \$\endgroup\$ – Dave Tweed Jun 9 '17 at 15:26
  • \$\begingroup\$ Part number and data sheet link of the part you would like to use, even if not in DIP format, please. \$\endgroup\$ – Brian Carlton Jun 9 '17 at 17:05
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You most likely will have to look solutions that are a bit more modern than your current strategy. DIP packages are really going by the way side and SMT technology is where most things are today. You are already stating that small size is to your benefit and an SMT part fits that requirement to a tee.

One part to consider is something like an SN74LVC1G74DCTR which is a D-Type Flip-Flop that has set and clear pins that can be deployed to operate as a SR Latch. An alternative would be an SN74LVC2G00DCUR which is a dual NAND gate part or an SN74LVC2G02DCUR (dual NOR gate part) both of which can be wired up to become an SR Latch.

If you really must have an 8-pin DIP look at getting a 555 chip in that package and using it as an SR Latch. It is very easy to use a 555 as a latch. Search on line for "Bistable Multivibrator Using 555" to find simple circuits that show how to do this. Here is one example:

enter image description here

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  • \$\begingroup\$ Small addition: LVC family can not be used with 5 V power supply, thus in this case I would think about standard LS74 which is available in SO-14 package (9 mm x 6.5 mm) ti.com/lit/ds/symlink/sn74ls74a.pdf (pages 21-22) \$\endgroup\$ – Anonymous Jun 9 '17 at 12:58
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    \$\begingroup\$ @Anonymous The "Little Logic" LVCxG chips work at 5 V. \$\endgroup\$ – CL. Jun 9 '17 at 17:33
  • \$\begingroup\$ @CL. thanks I overlooked it. I recall now that I did not select this family not because its power voltage, but because @ 5 V they output max 1.5 for logical 0 (0.3 * Vcc) and min 3.5 V for logical 1 (0.7 * Vcc) which are out of TTL and 5 V CMOS standards. \$\endgroup\$ – Anonymous Jun 9 '17 at 17:59
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    \$\begingroup\$ @Anonymous Those are the guaranteed input thresholds; the output are standard CMOS. \$\endgroup\$ – CL. Jun 9 '17 at 20:08
  • \$\begingroup\$ You are correct again, thank you. Input of 3.5 V min for logical 1 is > 2.4 V min defined for TTL. I think that's why I used HCT1G powered with 5 V, feeding it with 3.3 V level signal, to have 5 V level signal as the output. But it is totally different story :) \$\endgroup\$ – Anonymous Jun 9 '17 at 20:16

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