Audio Activated Relay - Circuit Help

[ADavis53]

Hi All,
I am working on a project, and i'm basically finished but i'm not sure that I don't have a problem with my setup. Forgive my ignorance here, I'm new with this stuff.

I have a radio scanner that has a fire tone-out mode, where it will listen to a county dispatch channel and only activate when my station gets a dispatch. I am using the audio output (4V AC) to activate a relay with a timer to ring a bell and flash a light for four seconds. Simple right?

Here are some facts.
* The radio is powered by a 13.8V DC .75A transformer that plugs into my AC wall outlet
* The relay/load are powered by a 12V DC 3A transformer that plugs into AC wall outlet
* The relay is rated for 12V, 5A
* My load consists of a bell (12V, .8A) and a light (12V, .2A)

My audio out from the radio scanner is a 3.5mm stereo jack that leads to 2x RCA connectors. When the radio is active, and i put my voltage meter on the male end of one of the RCA connectors and I put the ground lead on the metal radio case, I see 4-5V AC, when the radio is silent i see none.

I was dismayed when i originally set this up that the audio was not activating the relay. upon putting my meter on the relay I found that the (-) DC wire in my relay was not a sufficient ground for the AC voltage coming from the audio wire- I got no reading. Thus, the relay didn't detect any voltage change and never activated.

I spoke to my neighbor who has a background as an electrician and he immeditaaly identified the problem as a ground loop. He said that if i bonded the ground on the scanner to the (-) from the relay/power source that It would solve my problem. I threw an alligator clip between the two and sure enough... it works great.

or does it? Below is a simple diagram of what i have. Red is DC+, Black is DC-, Green is my audio cable and blue is the trigger wire for the relay.

I'm worried about a load (bell and light) pulling 1Amp of current and that current having the ability to ground through a radio that is probably only ever expecting to encounter .75A (or less) of current. I'm worried i'm going to ruin the radio.

In the diagram i have an X. Is this a sensible place to put a resistor to step the amperage down to below .75A? The load would still operate because any current in excess of .75A would go to the (-) of the power supply. Am I way off?

Any alternate suggestions for getting this setup to work other than bonding the grounds like this?

Thanks in advance!
A;ex

 

[kellys_eye]

A common ground is necessary and harmless in this application.

 

[ADavis53]

A common ground is necessary and harmless in this application.

Great news! For my own education, could you explain both why it is necessary and harmless? Trying to make sure I learn something while i'm at it. Thanks again!

Best,
Alex

 

[kellys_eye]

All the modules (and the control signal) require a supply (or signal) line PLUS a ground - this is where the voltage/signal flows to in any circuit (simply speaking).

It is standard practise to join all earth's (at a common point ideally) to prevent circulating currents which can manifest themselves as hum in audio circuits or problematic voltages elsewhere. In such a simple circuit as yours this is less of a problem but good practise anyway.

Although your drawing doesn't show it, the negative line from the radio could/should also be joined to the other negative lines and the 12V 3A supply could be used to power the radio as well as the relay and loads (light & bell) - saves you using a separate 13.8V supply (the radio will work ok on 12V).

 

[ADavis53]

Thank you! I think I'm following. Because the CASE of the radio is connected to the (-) for the radio, technically the radios negative IS connected to the negative for the relay/power supply/load via that jumper- so that makes sense.

I also grasp the concept of using the 12V 3A supply to power relay, radio and load. Load is 1A, relay draws negligible current and the radio must be sub .75A since that's all it's power supply so I'm covered from a power supply standpoint. If the radio runs on 12V, then that should be the ticket. Single power supply(+), single (-).

The way I might implement that is to just cut the cord off the 13.8v power supply and connect the wires to the (+) and (-) wires for my 12v power supply/relay/load, keeping the plug that fits in the radio in the radio.

Okay to use the wire/plug right off the 13.8v(.75A) source for that? Does throwing wire designed for .75A in to a circuit that has other components that will draw more current than that create an issue? My sense tells me no- if I have a power source capable of delivering 3A, surely not all of my devices must be rated at 3A.

I'm thinking typical 15A AC circuit, I can throw a 7 amp vacuum on it and a 1 amp light. The light will have smaller diameter conductors than the vacuum and the Current draw of the vacuum won't cause an issue in either the hot or neutral wire for the lamp. Valid analogy?

Thanks again!

 

[kellys_eye]

just cut the cord off the 13.8v power supply and connect the wires to the (+) and (-) wires for my 12v power supply

Correct.

My sense tells me no- if I have a power source capable of delivering 3A, surely not all of my devices must be rated at 3A.

Correct - the equipment draws only what it needs, no more.

 

[ADavis53]

Outstanding. Thank you very much for the help and information!