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Not enough current to trigger relay.

COffin

Sep 27, 2015
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Hi All,

I'm currently having trouble with a phone system and I just don't have enough electronics background to solve this problem.

I have a phone that is supposed to activate an external horn. The horn is part of a separate system and can be energized by a 24Vdc source. Inside the phone I managed to find a 12vdc source that was used to run the buzzer. I thought that this was perfect, I would just use a 12V relay to act as a switch for a 24vdc source that I have available and the horn would sound when the phone is ringing - easy. The problem is that the 12Vdc source inside the phone lacks the current to trigger the relay - not easy. I basically need any scenario that makes the horn sound when the phone rings.

Note: The original phone for this system that used to make all this happen is no longer available so I got to work with what I have.

Thanks for any help!
 
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duke37

Jan 9, 2011
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You could use a 24V relay and switch it with a FET or BJT, controlled from the 12V supply.
Put a catch diode across the relay to prevent high voltages ocurring.
You may be able to run without a relay, if ground isolation is not needed.
 

Arouse1973

Adam
Dec 18, 2013
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Hello
All you need is a transistor switch and an external power supply(wall wart). The wall wart powers the relay and the output from the phone triggers the transistor.
Adam
 

Arouse1973

Adam
Dec 18, 2013
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You could use a 24V relay and switch it with a FET or BJT, controlled from the 12V supply.
Put a catch diode across the relay to prevent high voltages ocurring.
You may be able to run without a relay, if ground isolation is not needed.
LOL, Trevor, you beat me!
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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I just don't have enough electronics background to solve this problem.
Hey, guys, I am schematic challenged at the moment. Would someone please post a simple wiring diagram for the OP?
 

COffin

Sep 27, 2015
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Please excuse the crude drawing but would this work?

Also these are the transistors I have on hand:
TIP120
IRL520N
LM7915CT
2N3904
D882-Y
LM317T
 

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duke37

Jan 9, 2011
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That circuit will not work and I have a stiff neck so it is difficult to see.:)

Measure the current drawn by your hooter when fed direct from your power supply.

I cannot draw a circuit at the present, I will try to get the equipment in the next week or so.

The components you list are not all transistors. Look up the data and list the primary characteristics of the transistors.

Data is always good, you cannot design an optimum circuit when you do not know what is needed.
 

COffin

Sep 27, 2015
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Thanks for all the help everyone but I don't think any of this will work anyway. When I was measuring the current for the horn circuit, I noticed that ringing wasn't continuous, it has its own circuit that creates a 2 second on and off sound. So if I want this to work, I would have to integrate a holding circuit for the 24V while the phone was ringing. Judging by how all this is going, It would all be way too far over my head.

All the best and thanks again for your time!
 

Martaine2005

May 12, 2015
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Hey COffin,
Don't give up so quickly. This may be a lot simpler than you think.
You may be able to connect a sound bomb directly to the phones ringer.
Here in the UK, the land line has 75vac when the phone is ringing and only about 4 to 7vac when not.

What phone is it? Model number?

EDIT: Your 'horn' can be removed from the circuit board and used separately too.
Martin
 
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AnalogKid

Jun 10, 2015
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Restating the problem:
1. Input: Pulsing +12 VDC signal, 2 seconds on, 2 seconds off.
2. Output: Steady +24 VDC switched to a horn.

Important question: Is the circuit ground for the phone the same different from the circuit ground for the horn? In other words, does the phone have to be kept fully isolated from the horn? If not, a corrected version of your schematic probably can be made to work. But my guess is yes, the two circuits must be electrically isolated from each other.. ebay to the rescue.

There are many 555 timer modules on ebay that have a trigger input and a relay output. Some are retriggerable. That is, if a new trigger pulse comes in before the timer is done responding to the previous trigger, the timer restarts without the relay changing state. A pulse that is 2 sec. on and 2 sec. off is a trigger once every 4 seconds. If the timer is set to 5 seconds, a string of input trigger pulses will look like one continuous signal at the relay contacts. The only possible drawback is that when the trigger pulses stop, the horn might keep going for another 4.99 seconds. The overall circuit function is called a missing pulse detector, and all implementations have the same characteristic.

For this to work you will need a small wall wart as noted above to power the timer circuit. But it's all modules and wires, no soldering.

ak
 

COffin

Sep 27, 2015
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Hi Martin,

The phone is a Vingtor DT-700 Phone Series that is manufactured by Dial-Tone Industries Private Limited. I couldn't find anything online resembling it when I searched for it. The phone system here operates via a phone exchange(Zenitel SMP 200) that has 48Vdc output constantly whether it is ringing or not. It is a 2-wire setup so I'm not sure what prompts the phone to actually ring, maybe a frequency sent down the lines?

To add, the horn circuit draws ~17.5mA
 

AnalogKid

Jun 10, 2015
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You said there is not enough current from the phone to drive a relay coil directly. How much current is available? There are other switching devices that can be used.

ak
 

Arouse1973

Adam
Dec 18, 2013
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Why can't the other system (horn) follow the timings of the phone ringing?
Adam
 

Tha fios agaibh

Aug 11, 2014
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In the US the ringer signal is 90vac 20hz.

I wish the country of origin was listed in the header of each post for clarification.
 

davenn

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Sep 5, 2009
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haha but , hell, I have missed comments an OP or other contributor has made and gone off on a tangent
Its easy to do, specially when the computer screen is just a blur for me :(
 
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