Ring Voltage actuated relay

I have searched the web and cannot seem to find a solution that I can just buy to meet a need I have. I did find a kit online but it is from the UK and I need to get this done quickly.

I have found solutions that are far more complex than what my needs are at the moment.

What I am trying to do is to design a circuit that will identify when a telephone line rings and then trigger a relay to close a contact. I have the first design and build done, but I have an issue everytime I connect the relay.

What I have is 4 metal film capacitors (250V 1µF) in parallele to each other for capacity but in series with the tip side of the phone line. This removes teh DC voltage and provides some torage of the AC voltage when the phone rings.

I then have a full-wave bridge rectifier to convert the AC to DC voltage. I have a 100µF electrolytic capacitor for smoothing and then a 7812 regulator.

Everything works fine. I have 50VAC ring voltage into the circuit, 54VDC out of the rectifier (after the cap) and a solid 12VDC out of the regulator.

When I connect the coil of the relay the voltage drops to 3.3V and is not enough to fire the relay contact. I measured the resistance across the relay coil at 200Ω.

I tried using a 7805 regulator with a 5VDC relay and the voltage when connected is 1.5 volts and I have the same issue of not engaging the relay contacts.

I know that the 7812 should only have 35V on the input, but I have not had any issues (with heat or not having the 12V output) since the load is low current and the interval is 3 seconds.

It has been a long time since electronics classes and I would very much appreciate any help in getting this to work.

You're not allowed to draw any real current from the telco line, you'll need a buffer and a power supply.

Thank you for your response. This is not connected to the phone line directly. This is connected to an FXS port on a PBX.

The goal is to configure a speed dial on the phone so that the FXS port rings the device (like an analog phone) which will trigger the relay and unlock the magnetic lock on the front door (relay contacts will complete circuit for existing door lock). Since it is an all IP PBX, we need some mechanism to allow activation from any phone location. So just adding a push button at the reception desk will not work in this scenario.

I am able to configure our FXS port to have a ring time of 5 seconds which will allow enough time for the visitor to open the door.

The preference is to have the device powered by the ring voltage and not need an external power supply.

You need to power the relay from a 6v to 12v supply.

Add a 56v zener diode to the circuit to get rid of the DC. That's how all our phone tapes work and we have sold thousands of them .

colin55, thanks for your answer. I am not an electronic tech so I may say some dumb things here.

Anyway, are you saying to use the 56v zener in place of the capacitors that I have? What is the benefit over using a capacitor?

Do you or your company have a device that would do what I am looking for? My preference would be to purchase something that I know will work or pay someone to make this happen. My challenge is that I have spent 5 days researching a solution and have not found one so I am in a crunch to resolve this.

Another challenge I have is that I am limited to the very few supplies that Radio Shack has. Ordering anything will take days to get and put together and spending time with testing and then buying then testing will be a challenge.

I have read through other threads on this site over the past few days to see if there was anything that would give me insight but I am a telco/software guy so piecing it together will be difficult.

I do apprecaite the input, if you were going to design this solution with an external powere supply, what would it look like?

The output of your rectificer currently has about 40v to 50v on it.
By putting a 56v zener diode on the output of this, the voltage will not be present until the phone rings. The 56v zener can be made up of any values totalling about 56v.
Now take the output and feed it into the base of an NPN transistor with 10k between the 56vZener and the base. Put 10u to 100u between the zener and 10k and the other end to ground. Put 100k between base and ground. Now connect the collector to the coil of a relay and the other end of the coil to 6v or 12v. Be careful to put the zener so that the line on the zener goes to the bridge.

When the phone rings, the relay pulls in.

I am a little confused. I just cheked the output of the retifier and it is at 0 VDC I also checked AC and the AC input. All measured at 0 with the phone line connected.

I will try to modify my drawing to match what you stated above.

Thanks for the help with this.

You are likely overloading the ring circuit. What is the rectifier output voltage when the relay load is connected and the phone is ringing?

You might try a relay with a 24V or 48V coil, which would have a much higher resistance.

What is the voltage and current of the relay load (the magnetic door latch), and is the load AC or DC?

You are right. With our circuit, we connect directly to the phone line.

You will not need the 56v zener.

crutschow said:

You are likely overloading the ring circuit. What is the rectifier output voltage when the relay load is connected and the phone is ringing?

Click to expand...

54VDC

crutschow said:

You might try a relay with a 24V or 48V coil, which would have a much higher resistance.

Click to expand...

So could I add a resistor to make this work? I have had a hard tie finding a 24V relay coil locally.

crutschow said:

What is the voltage and current of the relay load (the magnetic door latch), and is the load AC or DC?

Click to expand...

The load is 12VDC and draws .5A

You cannot power a relay from the phone line. It is only capable of delivering about 20mA, and the voltage falls to about 5v to 12v at this current.

The reason I thought this was feaible was due to this link:

**broken link removed**

I noticed that the relay is a 24V relay and in the user guide the design is pretty simple.

I emailed them to see if I could just buy the specs to buiild this so that I would not have to pay the high overnight shipping rates. I have not received an answer so I thought I would see if anyone else couls help.

I am open to using an external power source if it is required. According to your other post (colin55) I assume I can use the transistor and resistors as described to hookup an external power supply.

I forgot:

You can use a CMOS relay. We have 5v CMOS relays and they only require about 10mA for operation.

Again, thanks for the continued communication.

What would that look like (from a design standpoint)? I have not heard of one of those before.

Look on the internet for CMOS relay:

"Description: 5V high voltage, photo CMOS relay"


A point to remember:
Our CMOS relay needs to be connected around ONE WAY.

Here's an example of a MOS relay that will switch 700mA and requires 5mA max. at 2.3V to turn on: Digi-Key - 255-1546-5-ND (Panasonic Electric Works - AQV101).

Thanks Carl.

I know it has been awhile since I responded but I appreciate everyone's help.

I did get a CMOS and tried to make it work with an external power supply.

Radio Shack did not have much of what I needed but I did find a local electronics shop. When I explained what I was trying to do the gentleman helped me out and had the components needed.

I will try to get a drawing updated and posted for future users, but it is pretty much what colin55 tried to explain to me.

The device works and I tweaked the capacitor size a little to maintain output throughout the ring cycle so the door would stay unlatched as long as the line was ringing.

Thanks again for the help.