Circuit to take landline phone off-hook - Page 1
Forum section: electronics / projects

author: Peabody
author_rank: Super Contributor
timestamp: September 05, 2025, 12:48:09 am
content: I'm a hobbyist venturing into new territory, and would appreciate some guidance.First I should say that it isn't a real landline. It's connected to my Ooma Telo VOIP box, which just pretends to be the central office. I don't think the Ooma police will be comming after me if I mess with the line.I'm planning a project in which my device will connect to the line using a relay, take the line off-hook, "dial" a number using DTMF, and send an outgoing message. I understand that there are special telephone line interface transformers which are built to handle the audio stuff even though a significant DC current is flowing through the primary. But the transformer I have is the 42TM016, which is 600/600 1:1, but I can't find anything in the datasheet that suggests it can tolerate any DC current. So I will be protecting it with a series capacitor. That means I have to shunt current in some other way to take the line off-hook.https://www.mouser.com/catalog/specsheets/XC-600134.pdfThe line is 48V on-hook, and one of my extension phones when off-hook reduces the voltage to 6.51V. A 261-ohm resistor will also take the voltage down to 6.51V, which if my math is right sinks 25mA.I would like to shunt some DC current across the line to take it off-hook and get dial tone from the Ooma, but I don't want to shunt any of the AC audio component, either the DTMF or the outgoing message. So I have an AC signal with a DC offset, and I want to reduce only the DC offset, not the AC. My thinking is that a constant current sink would do that - one that would sink a constant current regardless of any changes in the "supply" voltage.I've found two circuits that might work, as shown in the picture. One uses a TL431, and the other uses two JFETs. I don't have any experience with either, and don't have either onhand. But it seems to me that both are feedback circuits, which might limit how well they would work. The audio frequencies involved would probably be 300 to 2500 Hz.Before ordering anything, I'm hoping to get some feedback from the POTS experts here as to whether I'm thinking correctly about this, and also whether some other circuit would work as well or better than these two.Thanks for any comments or suggestions.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 05, 2025, 02:49:48 am
content: The easy solution would be to just use the resistor and increase the transmit power to compensate for it dragging down the voltage some.

author: neillnz
author_rank: Contributor
timestamp: September 05, 2025, 08:44:48 am
content: One way to shunt some DC current without significantly attenuating the AC component is to use an inductor.You might think an inductor is not practical because the AC component in your apllication is just about DC so you would need an inductor like the size of a small car.This is where a gyrator will save the day. You can simulate the effect of a huge inductor to pass the DC to your shunt and more or less block the AC. I used this in a project which was a telephone line powered PIC micro.Attached is a circuit that uses a transistor, you could of course use an opamp.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 05, 2025, 08:51:25 am
content: Quote from: Peabody on September 05, 2025, 12:48:09 amHow much cable is connecting all the phone outlets, and do you want to avoid using the transformer?You can do this with a couple of opto couplers and a couple of NPN transistors.One opto gives the forward signal path, and the second opto gives DC bias path feedback to set a current operating point.

author: 2N3055
author_rank: Super Contributor
timestamp: September 05, 2025, 09:06:17 am
content: Try this...from a book "Telephone Projects for the Evil Genius".This is simple synthetic inductor with transistor.Put the diode bridge before it to not care for polarity.

author: Peabody
author_rank: Super Contributor
timestamp: September 05, 2025, 02:38:35 pm
content: Thanks for the replies. Nobody had any comments on my circuits, but I guess the transistor circuit will work well enough.Do I need a 12V zener diode to bring the 48V down to protect the transistor? It's not clear what happens when the line is first connected. It seems the transistor will be off until the capacitor charges to at least 0.6V. I mostly have 2N3904 and 2N4401, and 48V is in the range of some absolute maximums for them.

author: themadhippy
author_rank: Super Contributor
timestamp: September 05, 2025, 02:43:55 pm
content: chuck a solenoid under the handset,when triggered it lifts the handset clear of the hook switch,no messing with the phone lines.

author: Ian.M
author_rank: Super Contributor
timestamp: September 05, 2025, 03:01:36 pm
content: Beware of the ringing voltage which superimposes up to 100 VAC on the 48V DC!

author: Peabody
author_rank: Super Contributor
timestamp: September 05, 2025, 03:13:21 pm
content: Quote from: Ian.M on September 05, 2025, 03:01:36 pmThe device will only be connected to the line when it will need to make a call. But it's possible it might just by chance pick up the line while ringing is taking place. So I have that problem on my list of things to figure out.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 05, 2025, 10:27:51 pm
content: Quote from: Peabody on September 05, 2025, 02:38:35 pmThe JFETs will be hard to define, and the TL431 is very precise, but will saturate at a lower impedance. It needs a cap to give a HiZ at AC action, but not all TL431 like C loads.Quote from: Peabody on September 05, 2025, 03:13:21 pmTo tolerate possibleconnect while ringinga combination of low pass shunt and current limiter may be best.If you have any mosfets of enough voltage rating for ringing voltages, maybe something like thisIn normal operation M1 regulates the loop current ~ 27mA, avoids saturation at much lower currents, and lightly loads the AC signal, and M2 is just off.During ringing pulses, M2 limits the current - at 165V it is limited to appx 33mA. (Bridge to correct polarity is not shown)

author: Peabody
author_rank: Super Contributor
timestamp: September 06, 2025, 02:27:00 am
content: It seems to me that if I use a FWBR, then a 12V zener across the rectified line would protect against both the regular on-hook voltage and the ringing voltage.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 06, 2025, 03:19:34 am
content: Quote from: Peabody on September 06, 2025, 02:27:00 amIt depends how aggressive you want to be to your whole system.12V may be low enough to never go back on-hook, and 36~48V would mute any ringing on connected phones.

author: Peabody
author_rank: Super Contributor
timestamp: September 06, 2025, 02:16:30 pm
content: This will only be used to make outgoing calls, and will never knowingly answer an incoming call. So there will be a relay ahead of all this controlled by the MCU, and the phone line won't be connected at all unless a call needs to be originated. So coming in from Tip and Ring, there will be a relay, then a FWBR, then the 12V zener, then the off-hook circuit, and finally the transformer. The 12V zener will probably take the line off-hook, but then the other circuit will take the voltage lower, so the zener will no longer be in circuit. Then ultimately the MCU disconnects the line when the call is finished.So if it tries to make a call during ringing, I think the zener protects everything from the high voltage. It will probably mess up the incoming call because it will pick up the line, wait for dial tone, which never comes, and then hang up. That's going to be a very rare event. But I guess I need to think about a ring detector on the NC pole of the relay - maybe an optocoupler.If it tries to call out during an onging conversation, it will not get dial tone, and will hang up.That's assuming I can the Goertzel algorithm working to detect dial tone.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 06, 2025, 03:14:31 pm
content: Loop the line with a 150R resistor.This may interest you.It's an autodialer with line interface. Easy....You can still get the MT8880 from Mouser.Also see dial to dtmf using an Atmel tiny.http://robruark.com/projects/pulsetone/pulsetone.htmlhttps://github.com/inaxeon/rotarydialIn the UK pots is being retired on January 31, 2027.It's VOIP or nothing. For a bit of fun I've adapted a rotary dial phone to a VOIP line. Not quite there yet though, needs some improvement in the audio quality.I gutted the Wi-Fi phone adaptor that my ISP gave me. I found some weird bugs in it though. It won't dial out to numbers on the same physical exchange but works everywhere else. Freaky! ***kin* EE. I had a conversation with a BT engineer who got very angry and called me a f-wit. Pot kettle black I think. He really lost the pot! I invited him to come round and try it but the vicious little coward declined. B**t*rd. My guess is that I was the millionth complainer he’d had today while his buggy test system says it’s all working.

author: Peabody
author_rank: Super Contributor
timestamp: September 06, 2025, 04:15:41 pm
content: I found an arduino sketch called DTMFgen which generates the DTMF tones through an 8-bit R-2R ladder. Since I will also need to transmit the outgoing audio message, I think I can use the R-2R ladder to directly output the values in a WAV file. I haven't tried that yet, but I think it ought to work. They even make resistor arrays that are R-2R, so the wiring would be simple.Then that leaves the incoming call progress tones as well as the "to hear this message again, press 1" response. That's where Goertzel comes in, which appears to be very difficult to understand, but apparently pretty simple to implement, or so I'm hoping.The MT8880 looks pretty cool, but I think my Nano should be able to handle this.I wonder if my Ooma Telo will handle pulse dialing directly. :-)

author: Peabody
author_rank: Super Contributor
timestamp: September 10, 2025, 09:27:47 pm
content: I need to revisit this. I had planned to use a relay to switch in this circuit to take the line off-hook. But I may need the relay for something else, and would like to find another way. I was thinking I could use an optocoupler with transistor output. I could run a fixed current through the LED side to drive the transistor, and since that current is fixed, it might take care of the AC-shunting problem. But the transistor in the optocoupler would need to tolerate high voltage when off - up to 100V or more during ringing, so this may not be a practical solution.What would be other ways of switching in this 25mA DC-only shunt, controlled by the MCU on the other side of the transformer?Edit: It looks like high-voltage optocouplers are available, such as the H11D3, which has a C/E rating of 200V. And I could even put a capacitor on the base input to eliminate passing much AC.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 11, 2025, 03:15:34 am
content: Quote from: Peabody on September 10, 2025, 09:27:47 pmUsing a single opto coupler is a tough ask: check their output transistor ratings - they cannot deliver 25mA at many volts.You could use an opto to enable or disable a fet current sink as it is then the FETS that take the load.

author: Peabody
author_rank: Super Contributor
timestamp: September 11, 2025, 03:23:12 am
content: It will only need to sink 25mA at 7V. But when it's off, it will be subjected to ringing voltage at 100V, which it will have to withstand.But I'm a bit confused about the H11D3. While its voltage rating is 200V, which is fine, the minimum CTR (current transfer ratio) shows in the datasheet as 20%. That sounds terrible, but perhaps I'm understanding it wrong.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 11, 2025, 08:40:55 am
content: Quote from: Peabody on September 11, 2025, 03:23:12 amIt may try to turn on during ringing, you do not want it to 'pop' then.Quote from: Peabody on September 11, 2025, 03:23:12 amHigher voltage parts often have lower CTR, (and wider spread too). It's one compromise.

author: Peabody
author_rank: Super Contributor
timestamp: September 11, 2025, 02:47:20 pm
content: It looks like the answer is to use an optocoupler with a darlington output transistor:https://www.vishay.com/docs/83674/sfh619a.pdf

author: Phaseseeker
author_rank: Regular Contributor
timestamp: September 11, 2025, 03:28:04 pm
content: If you take a look at some old single-chip dialup modem datasheets, there's some examples of circuits to do basically what you want to do, i.e. controlling on-off hook status and adapting the signal levels to and from something you can manage with a 5V microcontroller; here's an example:https://images.100y.com.tw/pdf_file/39-TDK-73K322L.pdf.P.S: if you have an old dialup modem laying around, it might be an even simpler solution to your problem...

author: Peabody
author_rank: Super Contributor
timestamp: September 11, 2025, 09:22:59 pm
content: Yes, that example's circuit uses the method I started with - the line is switched in by relay. And it relies on the transformer primary's DC resistance (108R) to sink enough current to take the line off-hook. I was just looking at another alternative.I have an ancient XP laptop that has a "voice modem" in it, but it is getting very long in the tooth, so I don't want to spend time on that, only to have it crap out.