1. Home
  2. Electronics Design
  3. Need simplest circuit to convert momentary switches to latching "switches"

Need simplest circuit to convert momentary switches to latching "switches"

Hello,

I'm a volunteer in the exhibits department at the USS Midway museum in San Diego

formatting link
and I'm working to make some old equipment look operational. Among the things I need to achieve are to get a series of buttons to turn on various embedded LEDs (probably four per button).

The pushbuttons originally were push-on/push-off, but were very, very specialized, with about 25 contacts each. The button faces were backlit using bulbs, but we're converting to LED lighting. As a result, I stripped out all of the actual switching mechanism, and I now have a series of what could become momentary switches (I'm not explaining it well -- sorry).

Anyway, I've found various latching circuits that might work, but I'd have to have about 24 double-sided PCBs made, one for each copy of the circuit. That's way above our budget (in fact, I've already blown the budget and I'm now buying things out of my own pocket).

However, I found a circuit online that uses a 4017 CMOS that looks like it might work if I build just three circuits. The only problem is that the clearest reference I've found online is above my schematic reading skills (my level of electronics knowledge is just high enough to be dangerous). The circuit I found is at:

nov55.com/ther/latch.html

This one panel of pushbuttons is the one that's got me most concerned. I was going to just attach push-on/push-off buttons to the back of each button "facade," but I found that the push distance (stroke, I guess) of the old buttons is shorter than the new buttons I bought, so that when pressing the old button, it won't go in far enough to activate or deactivate the new pushbuttons.

Can anyone out there help with clarifying the circuit? I'm not familiar with some of the symbols, for instance. Free tickets or other Midway stuff for the clearest answer... : )

Thanks in advance!

Dave

Reply to
dabarak
Loading thread data ...

Do you know any programming? This is one of those rare occasions when something like a BASIC Stamp might fit the bill.

The circuit is very simple and should work okay. If you just want the indicators to turn on, ignore the AND gates and the OR gate, so you would need (5V regulated supply required):

1 74HC4017 (pin numbers as given) 1 74HC4049 (or ULN2003 for up to 7 outputs) (ground unused inputs on 4049 only, not necessary on ULN2003- leave 9 open on ULN2003) 1 74HC14 (ground unused *inputs*) 1 MCP101-450HI/TO to replace the 2 resistors and capacitor on the right. The output goes directly to pin 15 on the 4017.

2 resistors 30-100K (not critical)

1 resistor to determine LED current. The ULN2003 will allow more current and will allow you to drive multiple LEDs up to several hundred mA. 1 100pF capacitor (eg. ceramic) 2 0.1uF (100n) ceramic bypass caps (put from +5 to GN

Go to some place such as digikey.com and get the datasheets (I've linked most of them below), and compare with the schematic. If anything is not clear, you can ask here or in sci.electronics.basics and someone will be able to help.

You can check it out on one of those socket breadboards and then transfer it to a matching PCB available from hobby shops. Use DIP sockets for the chips and buy spares.

ULN2003:

formatting link
MCP101-450HI/TO:
formatting link

74HC14:
formatting link
4017:
formatting link

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Keeping it simple, you can use "latching relays".

Energize the coil once and the contacts stay closed, energize the coil again the contacts are open.

Digikey has lots of them to chose from.

donald

Reply to
Donald
[...useless and irrelevant background non-information snipped...]

No one can figure out what you want to "achieve" from that long winded and confused description. Does a single pushbutton turn on all four LEDs simultaneously or sequentially or in some kind of group patterns. I am somewhat amazed that you can believe a web circuit will do the job when you can't even understand how it works.

Reply to
Fred Bloggs

That sounds like a unique style of latching relay. Perhaps you can link us to a particular part number so we can know this gem???

Reply to
Fred Bloggs

Yes, please. IIRC the old X10 appliance modules had something like that.. a solenoid-operated mechanism toggling a hefty microswitch, all controlled by dodgy inadequately protected electronics, of course.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

So they were independent of each other, right? Push once, light on, push once more, light goes off, right? This is called a "toggle" switch.

The circuit is cute and simple, altough it suffers from some terrible design flaws that may or may not bite you when you build the thing. However, it has the most convoluted and awkward description that I've ever seen. No wonder you're confused. And then it doesn't even provide the functionality you need.

Because what you need might look like this:

. +12V . ,---------------------------------R1------, R1 = 12V-(N*UD)/ID . | | UD = LED fwd V . | spdt ,---------. R1 ID = LED fwd I . | microswitch | | | . | ,-----. | ,-----. | N LEDs . | ,o---+-----|4 | `-|12 9|-' | . '--o | | 1|----|11 | c . o---|--+--|6 | | 13|----4k7-b . | | `-----' `-----' e---gnd . 4k7 4k7 4013B . | | Pins 3, 5, 7, 8, 10 to gnd . | | Pin 14 to +12V . +--+--gnd

The venerable '13B is a dual D flipflop; one half is doing the switch debouncing and the other one does the toggling. The transistor can be any cheap TO92 npn part.

You need to use "microswitches" that are tailored for tasks like this. They have a flexible activation "tongue" and are easily mounted.

robert

Reply to
Robert Latest

Thanks for the answers everyone! Just to clarify, each button will control multiple LEDs (probably four per button). The LEDs will be incorporated into the old buttons, as they're backlit when activated. I was thinking about trying something like a BASIC stamp, etc., although I've never worked in that area before. I've been looking over Arduino (sp?) controllers for stepper motor control, and I think they might work here too. As far as the latching relay solution, I've looked into that but the cost is prohibitive as I have to do this for about 24 buttons. And regarding latching microswitches, I think that would be a good solution but I haven't been able to reliably identify which ones on Digikey or Mouser are latching. Any suggested part numbers?

Dave

Reply to
dabarak

If all you want is to backlight the switches then this technique is low parts count, the SW toggles with each press:

View in a fixed-width font such as Courier.

. . +-----------------------------------------------------------------+ . | MAIN 100k | . | BOARD .-----/\\/\\/\\-----------. .----. | . | | | | | | . | | CD4049 | | | | . | | |\\ |\\ | |- | | . | +-----| O----+----| O--+-|2n7000| | . | | |/ | |/ |- | | . | | / | | | . | | 1M --- | | . | | / /// | | . | | \\ | | . | | | | | . | | .-----+ Vdd [RLIM] | . | | | |0.22U | | | . | | | === | | | . | | | | | | | . | | | --- | | | . | | | /// | | | . | | | | | | . | | | | | | . | | | | | | . +---------------+------+--------------+---------+-----------------+ . | | | | . | | | | . x x x x . -------------------------------------------- . | | | | | | . | | SW | SWITCH | | | . | | | | +-|>|-|>|-+ | . | | --- | | | | . | '-o o--' '-|>|-|>|-+ | . | | . | | . -------------------------------------------- . . .

Reply to
Fred Bloggs

Hmmm, Ancient technology unknown by modern engineers.

"Latching relay

A latching relay has two relaxed states (bistable). These are also called 'keep' relays. When the current is switched off, the relay remains in its last state. This is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing coils with an over-center spring or permanent magnet to hold the armature and contacts in position while the coil is relaxed, or with a remnant core. In the ratchet and cam example, the first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting across a power outage. "

As has been said so many time before, Google is your friend.

formatting link

Reply to
Donald

Oh, I forgot. Digikey has 585 latching relays.

Not cheap, but easy to use.

donald

Reply to
Donald

I have a small change. I will edit it in:

Modified

Using the 2n7000 as one of the inverters would make my friend Less Partz happy because you now get 6 channels per CD4049

Reply to
MooseFET

Is it possible to recover the original switching mechanisms? You would likely be able to find contacts in them that would perform the alternate action function that you require.

--
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
Reply to
Peter Bennett

Well, although I have the original mechanisms, they're not usable for two reasons. First, they block access to the area I need to place the LEDs, and second (most importantly), when the ship was put in mothballs, various systems and such were "demilled" - demilitarized, which means they took hammers and cutters to a variety of things to make them inoperable. The switches are just one victim. The vintage Univac computer (1961) was trashed, as well as the inertial navigation system gyroscopes.

Reply to
dabarak

Sure, I just spec'd some miniature latching relays last week, but they were set/reset, not toggle. Most have two coils and a few work by reversing the current in a single coil, but I've not seen a toggle type in many years. They were some that were built in a similar fashion to stepping switches (the "ratchet and cam") but that was aeons ago.

Do you know of any contemporary types that toggle?

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

LOL- good one. That's what I get for mindlessly cutting and pasting a previous ASCII together...

Reply to
Fred Bloggs

Okay, here's an old P&B design available in surplus:

formatting link
Digikey has some similar ones in stock (11 pieces in stock) for $47.64 USD each.

Magnecraft's 711 series is still available from Allied for $15 each (120VAC coil) or $23 (12VDC coil). DPDT.

Priced mostly for the maintenance man market, looks like.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

This is quite sad, it's true - but if you can salvage even one contact on each switch, with some electronics you should be able to pull it off.

Did you say the switches are currently lit? You might have to do a little creative hacking to replace the current bulbs with LEDs, but it's not inconcievable that you could do so.

They're showing "Star Trek" reruns on Sunday nights here, and I get a kick out of the flashing lights on the "computer" panel. :-)

Good Luck! Rich

Reply to
Rich Grise

Yep, but the switch module (monster is more like it) gets in the way of where the LEDs would go (the plan is to use LEDs), and there doesn't seem to be a way to incorporate LEDs into the modules. These devices were built like tanks so you could continue to get navigation data despite having a nuke go off next to the system. : )

Reply to
dabarak

[...]

You mean an embedded processor could fit the bill to emulate a toggle switch? No, I think he needs an ASIC.

robert

Reply to
Robert Latest

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.