transistor question

[scott m]

Can someone suggest a relatively easy circuit or ic that can be used for switching 10 transistors on and then off, one after the other using one input? This may be hard or easy (or impossible), I don't know. I am new to electronics. TIA. Scott.

[Evodyne]

Scott,
What exactly are you wanting to do? Are you looking for a single "start" input that will then cause each transistor to sequence in turn, or do you want to advance from one transistor to the next each time you pulse or clock the input? Do you need each to be on for a defined period of time? Are you wanting any "off time" (no transistors on at all) as you switch from one to the next?

[scott m]

What I want to do is make a phone dialer for my computer that uses only one or two outputs from the lpt port. I thought I could use the transistors to make the connections on the keypad of a phone I tore apart today, and use one output from the computer for connecting the phone line (ie. "lifting it off the hook") all controlled with a input line from a sensor of some sort. Something like a 555 monostable circuit, just a momentary pulse to generate the tone of the key. I don't even know if this can be accomplished. Scott.

[Evodyne]

Hmmm, I don't know. I can see one output to take you off hook, but you are looking at four outputs to create the binary codes that represent the digits zero through nine.

Assuming you O.K. with this, there are simple integrated circuits called decimal decoders you could use. One of these puppies would have ten outputs (you can also get them with eight or sixteen) and four inputs. Put the binary code for a digit on the inputs (via wires from your lpt port) and the corresponding output goes high/turns on.

Example: I'll send out the binary code for "3" (0,0,1,1) and the third output will be "on", and all of the others "off". "6" (0,1,1,0) will turn on the sixth output. You get the idea.

You can see that a little programming would be involved.

But, without knowing more about how the phones keypad is wired to its electronics, it's hard to say if you could simply "short" across a button with a transistor or not. I'm guessing it's a row-column keypad matrix, in which case you are out of luck (at least in the keep-it-simple form).

Why not simply use a modem as a dialer? You don't have to build anything and it's cheap.

[scott m]

That's interesting, I didn't think of sending binary codes. I am ok with the software part. I will keep that in mind. Why I was thinking of transistors is I plugged the phone in after I tore it apart and it has two little bare copper "s" shapes on the ciruit board that do not touch, in a little circle where the keypad touches and makes the connection. Well, I cut out the little rubber pad on the underside of the keypad and touched it to this with the phone plugged in and it worked (made dially sound) so I tried making the connection with a small piece of wire and that worked so I instantly thought of using transistors. Hope I'm not too confusing.... BTW this is a junk computer with no modem that I got for 30 bucks. I don't even want to try to find one for it. Just seeing what makes it tick so to speak ;-). Scott.

[Evodyne]

Sorry I took a while to reply-got called away. Your absolutely right-shorting the two connections together for a particular button activates it. So, anything that does this shorting will dial a number-almost. A transistor can be used as a switch, but one side will need to be referenced to ground (if it's an NPN transistor) or the positive supply (if it is a PNP variety). For this to work, each digit on your keypad would need one of the copper "s" shapes grounded-and this is probably not the case.

A small relay, whose contact is a true switch, would work regardless of the keypad setup. Another possibility is an optically isolated transistor or FET (field-effect transistor). The previously mentioned decoder chip can drive the input with its logic-level input even if the output (transistor or FET) is not referenced to ground or power.

I'd still go for the freaking modem!

[Zippi]

I built a "dialer" card for an Apple ][e some years ago, slot fingers and all. I used some DTMF chip (can't remember which, it was a LONG time ago), and a relay to couple an isolation transformer to the line. It could grab the line, wait for the dial tone, then dial and hold the line open while a receiver was picked up on an extension. It could also detect ring. Man, I barely remember it! Hadn't thought about it for years until this post of yours....

First off, you want to isolate the line in some way, electrically, from your dialer (you want to do this for a number of reasons - some phone device on your line might be referencing ground in an unusual way, not to mention the lightning factor, or that your phone system might be referenced to your house ground by installation, etc). The phone may have a coupling transformer in it (probably not, who knows)- if it does, then no coupling transformer needed in your circuit. Personally, I'd scrap the notion of using the phone. But, if you want to go "ultra dinosaur", use some electromagnets driven by something like a digital Mux (74LS138 comes to mind, you'd need two) into your transistors to manually push the buttons (two 74LS138's, 3 control lines, add a fourth by using an inverted and a non-inverted chip select for each chip to select between the two with a high or low).

More elegant, optocouple the connection to the line. I've got a book around somewhere w/ the threshold specs for "pickup", tone levels, etc, but not at hand- though, for some reason, I'm wanting to say 300mA to trigger "on hook". Use one optocoupler (that can handle the line current) to couple the line, and another to transmit the DTMF signal (add some bias- also, be aware of audio response limitations of your opto..). Use some sort of DTMF chip to generate your signals (the one I had would also DETECT DTMF signals, even for a 4x4 keypad - anyone remember those? LOL). From here, you can pick up the line and dial. All needed I/O is within the capability of an LPT port.

But, I must say, using a modem would be a heck of a lot less work.... Open comm, send "ATDT XXXXX" where x represents whatever number you want to dial. Depending on the modem, there may be S settings you can manipulate to make it hold the line indefinitely. Not to mention, this would leave the LPT port open for "more important" things like running CNC machine driver boards..... Hmmmm... NC codes dialed in over the phone line and into your machine, perhaps? LOL

Take Care,
Paul

[lerman]

It will take more time to dial, but by connecting an opto-isolator to the parallel port with the other side across the phone line, you can just generate the pulses in software.

Ken

[scott m]

Hmmmm... NC codes dialed in over the phone line and into your machine, perhaps? LOL

Interesting............. Maybe I could make a robot arm that would load a pizza in the oven and turn the bake knobb on and call from work so my carboard is ready when I get home . Thanks you guys for all the help (most of it flew over my head). I will have to do a lot of learning, I think I have too many hobbies. I can just see a mechanical dialer! Tracking down a modem might be easier but it's all in the learning I think. Scott.

[Mat-C]

Assuming that you want to use your parallel port, and don't want to use a modem, and don't want to generate the DTMF frequencies from your soundcard....
The easy/hacky way is to use good old pulse dialling. You've already got the lift off/on bit... just make the LPT port do the pulses. You can practive by hitting the hangup button repeatedly to dial people. Make the clicks at about 5 Hz and you're all set. Tip: use 10 clicks for 0
Disclaimer: be aware of what is legal to fasten to a phone line in your area!