acm@bu-cs.BU.EDU (ACM) (09/08/87)
Ok you wizards out there. I have 4 phones (on one line) in my suite
which are being used by 6 people. Obviously there is some contention
for phone-time. It's a pain to have to pick up the phone (and bother
someone) to see if someone is on the line.
Now, I know there is a voltage differential between on-hook and
off-hook (I believe it's 24 volts on-hook and 10 off-hook but this is
purely from memory and I could be wrong). How can I build a simple
circuit to light an LED when the voltage drops? I'd prefer the
simplest curcuit possible that's pretty reliable.
Thanx for any help. Please email replies to madd@bucsb.bu.edu (or if
that doesn't work try acm@bu-cs.bu.edu or if that doesn't work just
post it although I don't read this group often, being a software
person).
jim frost
madd@bucsf.bu.edu
madd@bucsb.bu.edu
"But God favors drunks, small children, and the cataclysmically stoned...."
-- Stephen King, _It_gene@cooper.UUCP (Gene from EK Enterprises) (09/09/87)
in article <12660@bu-cs.BU.EDU>, acm@bu-cs.BU.EDU (ACM) says: > Now, I know there is a voltage differential between on-hook and > off-hook (I believe it's 24 volts on-hook and 10 off-hook but this is > purely from memory and I could be wrong). How can I build a simple > circuit to light an LED when the voltage drops? I'd prefer the > simplest curcuit possible that's pretty reliable. Well, I've always had problems with emailing, so I'll post... If memory serves me correctly, on-hook voltage is up to 48V, with off-hook voltage being close to 4V or so. _This_varies_with_your_particular_site!_ In fact, it's possible (I've done it) to make a primitive listener-detector by monitoring the voltage. Something like 6V with just one phone off-hook; 4V or so with en extension lifted. Anyway, a nice IC to use is the 3453 or whatever low-voltage detector. While it's not exactly micropower (or is it?), it can be either run off the line voltage (*REGULATED*) or off a separate battery. just set it to about a 10V cutoff. Over 10V is definitely on-hook; under is off-hook. (The 34xx whatever is a Motorola IC.) Experiment and have fun! Gene ...!ihnp4!philabs!phri!cooper!gene "If you think I'll sit around as the world goes by, You're thinkin' like a fool 'cause it's case of do or die. Out there is a fortune waitin' to be had. You think I'll let it go? You're mad! You got another thing comin'!" - Robert John Aurthur Halford
plate@dicome.UUCP (Douglas B. Plate) (09/10/87)
Here is a circuit I thought might work with no external power supply or batteries. I don't know the on/off hook voltages of phone lines, but with a little bit of figuring, this should work for any voltages. I just thought this up, I HAVE NOT tested it! Maybe someone can confirm/reject my theory of operation. Here it is: VCC=phone line voltage o o | | \ \ / R1 / R2 \ \ / / | R3 | A o--------/\/\/--|<----- <--LED | | --| --- / | / \ ZENER DIODE \ | | /<| VR1 | \ | | | --- --- /// /// THEORY: The phone line voltage is applied at the top of R1 and R2. The zener diode selected must have a reference voltage lower than the lowest (offhook) phone line voltage. Adjust Variable Resistor 1 so that the voltage at the cathode of the LED is the same as the voltage at the anode (zener voltage) when the phone is on the hook. When the phone is taken off the hook, the voltage at point A will drop, while the voltage at the zener cathode will remain the same. This should create a potential that will foward bias the LED, causing it to light. R1 and VR1 should be of a high enough value not to load the phone line, R2 limits current through the zener, and R3 limits current through the LED. Varying the value of R3 will determine the brightness of the LED. I am ASSUMING that the phone line will supply enough current to light an LED. If I get a chance I'll try this out myself. Doug Plate
dunn@ihlpf.ATT.COM (09/11/87)
>Subject: Re: How do you build phone off-hook indicator
AT&T sells a device which does just what you want (I don't have the part
number) at their Phone Center stores. Of course we charge ~16 dollars (high in
my personal "views are my own, not my company's" opinion). I have read that
GTE sells a phone with this device built in also.
This must be a common annoying occurence in many households, picking up the
phone to make a call only to hear a conversation already taking place, saying
"sorry....", and not knowing when to pick up again. A good friend of mine was
talking about making such a device many months ago ( and selling it for
millions of dollars) but never did get around to it. If you come up with a good
cheap design that works only on line power (no external battery needed) you
will have a good product. good luck.
Bill Dunn
ihnp4!ihlpf!dunnrobert@uop.UUCP (Robert McCaul) (09/13/87)
In article <2172@ihlpf.ATT.COM>, dunn@ihlpf.ATT.COM writes: > > This must be a common annoying occurence in many households, picking up the > phone to make a call only to hear a conversation already taking place, saying > "sorry....", and not knowing when to pick up again. well in my house we had a great remedy for the problem... whenever my dad wants to use the phone, he picks it up and says, "GET OFF THE PHONE, I WANT TO USE IT NOW!!" this goes over *real* good with others ;-) but i do have him trained not to pick up the reciever when i am on the computer, he will come in and tell me faithfully, (set your rolex by it) that his girlfriend will be calling at 9:30, and could i please be done with the computer by then..this usually happens at 9 or so, now on to more funnies...will this still work? (oh shame on you for digging up the past!!) / S1 -------/ ---- RED | |<--R2 goes here - | R1 | O-----------------|-----^^^^^^--|-----------O--\ | | \ | C1 | \PHONE ------| |------ / / / O---------------------------------------------/ + GREEN S1=SPST R1=10k .5 watt R2=10 resistor (listed as brown-black-black) C1=100 microfarad electrolytic @ 50 volts S1 should be opened for incoming, and closed for outgoing. this is from...yep you guessed it..."Steal this book" (ok so i collect old things...) it is supposed to cause a payphone to barf out the money when someone calls your house at a predetermined time. these things always have my curiousity going. i figure by now the problem is solved (20 year old book)