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Help with a momentary contact switch Answered

Hi everyone. I'm currently working on a new Instructable, and I've hit a small wall. What I'm looking for is a momentary contact switch that closes for a second, then opens again on its own independently of the actuator -- and I can't quite figure out how to do it.

To clarify, here's the operation:
You lift the receiver (or handset, or whatever you like to call it) on an old telephone, and the buttons -- normally held down by the receiver -- lift up to create the contact so the telephone knows you've answered and the call connects. Well, this phone isn't going to be connected to the telephone network any more, but what I want is for a contact to be closed momentarily when you lift the receiver, and then opens again automatically on its own even if you haven't put the receiver back down.

I also want the same contact to be made when you eventually do put the receiver back down -- so you replace the receiver, a contact is made momentarily, and then opens again on its own, even though the receiver buttons remain down.

I hope that makes sense, and that someone has a bit of advice on how to achieve this. Thanks in advance!

EDIT: Been continuing my research, and found the following: http://www.simprojects.nl/toggle_to_momentary.htm
Can anyone confirm if this will achieve what I'm looking for? Thanks again.

Discussions

Yes, I have actually used something similar. There are mechanical "mometary switches" (not thinking of the kind you push then release) that make contact but as you push them break the contact, but that would be harder to make last for a full second.

Well, I say a full second, but that's kind of misleading now I read it back. It's more a case of creating a "quick button press" from a switch that might stay open or closed for extended periods. Do you have any links or details to the mechanical switches you mentioned? Thanks.

The switch I was thinking of, isn't used much any more, and would act similarly to a "keyboard" switch, where there is not "repeating" when pressed, it makes contact then releases immediately. I don't think that is what you are looking for.

Circuits that latch "until a condition is met" or "until a condition changes"  are not difficult to create with a capacitor, a few resistors and a transistor (or maybe a triac).   Or, if it needs to latch, maybe like this one?  

Goodhart,
It may be that the mechanical "push through" switch you mention is exactly what I need. If you push and hold, it makes a momentary contact. Once you release, it makes a momentary contact. Do you have any suggestions for such a switch action? Who to contact?

What you're describing is exactly what I'm looking for, but so far it would appear they don't exist. I'm planning on testing out the simple circuit that activates a relay momentarily through an electrolytic capacitor whenever the switch is activated (in either direction). I'll report back when I've given it a test.

The downside of the relay approach means I'll now need to power my Instructable, but that's not a particularly big deal.

depending on how much power you have a available or how much is needed, you may be able to accomplish the same effect with two transistors (one NPN one PNP).

It sounds like such a simple (and unusual) mechanical switch does not exist -- although I'm surprised and will not stop looking. An electronic implementation would work, but does require power. Thanks for commenting.

Well, I am trying to conceive of why you'd want a switch to make contact a second time as you release the button?

It seems that most Android phones short the microphone contacts to indicate a headphone cord button press. This makes the corded mic inoperable if used in the familiar PTT format. I want to make an adapter cable that can signal the start and end of speaking w/o a second unnatural button press. Might still be something we can do within the Android programming for the only 4 contacts we can easily access. It is only a game. What fun.

I was thinking, something "could" be rigged/built to accomplish it easily enough (well, fairly easily) but it might be bulky and cumbersome.

DoublePulseSW.bmp

That's how it could be done all right, but unless someone like Cuttler-Hammer has already done it and tested it for a gazillion cycles, it might turn into one of those pesky intermittent problems. Don't like those. At this point I'm headed to astable mulitvibrators or FETs with some juice coming from somewhere (USB?).

I was just thinking, a plunger type button switch, with a long "keystroke" could be rigged to make contact while passing a point in its trip down to the "stop:" (make then break) and it would make contact again on it's way back up to it's original position.

Been through that thought process and if I want to stay mechanical we use a plunger with a hump in the center of the shaft. Can use a micro-switch half-way down the throw range to click when the hump passes. Would not be too bad.

It might wear out quickly depending on how it was done. I was thinking of contacts passing each other, but then that might create an arc and would quickly NOT make contact anymore

Hmm, this is a very interesting idea, and I think it merits some experimentation. My initial thought was that perhaps some kind of brush (like you see in a motor's contacts) could potentially be employed for this, but as you say, there's a good chance of arcing and a lot of button bouncing.

However, it did give me another idea. Perhaps a magnet-activated reed relay could be used. If a small magnet is attached to the plunger it could momentarily activate the reed relay when it passes in one direction, and then activate it again as it passes when coming back the other way. The only requirement would be that the reed relay is situated in the middle of the plunger's throe, so the magnet doesn't hold the relay in one state or the other.

No physical contacts to wear out that way (other than inside the reed relay, but that's neither here nor there) and the contact should be good and solid. And it wouldn't require any external power either, I suppose.

Ah yes, very good ! The magnet / relay idea would eliminate the arch for sure. And you are right, if you use the magnet actuated reed relays, no external power would be needed.

I think I might butcher up a burglar alarm magnetic switch to experiment - they're dead cheap, and should work just fine. I'll report back.

The magnetic reed switch may require a long throw for assured operation with a fast button push. You can diddle with orientation and magnet size to adjust. Also, a simple plunger on a tube with a N.O. microswitch slightly protruding into the tube and then a plunger with a lump on the end (like a bead from Michaels -- or one of those old swizzle sticks with the ball on the end) would absolutely create a reliable one-shot as it passes. A pen spring could force the return. Pretty simple.

Sounds like the simplest solution all around at this point.

Hmmm, I know of switches that make contact (momentary) when pushed, but not any that "remake" contact on release. It doesn't mean they don't exist, but I am not sure where to look for them. Jameco? or some such outlet.

I think the keyboard switch mechanic is the way I need to go -- in many way, it currently latches as the phone's receiver holds the switch in an N/O or N/C position (depending whether you've lifted it or not). But I'm looking to change that latching action to a momentary contact, whether the handset is being lifted or replaced.
Thanks for the link, though -- it's still helpful.

the "electronic" switches mentioned (using a transistor) can be "timed" so that they hold only for the duration of (say) a capacitor discharge time.

What are your triggering with the edge event ? A resistor and capacitor will do what you want.

Steve

The required action is only to replicate a button press -- essentially replacing the button press on bluetooth headset to answer and hang up a call.

When you lift the receiver on a telephone, it opens a switch while the call is in progress, and closes the switch when you hang up (prolonged states in both situations). But a headset requires a brief button press to answer, and another (identical) button press to hang up.

So I'm looking to combine the two. Lift the telephone receiver, and there's a quick button press to answer the call. Put the receiver back down, and there's another quick (identical) button press to hang up.

Can you explain how this might work with a resister and cap? If it can eliminate the relay I was currently thinking of using, that'd be very handy. Thanks.

Thinking about this, here is a better way:

Get yourself a 4030 XOR gate. Connect pin 14 to supply (3-18V) pin 7 to ground

Connect pin 1 to your push button. Connect a 100K resistor to the pushbutton, and pin 2. Connect pin 2 via a 1uF cap to ground.

The output (pin 3) should do what you want.

Steve

looks like it would work