Intro: Intercom Remote Buzzer - Italian Citofono URMET (1130/1 or Others)
Hello guys, in this manual i will show you how to convert a CACAZI transmitter in order to remotely deliver an intercom sound. The transmitter will be plugged directly and permanently inside a URMET intercom (mainly used in europe), stealing power from the buzzer itself.
Step 1: CACAZI Transmitter and Recieving Stations
Any CACAZI set - like similar products of different brand - is sold in pair of at least a transimitter unit and a recieved.
This is the ALIEXPRESS store of CACAZI, the brand
My choice is not important: any battery powere trasmitter will work for this project (it took one month to have the items delivered to italy). I bought two transmitters and three revievers to help my old mum to hear the intercom when someone is at the door of the building: that was my mission!
Transmitter is usually powered by an internal battery while the recieved is powered from main, fitting different plugs to adapt to the international standard of any country. We will not touch or dismount a reciever unit in this tutorial but we will have to deal with the electronics of an intercom that works on low alternate tension inside (the system is separated from main by a simple transformer).
Let's say that you will not risk your life while handling the parts but working in a minimal safe environment is strongly reccomended (don't operate on electrical devices barefoot, at least).
Step 2: The Transmitter Unit
In the first picture you can see a transmitter unit as it is after unpacking. On one side of the unit you will find a hole. Stick your flat screwdriver in the hole, apply a little pressure towards the inner part of the unit and then turn the screwdriver a little. The cover should pop-up and i hope you don't brake the plastic mechanism that is holding the covers together.
Anyway, you will never have to use the cover again, unless you really need to revert the unit to its original purpose. As you can see from the third image, the unit is shaped so that it can host a small battery...
Step 3: Battery in Use, Alternative Power Source
The battery that powers up the unit is used only when you press the RING button. There is no sleep mode here: if the button is not pressed, the circuit is open and the battery is not in use. Transmitted signal is RF, not wifi!
The original battery required to power the unit then is a 12volts 23A lithium dry cell, a not rechargeable one, commonly used for remote controllers and other small devices when a low self discharge ratio is required to provide a long lasting usability of the device.
I tested the circuit and discovered that the device is operating correctly at 7volts at least. It means that there should be a voltage regulator inside somewhere, but that is not the point.
For us the important aspect of the situation is that we can count on the presence of a voltage regulator inside, somewhere and so - even if for just a short time - we can overload the unit by 20-30% the rated value.
We will not use the battery then...
Step 4: Removing Button and Battery Slots
Opening the unit we will have to remove two pieces that will not be needed for now on: the battery slot and the push button on top of the unit.
Please, this is a circuit: don't use a furnace to unsolder the components. A good 30W solderer will do the job: use a sucker for the pond in excess an be patient. Don't overheat the components and the board!!!
Both the terminals and the button will have to be removed, six soldering points in total.
Once the circuit will be cleaned, we can start to solder the new components.
First we will have to shortcut the push button leads to simulate a button ALWAYS PRESSED. After you will have sodered those two small pieces of wire in the center of the image, powering the unit will immediately activate the transmitter, sending a ringing signal to all the available reciever.
Then we'll have to plug the TWO NEW COMPONENTS on the power leads of the board.
- One DIODE
- One electrolytic CAPACITOR
For the diode you can use any scrap part you have there hanging around. As you can see i used a very powerful one: i would say the bigger the voltage drop the better (because we'll have to rectify a 15v AC power source to feed a 12V CC powered equipment and if the diode will drop one volt from the equation it will be safer for all the components).
The capacitor should be an electrolitic one, rated AT LEAST 25V. I used a 1000uF capacitor that will give me at least one second of power for the unit once charged (i checked the discharge time of different capacitor by simply observing the leds on the unit that are powered while the signal is transmitted, switching off when the capacitor is discharged).
The presence of the capacitor will also make sure that - even if a person at the door will press the ringing button of the intercom for just a fraction of a second - the transmitter will have enough power to send a signal to the recievers for a second at least.
- THE NEGATIVE terminal of the capacitor is the one with the white line painted next to it (check the capacitor image) and should be attaced to the ex-negative terminal of the battery, marked in picture one with a - sign on the board, together with the anode of the diode.
- The positive terminal of the capacitor will be attached to the positive connector of the board (marked +), together with the a new connection cable, the red one in the image that will be used to power the unit.
- The correct orientation of the diode is with the anode soldered together with the negative terminal of the condenser on the negative connector of the board. The cathode instead will be attached to a piece of wire that will be user to power the unit, the black one in the image.
Step 6: Flattering the Back of the Unit and Add Some Glue for Stability
Once the little modifications are made, you can now adjust the profile of the back side of the unit. As you can see there is a quartz on the back of the board that would prevent the board from fitting in a stable position on a flat surface: problem is solved adding a spacer. I used one of those adesive pads that you can attach under the leg of an old chair to make it more stable: it was the right depth!!!
Once you've done with this very complicate (aahhahaha) task, you can add a very small amount of hot glue on the new wires connected in place of the old button and on the base of the capacitor, to make it stand not only on the soldering points.
Step 7: Schematics of an INTECOM
As you can see from the schematics of a URMET intercom, the buzzer that is activated directly when someone is ringing the door is powered by a 12vac or 15v ac power supply. The intercom ringing button CLOSES a circuit that will bring ac current straight to the buzzer coil.
"THE BUZZER" is fed with an alternate current that powers a coil back and forth, pushing a small piece of metal back and forth, creating that horrible sound that most of the european citizen know so well when it knocks the end of its ride.
- We will install our modified unit in parallel with the coil so that, when the coil is activated with the right polatiry our capacitor will be charged, providing tension straight to the CACAZI unit.
- Then the AC power will revert the tension and our diode will stop the new power source from (at least) discharging the capacitor (we installed just a capacitor as a half rectifier but it is just enough for this purpose)
The unit will then be plugged inside the intercom in our apartment.
Step 8: Fitting the Unit Inside an Intercom
Opening an URMET intecom is very easy. You just stick something between the unit and the cover and then insult a couple of divinity (if there are no screws there are divinities holding everything together, that's my theory).
Once the cover is opened, in figure one you can see a strip of contact points, the connections of our intercom with the outside world.
Pin 6 and 7 are our plug points for the two cables of our brand new handmade transmitter.
As you can see, pin 6, 11 and 10 are connected between each other in most of the application. That's why in image 2 i attached one cable of our CACAZI on pin 10 instead of 6: it doesn't make the difference but in case you have a different configuration or a different intercom, you will only have to find out which pins are the one of the buzzer and plug the CACAZI in parallel with those two pins.
As you can see from image two, the board fits perfectly inside the case. I didn't even have to use glue of screws to hold the board in place!!!
Polarity doesn't count in this application because our circuit has an half rectifier on it. Anyway, if your intercom system (or telefone, or whatever) is different and uses continuos current to power a buzzer, you can easly attach the CACAZI the same way BUT by following the right polarity: that's up to you. Just remember to use a boost converter (with a capacitor in parallel at the input) to provide at least 7volts on the CACAZI board if your source provides a lower tension.
Once you've done, plug the reciever and close the intercom: if someone will ring the intercom, ac current will be applied to your half rectifier, will be stored in the capacitor that will feed the CACAZI transmitter board with current. The shortcut ex-button will immediately activate the signal transmission that reaching for the recievers will make them sound with the selected melody and volume.
Inside the intercom the leds of the transmitter will blink while the transmitter is activated: you can bring a led outside the case of the intercom if you want (for aesthetic purpose) but those leds are useful for diagnostic and throubleshooting in any scenario.