latching LED array 2

[RyreInc]

I am in need of a simple circuit that will latch on an LED after a momentary digital signal is produced (dry contact), and stay on until power is removed. I need an array of 60 of these, so the simplest/smallest footprint/least wiring possible is desired. One or more of these LEDs is expected to light per use.

The ideal device as far as I can tell would be something like a light-emitting SCR (LESCR, SCLED, Thyristor LED?), however there does not appear to be such a device. An SCR in series is another option, but a multiple channel IC would be preferred instead of an additional device per LED.

I haven't seen anything higher than a 4-channel S/R latch IC (e.g. 4043/4044), and besides, the reset function is superfluous (although could be used instead of removing power entirely). Is there anything out there either with more channels, or with a single reset for all channels?

The LEDs will be arranged in a 6x10 grid, so I am also considering the idea of turning on a row & column to activate these. However, with multiple activations there will be one or more extra LEDs lit, which will add confusion.

So what are my options? Thanks for your help!

 

[MacGyverS2000]

What's wrong with multiple latching ICs?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

A small FPGA or CPLD with sufficient I/Os would seem to me to be simplest in terms of achieving the number of I/Os, even though from the functionality perspective, they're gross overkills.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[RyreInc]

MacGyver - nothing wrong per say, just hoping to find a more elegant solution. Or at least a latch IC with more channels and/or a single reset pin.
IRstuff - not something I had considered, I'll look into that; but again, trying to find an elegant solution that does not have superfluous features.

 

[itsmoked]

Are we talking 60 buttons too?

If so, hands down, a large pin count microcontroller. Say, something in a 100 or 120 pin count region. Perhaps something off this list:

List of micros

You can get a free compiler for this little job.

Use a couple of pins to scan ALL the buttons and then directly drive all the LEDs from the raft of remaining pins. You're talking a regulator a single chip and a handful of resistors.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MacGyverS2000]

If the functionality is simple (sounds like it) and the pin count is high, FPGAs are usually the best bet... dirt cheap (comparatively speaking), and the software for simple functionality is free (Xilinx, et. al.).

If you plan on adding a bunch of functionality, high-pin count micros are likely better, assuming you can program.

If you can only wire together chips, latching ICs is best.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Skogsgurra]

The "ghost" indications that are likely to occur if more than one input is activated ("The LEDs will be arranged in a 6x10 grid, so I am also considering the idea of turning on a row & column to activate these. However, with multiple activations there will be one or more extra LEDs lit, which will add confusion") can be eliminated if you multiplex rows and columns and strobe x high and y low when the desired LED is addressed. That will be a little more complicated in a simple FPGA, but very simple in a micro. The needed pin count will then be 60 (inputs) and 6 + 10 outputs = 76 pins.

If you multiplex the buttons (6x10 is a logical way of doing it - then your "data processing" will be very simple) then you only need 2x(6+10)=32 pins. Not unheard of and available in most micro families.

Even more reduction of pin counts if you use one set of outputs to scan both inputs and outputs. You will then need 10+6+6=22 pins. Sounds like you will be using a small and simple micro.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[VEBill]

You've described an SCR in a DC circuit. The same latch function can also be implemented with a transistor. In large quantities, impractical.

In the hobbiest Arduino world, there are plenty of cheap LED arrays andcrelated drivers available. Whatever you're doing could probably be built from scratch in an evening or two using the building block modules readily available from the Arduino sellers.

 

[djs]

I have seen SCRs in a TO-92 case. Not much bigger than a standard LED.

 

[hrcooper]

Ever looked at flip dots?

http://www.flipdots.com

You would still have to do the circuit to drive them, but another potential solution

 

[Skogsgurra]

That's a truly innovative solution, hrcooper!

Actually, there may not be any electronics needed at all. Just connect the input switch to the "show" terminal. Then tie all "unshow" terminals together and reset wit a simple contact closure. Sooo simple.

Of course, it depends on the length of your input signal. Microseconds won't work. But ten or twenty milliseconds may work and 100 ms will work fine.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[RyreInc]

That is indeed a pretty neat, novel solution hrcooper!

Its close, but not quite there, as the flip-dots appear to require at least 250mA to drive them, and my switches are limited to 10mA. I have so far been unable to find a good IC driver solution, and discrete drivers are no better than discrete SCR/LEDs.

Also, at 250mA and 5V that comes out to 75W total to drive 60 of these, which seems like a lot.

 

[IRstuff]

True enough, but the datasheet states a minimum pulsewidth of 1.5ms, so total power is dependent on what your duty cycle requirement is. Assuming that it's no more than 1 Hz would put the average power consumption at a much more palatable position. Then, it depends on how many LEDs are on/off. Even then, if you assume that you have N LEDs on, you would assume a worst case of say 2N flipdots changing state, that would result in an average current draw of 640mA vs. 72mA for the flipdots.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[Skogsgurra]

An emitter follower transistor or a ULN type coil driver takes care of the limited current that the switch can handle. It can't get any simpler, RyreInc. Problem solved - simple as that.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.