Help with Telephone Circuit

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The first thing I'd do would be to put about 1000µF across the 5V
supply where it comes into your box (I'm assuming you're using a
wall-wart of some sort) and the second thing I'd do would be to put a
0.1µF ceramic directly across the Vcc and ground pins of the chip.

One other thing you might consider is replacing the chip with HC,
since the HC versions usually come with Schmitt trigger inputs (check
the manufacturer's data sheets) and your day/night detector has _no_
snap to it at all. Also, the HC will provide you with substantially
more base current for the relay driver than the 123 can.  Lastly,
since the 123's low output voltage will never go above 0.4V, I'd get
rid of the 2.2k resistor on the base of the relay driver.

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The power supply is a 9 Volt AC/DC adaptor and then I use a 7805 to bring it down to 5 Volts (although when I put my meter on it it appears to be more like 4.8 VDC)

I think ( and I have the whole thing in a junction box right now functioning) if I recall the caps I used with this are 10uf ?? Eg

(12-9v) [7805]

+Vin --------1 2 3--------Vout (5v) | | | +10uF | +10uF | | |

- ---------------------

Are you suggesting that I change these Capacitors ??

i don't see what your 5 Volt supply is, but its common to have electronics perform resets and do funny things when powering up. its called oversight. :) any ways, you could try using a large capacitor on the 5+ line to help hold the charge for those short minor pulses in the line.

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Thanks for your advice. In trying to understand this a little bit, so that I can apply this to other circuits I make.

Why do you think a slight spike or oversight (if those two words are synonyms in this case) would cause the relay to trigger. An increase in voltage "shouldn't" cause the relay to trigger. If the voltage dropped I could see this. Eg. when I plug in the DC adaptor it understandably turns the light on for its period of time because its charging the circuit, if I can put it that way. Perhaps I'm confused, I assume that when the Furnace for example turns off the voltage surges for a moment. Maybe it does drop. As if its turned off then on. I suppose lights sometimes flash when motors are turned on indicating that there is a drop. But as well shouldn't the AC adaptor which is 9 volts hold some Inductance keeping the circuit from dropping. Unfortunately I don't have a scope to test this, and would not know how to use one if I did.

Sorry for writing like a stream of consciousness .

Appreciate your help in understanding this phenomenon. For future projects.

yup!

10 is very small, try something in the line of 200 uf and up.

i would put like a 200 min on the input side and a 500 min on the output side.

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Thanks for your Ideas. I should say, (I didnt before for brevity and I didnt think it was the issue) I increased the R1 to 100K and I added a diac to the ring line inseries. The reason I did this was because when ever someone picked up a phone the light came on. The diac and 100K solved this problem. The break over voltage interestingly enough I think was 27-32 Volts. About what you wanted. Your use of the two diodes together are an interesting way of doing this. I think (I'm strickly amature so I may be wrong) but the diac does the same or similar as the two diodes together head to head. I have sometimes wondered why in some circuits people had diodes anti-series I see now. Your using the break down voltage in your favor. IF IM RIGHT.

Regards

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Thank you for your responses and advice.

For full disclosure (I didn't put all the details in my original post for brevity)

1. I put a diac in the ring line with a break over voltage of about 27-32. I did this because when people picked up the phone the light came on . I also put a 100k Resistor at R1. This seemed to solve the problem of the light coming on because of phone line issues. eg people picking up phone or turning phone on in the case of wireless handsets. I didn't mention these changes because I didn't think they were the problem. or that the phone line was the issue. I think based on what you have said you would agree.

1. I actually did change the LS123 to a 74HC123. I did this because an electrical engineer acquaintance was nice enough to look at the schematic before I made it and made the suggestion. He said something about it being more resilient to voltage issues. It appears you two are singing from the same song page.

I don't know if this will change your advice;

Back to your first bit of advice

I am using a 9 Volt AC/DC adaptor ( Wall Wart as you call it) I then am using the following

(12-9v) [7805]

+Vin --------1 2 3--------Vout (5v) | | | +10uF | +10uF | | |

- ---------------------

One poster has already suggested that I "i would put like a 200 min on the input side and a 500 min on the output side."

I think (and verbal descriptions are always hard to understand) Your suggesting the same kind of thing. Using a 1000uf on the in line from the + to the -. I'm not sure if the mis leading info I gave you regarding the HC / LC would change your advice on the 0.1uF from Vcc to ground on the chip.

Regards

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PS. I did ground Pins 6 and 14 as per datasheet for HC123.

--- The reason the light was coming on before was because of the pulse created by going off-hook. When the phone is on-hook, the phone line sits at about 45VDC and then when you go off-hook that voltage drops down to about 6V, so you were coupling that ~ 40V level change through C1 into the opto's LED, causing it to flash momentarily, but long enough to trigger the first one-shot. What you've done with the diac and the resistor change is to limit the amplitude of the pulse into the opto to the point where it no longer generates enough light out of the LED to turn on the transistor and fire the one-shot. However, the phone line could still be part of the problem if what's happening is that the fan cutting off is generating a magnetic pulse which is being radiated by the power wiring, picked up by the phone wiring and being coupled into your first one-shot. Not likely, but it _could_ happen, depending on how close the phone line is to the fan wiring... The easiest way to find out if that's the problem would be to disconnect the phone line and exercise the fan a bunch of times to see if the problem goes away.

--- No change; with the HC123 in there there's even less need for R8 since Q will, essentially, go to the ground rail when it goes low.

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--- Yes, but thinking about it a little bit more makes me think that this would be a better way of doing it:

WALL-WART +9-12V>----+-----[82R]------------->RELAY +5 | +----[78L05]---+-------->LOGIC +5 +| | | [1000µF] | [0.1] | | | WALL-WART GND>-------+-------+------+-------->LOGIC GND | +----------------------->RELAY GND

While there's nothing wrong with the 7805 you're presently using, the

78L05 can easily supply all the current the logic needs out of a nice little TO-92 package and, this way, the relay's operating current is taken from the dirty side of the regulator and any transients or junk it creates will be sucked up by the 1000µF cap and also blocked by the regulator. Also, a separate ground line for the relay connected right at the cap to keep the relay's ground current out of the rest of the circuit couldn't hurt

The 82 ohm (1/4 watt) resistor is based on your having a 5V relay with a 100 ohm coil drawing 50mA from a 9V source, the resistor wasting the other 4 volts from the [9V] wall-wart. _But_ since you say "9-12V" I suspect that what you have is a 9V wall-wart, but that it's being so lightly loaded that its putting out 12V. If that's the case, then the resistor would have to be 150 ohms, 1/2 watt in order to drop the other three volts.

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No, that 0.1µF is always a good idea since it heads troubles off at the pass...

-- John Fields

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Thanks again for your advice. Actually, and again because I'm learning the the >(12-9v) may have been miss leading. I copied it from the little diagram that I had in how to use the 7805 and it meant to me anyway that I could start with anything from 9 to 12 volts. and the regulator would bring it down to 5 volts. I know that you could actually start with a higher input voltage but for us home bodies who has a 30v AC/DC adaptor. Most everything is 12 or under. (my experience anyway) I have not or don't recall anyway actually checking the voltage on the AC/DC adaptor accept what its rated for which is 9Volts.

Again thank you for the advice I must now think it through. I have not seen a 78L05 but looked it up and sure enough there it is. It does look like a better regulator. Or at least more suitable for this application. I can't imagine anything I would do that would actually need 5v, 1 amp that can come from the 7805.

Thanks again I will digest it all.

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Looks to me as if a glitch comes in from the phone line itself (the "ring" terminal) which triggers the first '123. Anything of size say about 2V *will* trigger IC2 and thus IC3 and so on, meaning the circuit is built way too sensitive to just detect something like a ringtone.

As far as I see it, I'd increase the level at which IC2 'passes' anything. Might do it by connecting 2 zener of say 27V in series with R1, making sure the zeners are "anti-series" (cathodes connected together). Motivation : Since a 'ringtone' is at least about 60Vac it will pass, but anything smaller will be blocked by the zeners.

Another way would be to "slow down" any glitches at the output of IC2. Motivation : Since a "ring" always lasts for at least say 0.5 sec, removing any glitches shorter as say 0.2 sec might do the trick there, so no more false triggering occurs.

I wouldn't recommend that. I've been advised in the past to use a smaller cap on the output side. The reason being that when power is removed, the output cap can hold the output pin voltage above the input pin voltage since the input cap will discharge faster. I haven't sacrificially killed any 7805's yet to prove that out, but it seems reasonable.

I've seen quite a few schematics showing a diode connected from the output pin to the input pin. It remains reverse biased as long as power is applied. When power is removed, the diode only allows the output pin to rise a maximum of .6V(or whatever Vf is for the diode) above the input pin. This should allow any size capacitance to be used on the output side.

I would also recommend adding a .1uF cap to ground on the output pin to help prevent oscillation.

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your use of the diac is about identical as the two zeners, only difference is the diac voltage decreases to about zero if triggered while the zener-voltage remains at its level

Increasing R1 to 100k makes the internal led of IC2 "glow" less for lower voltages, might also reduce small glitches however is less reliable

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Whenever *any* circuit gets powered there are *always* transient effects, even in integrated circuits Increasing/decreasing capacitances will not cure those effects, increasing power supply caps might even get things worse since longer transients exist Designing circuits so that transient effects cannot harm anything does the trick Just look at C2/C8 timing caps, and imagine a transient on the power supply...

Avoiding "power-on-light-on" might be done for example by slowing down the base of T1

One other thing : I'd reduce R8 to say 560...680 Ohm, since a TTL out

*never* reaches 0V at "low" (usually somewhere between 0.4 and 0.8V, bringing Vbe of T1 at say 0.5V worst case thus *possibly* conducting T1 somewhat unwillingly if IC3out is "low")

Some of the usual suspects not seen in responses. Do you have a bypass capacitor adjacent to the 74HC123? Bypass cap is more critical with the HC version compared to the LS version. And it must make a short connection to that 74HC123 chip. Schematic does not show it.

You did connect power and ground to pins 16 and 8. HC chips can be powered, unstable, via other pins if the Vcc and Gnd pins are not connected.

Another has recommended the relay be grounded separate from HC ground. Remember, wire is an electronic component and not a perfect conductor.

Many responses assume the phone line is electrically isolated from the rest of that circuit. Maybe. But then separation between phone line components and wires is important for proper operation.

You are suffering a transient problem. But too many post as if a transient enters on one wire, causes the problem, and terminates. View transient problems in terms of both incoming and outgoing path. For example, the transient may enter on one AC power wire and leave on the other. Or it may enter on one AC wire and leave via a not so well separated wire to phone line.

Remember, there is no complete isolation between relay wiper and relay solenoid. We have seen where a transient on the HC output will actually trigger that IC to change state.

An oscillosc> Thanks again for your advice.

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Thanks for you tips. I keep adding data to my knowledge. The idea of grounding the relay etc is an interesting one suggested by you and another poster. Im a little unclear on this exactly. I would not have thought that on such a little circuit it would be much resistance. If I understand what people are saying I asume what people really mean there should be a separate negative - connection put closer to the emitter side of the T1 close to it to prevent the electricity traveling as far to the relay. And absorb some of the voltage fluctionations. If you put it on the collector side it would trip the relay. And not work properly. If people mean someone else I would apreaciate knowing. Another respectable answer was to "separately" power the + side of the relay of a sort. see "John Fields February 14, 2005 1:42 PM" in this thread for details. I would have never thought of that and find it a facinating way of doing it.

Its a lively news group with some thoughtfull answers.

Regards

In electronics, noise on the ground pin will cause more noise problems as compared to noise on the Vcc line or other IC pins. Therefore we may create separate digital ground, analog ground, and periperhal driver ground that only join at one point. This longer distance in copper between the different family of components creates reactive electronic components - the copper traces - to separate the different grounds.

looked at your schematic. I suggest you replace the opto-coupler with a schmitt-triger type opto-coupler to avoid multiply spikes during the ring-cycle.

Since ringtone is about 60Vac there'a always multiple triggering during the rincycle... Replacing the opto with a schmitt trigger type would only be helpfull if a slow changing dc voltage would be applied at the 'ring' line