Battery selector switch for in-dash stereo?

[Pig_Pen]

Hi All,
Our "new to us" MH has only an in-dash radio that powers the overhead speakers and the bedroom speakers.? I don't think the stereo is original equipment since the "radio off" switches in the kitchen and bedroom do not work.? I'm guessing there is a relay that was bypassed when the stereo was replaced.? Does that sound correct?? How do the remote radio off switches work?

I am going to switch out this am/fm cassette for a stereo with a cd player, but would like to also add a switch to select the house batteries over the main.? Is this possible?? When switched to main, I would like the radio to operate as normal, turning on and off with the ignition key.? Once parked, I would like to switch to the house batteries and use the radio without the key.? Is there a switch like this made? If so, any idea where to find one?

I'm assuming this depends on the specific radio, but, is there a way to keep the clock and radio pre-sets from resetting when switching from house to main batteries?

Thanks in advance,

Paul

 

[Ned]

You could run a fused 12VDC wire from the house batteries or DC distribution panel, and use a make before break toggle switch to select the power source.

 

[Pig_Pen]

Thanks for the quick response Ned,

I didn't know they made such a switch, I will do some searching.

Thanks

Paul

 

[Ned]

The make before break switches are usually rotary and not toggle, sorry if I confused you with that.  I don't know if they make MBB toggle switches, but I'm sure Karl will know

 

[King]

Correct me if I'm wrong, but I think most digital receivers with program memory have 2 power connections.  One is directly connected to a fused hot connection to maintain the memory and the other is connected to the "accessory" terminal of the ignition switch.  The memory requirement is quite small and does not vary when the receiver is in operation.  This is usually such a small amount of current that it would take the better part of a year to discharge the chassis battery.  The other lead may be connected to a simple single pole double throw switch to transfer between chassis and house, since it is normally off when the ignition is off.
Art

 

[Ned]

Art, good point.  If there is indeed a separate hot lead for the memory, then a simple SPDT toggle switch will do it.  Actually, there would have to be a separate memory power lead as the power to the radio is normally off when the ignition switch is off, unless they used NVRAM to store the settings.

 

[Jim Godward]

In our RV, the radio is powered from a relay, normally connected to the house batteries but when the ignition is turned on, the relay switches to chassis power.  There is a separate wire for the memories in both the MH and Jeep.

 

[Ned]

Jim, that is an elegant way to wire it.

 

[Jim Godward]

Wait till you see the re-wiring job for the headlights!  VBG  We can see at night now. 

 

[Pig_Pen]

In our RV, the radio is powered from a relay, normally connected to the house batteries but when the ignition is turned on, the relay switches to chassis power.? There is a separate wire for the memories in both the MH and Jeep.

I wonder if that is how ours was originally wired since there are radio off switches in the middle and rear of the coach?  Since they are not working now, I'm assuming that the aftermarket radio was just wired in like a car.  I will keep my eye out for a stereo that has the constant power wire too.

Thanks for all your help everyone!

 

[Karl]

Paul,

I've attached a circuit diagram for the switch you want. You can use a simple single pole double throw switch rated at 10-15 amps  (slide, toggle, whatever) and the capacitor size is not critical - could be 50uF, 200uF; 25 volt, 50 volt, etc. Just make sure you wire the positive side (it will be marked with a "+" symbol) to the positive radio wire and the other side to ground. The capacitor will maintain plenty of power on the radio during switching from one source to the other provided that it's not turned on at the time.

I would recommend NOT using a make-before-break switch as there may be a significant voltage difference between the house battery and the chassis battery, which could damage the switch.

 

[Ned]

I would recommend NOT using a make-before-break switch as there may be a significant voltage difference between the house battery and the chassis battery, which could damage the switch.

I thought about that but since most motorhomes have a switch to bridge the 2 battery banks when needed for starting, I don't feel a momentary bridging would be a problem.  But since the radio probably has a separate power feed for the memory, it's much simpler to just use a SPDT toggle switch.

 

[Karl]

Ned,

You're right about the bridging switches, but dem be some pretty hefty guys, whereas a little SPDT may not be able to handle the current.

 

[Jeff]

Our Tradewinds uses a double pole double throw switch to select the stereo while driving powered off the hot side of the ignition bus (Radio stays on when M/H is shut down).

The other throw is marked TV/Stereo and uses power from the coach batteries. The reason for the double pole is that when powered from the coach the output not only goes to power the stereo but also powers the FM Modulator that feeds audio from the TV into the stereo system.

By using separate power when driving the modulator is off and does not interfere with the stereo freqency utilized by the modulator.

The memory power is wired to the chassis hot bus with an inline fuse.

 

[Karl]

Paul,

The "radio off" switches probably worked like the water pump switches - a pair of small ga. wires that toggled a latching relay. You could use one of those if the original relay was removed, or you could do it electronically with a power MOSFET, a CMOS flip/flop, and a few other components. No biggey; couple of bucks.

Jeff,
Makes sense. 

 

[King]

If I were to design two "radio off" switches with a "chassis accessory power/house power switch" I would use a single pole double throw switch to choose chassis or accessory, and two single pole double throw switches supplying house power wired as a light wired for two switches.  The armature of the first connected to house power, the two contacts connected to the two contacts of the second switch, and the armature of the second switch connected to the house power contact of the chassis/house switch.  Then the radio would work normally when powered from the accessory connection of the ignition switch, and when the the radio was powered from the house source, it could be turned on or off from either switch.  No need for any relay.  As to the need for high power rating on the switches, the rating of a dash stereo without power amps is relatively low, so as not to damage the ignition switch.
Art

 

[Karl]

Art,
I think you're missing the points of my suggestions:

No need for any relay.  As to the need for high power rating on the switches, the rating of a dash stereo without power amps is relatively low, so as not to damage the ignition switch.

The purpose of the relay or electronic equivalent is to enable the user to power on or off the radio from a remote location. When dealing with low voltages (12V) it's always better to do the switching at the source; not the remote. Much better to have the remote wiring carry a relatively low control current than a high power on current.

I would use a single pole double throw switch to choose chassis or accessory, and two single pole double throw switches supplying house power wired as a light wired for two switches.  The armature of the first connected to house power, the two contacts connected to the two contacts of the second switch, and the armature of the second switch connected to the house power contact of the chassis/house switch.

We already were talking about using a SPDT switch to select chassis or house battery for initial power. Also, my capacitor shunted switch would allow memory retention in the case of a single wire radio without NVRAM, and the standard switch as opposed to a make-before-break has to do with battery banks at different levels of voltage or charge level. The SDPD switches at the remote locations are a given but, as stated above, it's better to do the (relatively) high current switching at the load; not at the remote location. I hope this clears up any confusion.

 

[Just Lou]

My old Bounder, simple machine that it is, has a DPST switch that switches  radio power from the house batteries in "PARK" mode to the chassis battery in the "DRIVE" mode.  The memory power lead is (fused) direct from the chassis battery.

BTW - the memory circuit remains hot even when the battery disconnect switch is on.. or off... (which ever disconnects the battery)..  Are we ever truly disconnected?

lou

 

[Karl]

Lou,

My old Bounder, simple machine that it is, has a DPST switch that switches  radio power from the house batteries in "PARK" mode to the chassis battery in the "DRIVE" mode.

I think you meant SPDT, but what do I know about your rig?

 

[Just Lou]

Karl,?

You are probably right about the switch nomenclature.? I'll have to do some research and refresh my memory on DPST and SPDT, etc...? ???? ?I just tried to understand the logic of it in my mind and took a "swag" at it..? ;D

lou