OK,

I need a solution as quickly as possible, of course I'm always considerate of those that lend a hand, so take your time! ;-)

I was going to use a NO relay that would be energized for two seconds to close the "1 touch dialing" keypad key of an old cell phone thus making a call, until I realized that their "1 touch dialing" requires you to touch two keys. (go figure!)

I thought I might get away with closing both keys at the same time, but the second key press requires approximately a 1 sec delay after the first. I have another relay available that energizes exactly the same as the first, that I could use to close the second key, but It would have to have a 1 sec delay, and I can't modify that boards circuit or I'll void it's warranty.

Of course I'd rather only use one of those two relays (power consumption on that board) to close both keys, so I thought I might build a small, simple circuit on a separate perf board, that I'll wire wrap and solder, that could quickly switch the second key within that 2 second window.

My supply voltage for this circuit is 12v DC. I'm looking for the simplest power conserving solution. One without a relay would probably be better from a power conservation standpoint.

Help! I really don't know where to begin.


My background:

In 1992 for just under a year I was an Audio Amplifier repair technician (and I use the phrase loosely). For the most part all I did was find fried transistors, resistors, capacitors, amp chips and cracked solder connections but wasn't very good at it. However my soldering techniques are superb ;-) I have a really nice fine point soldering station (though not a Weller it's a Tenma and a decent Radio Shack (Micronta) DMM, sorry no Fluke here. ;-) Oh, and I can read schematics pretty well.

So that's what you have to work with ;-)

Any takers?

TIA

 

Well, I don't know if this will apply in your case but I just built a delay timer using an RC pair and a comparator. The cap charges thru a diode (quick) but can only discharge thru the resistor (slow and variable). The comparator then looks at the decaying voltage and switches state at whatever reference threshold you've set. I can supply a picture if this isn't clear. There are many 555 timer examples that would work too.

 

Thanks wayneh,

A schematic of your project would be great, and links to any timer circuits that you think will work.

You understand that all I'm trying to do is close a switch one second after the first switch, and that what initiates the sequence is a relay that will only be energized for 2 seconds.

Again Thanks!

 

Wayneh's suggestion of a 555 is the simplest and cheapest method. Do a search on here for 555 circuits and you will be inundated.

 

OK, 555 will be my search criteria.

I know that here I'm surrounded by many experts, while I'm just an electronic newbie in a pinch.

I really do appreciate any help and will reciprocate if at all possible. I'm desperate because I knew my idea would work until I found out that "1 touch dialing" really meant "2 touch dialing." So now I have to scramble.

This will keep you all guessing, the urgency with this project is foreclosure related.

Again, Thanks!

 

Thanks wayneh,

A schematic of your project would be great

OK, here it is. I've made some changes but the part you're interested in is fine. See D1, C1 and R8. When the comparator on the left turns on, C1 charges "immediately" and turns on the second comparator. When the first goes off, the charge on C1 decays thru R8 and eventually falls below whatever threshold voltage is at the inverting input. As shown the reference voltage is only ~.25 volts and the cap gets charged to ~12v, so it has to go thru several RC time intervals to fall below threshold ~20 sec later. Your needs are slightly different but the ideas still apply. You don't need a quad comparator!

I already had a spare comparator in my circuit, so adding very few components gave me a nice delay. The 555 is more specialized for doing this kind of thing and a better choice if you're starting with nothing.

 

I was wondering what type of relay I should use to short the cell keypad keys to close them. I don't want to inject any voltage into that part of the cell phone circuitry, because no doubt it connects to a microprocessor or some other CMOS chip. Should it be an optically isolated relay?

I have a bunch of scrap PCB's that I've collected over the years and decided I'll rob as many components as I can.

I found an old reed type SPST-NO 12v relay (RC108T). The coil has a resistance of 550 ohms. I found a RC108T in a Jameco Cat on the internet that had a 1050 ohm coil and it's dielectric strength was quoted as 1500 VDC (between contact and coil). When I powered my relay up I checked between ground and the contacts and got 20-30 millivolts. Do you think this would cause a problem with the cells circuitry?


Thanks!

P.S. Wayneh, thanks for the schematic, I really appreciated it, but after looking at some of the 555 timer schematics I decided they looked easier to make. Not only the number of parts but some seem to be more complete for my needs.