Analog Switch Audio Switching Circuit.

Thread Starter

SmithsRadiomobile

Joined Feb 9, 2024
7
Hi All,

Can anyone assist with helping me design a circuit to switch between 2 audio signals using a DG419? I’ve never designed a circuit before and am only attempting now as there isn’t any pre-made electric boards breaking out this chip into an easy to use plug and play form.

The power source I have is 240v A/C, which has been stepped down to 5v through a transformer.

Is it as simple as I think below?

GND -> 100nf Capacitor -> GND
V+ -> 5v from power rail.
IN -> 5v from switch to toggle input
S1 -> Audio (Context: Radio)
S2 -> Audio 2 (Context: BT Audio Module output)
D -> Switched output to amp.
VL -> Unknown ?? Not needed
B- -> Unknown ?? Not needed

Also do I need any additional components.

Sorry for the spoon feed me question but I’m a complete novice and I only ever repair things which is that’s burnt replace it or bundle bunches of eBay modules together to achieve a desired result.


IMG_6497.png
Thanks,
Tom.
 

LowQCab

Joined Nov 6, 2012
4,075
The Minimum Power-Supply-Voltage is ~10-Volts, 5-Volts won't get it.

You need to explain and provide pictures or documentation for
the Audio-Source(s), and the Audio-Input(s),
and WHY You want to switch them.
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Thread Starter

SmithsRadiomobile

Joined Feb 9, 2024
7
The Minimum Power-Supply-Voltage is ~10-Volts, 5-Volts won't get it.

You need to explain and provide pictures or documentation for
the Audio-Source(s), and the Audio-Input(s),
and WHY You want to switch them.
.
.
.
Hi,

Thanks for your response, I couldn’t see on the data sheet the minimum voltage so was hoping this would work as it small and neat.

Reasons for switching the audio signal are, that it’s a 1950s 4100 Smiths Radiomobile, where a schematic can be found online. The radio is a positive earth and is valve controlled so the radio generates anywhere around 240 - 120 volts depending on where you measure A/C. I have therefore taken a transformer off this A/C circuit as the radio needs to remain postiive earth, this then gives me a stable 5v which is powering a Bluetooth module which can be connected to my phone to allow me to use Bluetooth to play music in the car easily. It also keeps the radio original as it sounds really nice anyway.

Thats the high level view so additionally to the above the 5v signal to the Bluetooth module is turned on / off by a SPDT switch (could also be SPST its just what I have). This means when the radio is taken out of Bluetooth mode the module turns off so audio automatically goes back to coming through the phone should it ring.

There is also a second SPDT switch activated the same way currently, which has the common terminal going to the volume control, and one throw goes to the BT Audio Signal and the other goes to the Radio.

The issue is to fit the second switch I had to find a smaller volume control knob, which works but it has less range and a greater resistance and I’d like to put the original volume control back in which I could do if there was only one switch. Which is why I thought the DG419 would work as I could then use the switch to turn on the Bluetooth module and send an on signal to the chip to switch audio signals. It’s also quite small and compact as I’m really limited for space in the radio.

I’m guessing a really small 5v relay would also work but I need something really small, or I just live with the poor new Chinese’s volume control .

Sorry for the essay and if you need images or anything please let me know.
 

LowQCab

Joined Nov 6, 2012
4,075
Or,
there are other ways of achieving the same results,
such as using "Opto-FETs".
The only problem with these parts are a higher "On" resistance of around ~100-Ohms.

The Circuits below are NOT designed for this particular application,
but are to only show their general operation,
the actual purpose of the below Schematics are for DC-Linearization, not Audio-Switching.

The situation with the Thread-Starters desires will require Schematics and Specifications in
order to come up with a solution that is likely to be reliable, and function exactly as expected.
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.
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Linearization .png.
Linearization 2 .png.
 

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Thread Starter

SmithsRadiomobile

Joined Feb 9, 2024
7
Thank you both, I was looking at a different datasheet which unfortunately didn't specify this, it says DG417 but includes DG419 dg417.pdf (vishay.com).

I cannot draw a circuit diagram in detail as I don't know what I have and usually piece modules together, switches and pre made modules make electronics more accessible and this simple tasks appears to be outside my scope as I don't understand. I could likely just about reverse engineer the AC Transformer board to draw a diagram but it's sealed in heat shrink currently to keep it from shorting out the radio which if needed I can do as the exact module I have the seller no longer sells. I wouldn't be able to do any reversal of the Bluetooth board though as it's way beyond m
eLike I say I usually fix things so the radio I have was missing a bunch of components and wires and getting it working from the diagrams I have was likely a miracle for me.

I've attached the best I can do currently anyway and will also attach photos of the insides of the radio itself later today. Sorry to be a complete novice.
IMG_6498.jpeg
 

Attachments

Last edited by a moderator:

Thread Starter

SmithsRadiomobile

Joined Feb 9, 2024
7
I'm guessing I couldn't use the positive earth 12v to power the chip and the 5v connection through the transformer toggle the chip? Electronics not liking positive earth is why I decided to go down the transformer route, which it also appears to cut out some of the noise generated by the vibrator circuit in the radio. I'm open to all suggestions though.
 

Thread Starter

SmithsRadiomobile

Joined Feb 9, 2024
7
Looking more into it, with the above advice I think MAX4624EZT+ Analog Switch, will do what I need. Seems more fit for the application. I would edit a post but there’s a 10 minute timer on edits so sorry for spam. The chip makes much more sense also, just power, ground, switch signal, then the 3 normal contacts like on the existing switch. I’m thinking this is my solution and I should have done more research in the first instance.

I did try to take pictures of my radio and circuit but its so tightly squished in you can’t actually see what’s happening but I also feel the new chip I understand and the circuit will be quite simple.

https://www.mouser.co.uk/datasheet/2/609/MAX4624_MAX4625-3129513.pdf
 

LowQCab

Joined Nov 6, 2012
4,075
You need to put the BT and all switching in a external Aluminum-Box.

There may not be any "requirement" for switching the Radio-Audio
since the Radio-Impedances are so high, and the BT-Card-Output-Impedance is quite low.

This could be a situation where the Radio-Wiring could remain undisturbed,
and the BT-Card can simply be "plugged-in",
which would swamp the normal Audio-Chain in the Radio.

A Miniature-Signal-Transformer, and a Stereo-Jack-and-Plug pair may be everything that You need.
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Thread Starter

SmithsRadiomobile

Joined Feb 9, 2024
7
You need to put the BT and all switching in a external Aluminum-Box.

There may not be any "requirement" for switching the Radio-Audio
since the Radio-Impedances are so high, and the BT-Card-Output-Impedance is quite low.

This could be a situation where the Radio-Wiring could remain undisturbed,
and the BT-Card can simply be "plugged-in",
which would swamp the normal Audio-Chain in the Radio.

A Miniature-Signal-Transformer, and a Stereo-Jack-and-Plug pair may be everything that You need.
.
.
.
Thank you, I agree there is much simpler ways but I like the elegance of it being completely hidden and you wouldn't know unless you turned the tuning dial all the way to the left or the right which activates a switch. I also feel as this car is my daily driver it's worth the added effort, as previously I've used a BT Speaker which I never charge so it's aways dead, and my phone doesn't have an AUX. I've done this to two other radios where normal switches are just fine as there is space to have both of them but this one is so tight for space (it will all safely fit). Plus this can also be easily removed at 4 solder joints, and 2 screws.

I'm only trying to reinvent the wheel with something more complex this time, as I can't find a decent volume control as each Chinese one is high resistance and doesn't have the same control range as the original (which is much larger). I'm going to try with a 5v chip not decided on which one yet going to do a bit more looking plus I can also learn a bit hopefully as I like to learn with every project.

If I fail I think I'll just use one switch to toggle the module on and off and as you say have both audio signals wired with no switch, the only issue I find with this is when the Bluetooth is playing there will tend to be additional noise on the audio without the switch. This strangely only seems true on this and the other valve controlled radio I've done, and not true on the later 1980's Motorola radio I've also done so I've put this down to the way the radio is generating the high voltage as it tends to be harmony with the vibrator in the radio.

Hopefully by next weekend I'll update you with a positive post :).
 
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Thread Starter

SmithsRadiomobile

Joined Feb 9, 2024
7
Just wanted to say thank you for your help, I ended up buying a 2n3904 transistor, 1k resistor, and MAX4624EZT+ and as that chip is tiny a breakout board for prototyping.

The chip draws basically no current the transistor pulls more and that’s 0.001 amps on a power supply gauge. It’s also very low resistance and works perfectly without distortion to the audio.

I did a bit of learning along the way though which I can pull into my next project so I’m glad I never just gave in.

Thanks Tom.
 

MisterBill2

Joined Jan 23, 2018
18,538
For this application it will probably be better to use an enclosed electomagnetic relay. Those are available in fairly small packages, and then the relay coil can be selected to operate from the same supply voltage as the radio. More details later.
 
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