Modified Phone Intercom


Active Member
We got this idea about a phone intercom system from the other tutorials section. we wanted to modify it a little but i'm not an expert on wiring i do know some stuff, and we wanted to know if we have to change anything or add anything to actually make this work. So if you can look at the diagram below and see if something needs to be changed or added. I appologize if i didn't draw something right I'm not that familiar with drawing diagrams, so if somethings needs to be interupted due to my inability to draw please ask.

I also wanted to add in that one side will be staying in the booth while the other side will go out right next to the booths door to make a intercome for nontech people who might be from another group or when we don't want people wondering into our booth before or during show. mostly before.
in case it isnt obvious what sombra2 didnt mention is that we added in indicator lights in the booth and at the door and i think we took out the buzzer by the door so there is only one in the booth. the point of the lights is to eliminate noise during the show and the lights are like a call button. the point of the intercom is to have one by the door and one in the booth so we can lock people out and still know if they are at our door.
btw our main concern is the amount of power needed for the lights and buzzzers.
I would also recommend that you set up a security camera that you guys have lying around in "the cage" on the back of the stage. BTW for you who don't know me, I work with these guys.

Hi - new to the forums, but not electronics :).

There are two problems with this design.
The first is with your choice of lights. They pull 12v as you indicated but, after the zenner diode pulls its 15v, there is only 9v left for the light. The 12v would light up, but it would be dim. Try and find a 9v light.
Secondly, your buzzer/light system for in the booth would short itself out, because of different resistances. The easiest solution would be to have the light or the buzzer as below:

----------- 9v light ---------
-------* +------------
--------- sonalert -----------

The * indicates a SPDT switch for toggling between the two components, like this one

Also, make sure all the components (resistors, capacitors, zenners etc) are one watt (or larger) versions. This way, if the components are ventilated, there is no danger of overheating and failure.

Hope this helps. Email me if you want a better diagram.
the buzzer is suppose to be conected to a on-off switch so if buzzer is on both light and buzzer goes off, but if buzzer is off then light goes off. is there a way to do that?

oh and rudy to use the camera we need a monitor that can view bnc, which cheapest i found is 99 dollars.
just get an adapter for the bnc, thats how all cameras and moniters are hooked up.
i just looked at radioshack website. i can't find a 9v light. is there a way i could use a 12v light? since the sonalert uses 12v isn't there a way to do that?
a 12v light would work, but it would not be as bright as it would be otherwise. The sonalert also could work on the lower voltage but having both a light and sonalertwould need too much power - they cannot both be on at the same time. The SPDT switch alternates between the light and sonalert. What light and sonalert are you using? If the total current draw is less that 30mA, you may be able to wire the two in parrallel.
the phones themselves only need 25mA is that helps. if i add more power instead of 24v make it like 33v or 42v would that make me able to use both the buzzer and light at the same time? the sonalert which i'm not sure is a sonalert is either 95dB Panel Piezo or Piezo Pulse Buzzerboth from radioshack.
Those sonalerts use alot less power than i thought. In that case, your origional scheme will work but make sure the resistors and capacitors can handle the total current draw - they may need to be 2 watt or larger.
ok. then i'll set everything as it is planed out. and i guess if it doesn't work i'll increase the resistor. i'm guessing it would be a good idea to borrow a multi-meter from someone.

If someone as any ideas that could improve this system please speak up.

Thanks headhunter for your help.
propmonkey said:
just get an adapter for the bnc, thats how all cameras and moniters are hooked up.

To clarify this, BNC is the same signal that the RCA composite video input on any TV with A/V inputs uses. Any cheap TV or monitor with a video input (ie, not just a coax modulated video input) and an appropriate adapter (which, if necessary, you can get from Rat Shack) and you're in business.

ok. I went to radioshack yesterday and they didn't have the 15v zener is there another thing i could use like maybe 3 5.1v zeners in series? or must i need a 15v zener.
the problem is that i only need two of them and it not worth spending money for shipping unless they don't charge for shipping.

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