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Jul 08, 2012, 08:52 AM
Wallop!
Martin7182's Avatar
Mini-HowTo

Cheap TFT LCD Monitor DIY audio upgrade


The well known cheap Ebay TFT LCD screens from China usually come without audio. Here's a cheap and fast way of adding audio. Total costs are about $8.00 but when using 1 speaker instead of 2 it's only $4.00.

Recently I purchased the well known 7 inch screen from a HK seller. It was only $28 but it comes with 2 video inputs and no audio.

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Description: 7" TFT LCD Headrest Color Car Rearview Monitor VCR DVD
7" TFT LCD Headrest Color Car Rearview Monitor VCR DVD

Below are the parts needed for this upgrade. I used a 8-dip socket that was laying around for years, but it's not necessary. For volume control I used a small 10k potentiometer from an old VCR.

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Description: Parts for audio upgrade (minus extra 100uF cap for input voltage stabilization).
Parts for audio upgrade (minus extra 100uF cap for input voltage stabilization).

The monitor's back lid features two 40mm empty speaker bays. I've purchased two very thin 8 ohm speakers from Ebay. They're just 4mm thick and fit perfectly. Any small speaker will do as long as it's not thicker than 5mm or so. Or just use the right bay, then you'll have about 1cm of free space. The left speaker is above the pcb so there's less free space there.

http://www.ebay.com/itm/180904080824

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Description: Inside the back lid.Name: DSCN3022.jpg
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Description: Back lid with speakers glued in.
Inside the back lid, Back lid with speakers glued in.

The audio amplifier is a LM386. Rated at 1W max and needs very few external parts. Found on Ebay for about $0.33. I just used one of these to drive the two speakers in series. This results in less output power (load is 16 ohms instead of 8). You could try it with just one speaker, but beware not to fry it.

http://en.wikipedia.org/wiki/LM386

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Description: LM386 audio amplifier.
LM386 audio amplifier.

This is the simple schematic I've used. It can also be found in the amplifier's manual. L is the load consisting of two 8 ohm speakers in series. Note that I've added a 100uF capacitor across the input voltage to stabilize it.

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Description: Schematic for 20 gain.
Schematic for 20 gain.

The end result looks messy but it works ok. I used the V2 video input i.e. the white wire as audio input, but kept the wire in the socket. So I can still use a second video signal (but then without audio of course) and pulled the wire out of the socket. The video input has an impedance of 75 ohm which attenuates the signal too much. Potmeter CA-glued on top of two screw mounting nipples that were cut short and filed flat. Below-right there are three transparent led mounting places. I've pushed out the middle one and used it for the potmeter.

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Description: The end result; 7'' TFT LCD Monitor WITH audio.
The end result; 7'' TFT LCD Monitor WITH audio.

Cheers,
Martin
Last edited by Martin7182; Oct 25, 2013 at 01:25 PM. Reason: Pulled the V2 video wire for better audio signal.
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Jun 22, 2013, 01:55 PM
Registered User
I just found this post and hope that you see this. I bought a similar monitor. It also has the holes for sound but no speakers. Looking at the parts, it seems that there are; 2 electrolytic capacitors, a ceramic resistor, a film resistor, potentiometer, 2 speakers, and LM386 amp. Is this correct? Could you give exact specs for all parts? Also, I am not very good at reading schematics. I have done some simple circuits but am a little confused. Could you provide a parts diagram showing more detail. The picture is good, but I cannot make out all connections.
Jun 22, 2013, 05:42 PM
Wallop!
Martin7182's Avatar
Hi Joekurm,

All components are non-critical, this is the complete parts list:

- 2 speakers, 8 ohms each, 16 ohms in series
- LM386 audio amplifier, DIP-8
- 10 kohm potentiometer (probably logarithmic but linear would be ok too)
- 100 μF 35V electrolytic capacitor. Any voltage >= 16V would do.
- 250 μF 16V electrolytic capacitor. Any voltage >= 8V should be ok.
- 10 ohm 5% resistor, any type, any wattage, any tolerance will be good.
- 0.047 μF 100V 10% polyester film capacitor. Any other type, voltage > 8V, tolerance will be ok.

The schematic can be found here: http://www.ti.com/lit/ds/symlink/lm386.pdf, in the "Typical Applications" section. 20 gain is plenty enough for this application and it keeps things simple. All parts are places as per above schema. The 100 μF cap is placed across the supply voltage.

Cheers,
Martin
Oct 24, 2013, 04:26 PM
It's all about Quadcopters
I really want to do this too but it goes right over my head! Do you have anymore detailed pictures?
Oct 25, 2013, 01:58 PM
Wallop!
Martin7182's Avatar
No more pictures, sorry! Easiest is to use a breadboard and try the amplifier beforehand. When that works it's very easy to build it into the monitor: just fork the existing power wires (red and blue in my case) to feed the amplifier and pull out the white V2 wire and connect it to the amplifier's input. I don't know if all monitors are identical on the inside. You better check the +12 and ground connections first with a multimeter.

The LM386 pinout goes (as always) counter-clockwise, starting below-left. So below-left is pin 1, next to that pin 2, pin 3 and pin 4. Top-right is pin 5, followed by pin 6, 7 and 8. The schematic also mentions these pins. So the outer potmeter's pins are connected between the input signal (white wire) and ground. The middle potmeter's pin is connected to LM386's pin 3 and so forth.

Martin
Oct 26, 2013, 02:04 PM
It's all about Quadcopters
Mines slightly different that yours as you'll see from the attached image. I'm going to give it a go...might break something in the process though
Oct 26, 2013, 02:52 PM
Wallop!
Martin7182's Avatar
Ok, good luck! Just check beforehand with a multimeter that red is positive, black is negative and white and yellow are the video inputs and you're good to go. You never know how these guys wired things on the inside.

Martin
Oct 27, 2013, 07:54 AM
It's all about Quadcopters
Finding the right capacitors is turning out to be a nightmare! I've got the first one off the list and the rest of the components from my local Maplins but I can't track down the rest without buying them individually on eBay :-(

Would you recommend any alternative to a 250 μF 16V electrolytic capacitor as I'm struggling to track one down
Last edited by ecreative; Oct 27, 2013 at 12:09 PM.
Oct 27, 2013, 04:18 PM
Wallop!
Martin7182's Avatar
Hi Ecreative, the capacitor's value isn't critical at all. At any given output power, it depends on the speakers' resistance how much it should be at minimum. At max. output power they specified the value to be 250 μF. I'm using 16 ohms (2 x 8 ohms in series) so I could use 100 μF or so because the output current is twice as low. A too high value will plop the speaker if you turn it on, a too low value will cause worse sound quality. But I wouldn't worry about it. My guess is that any value between 100 and 500 μF would be perfectly fine.

Instead of buying capacitors you could use an old vcr or tv. They are good sources for these.

Martin
Jul 03, 2015, 04:13 AM
Registered User
Hello to all. How to find audio inside screen's plate? It has audio input, audio cable, volume keys and OSD displaing volume ajusting, but has no speakers inside.
Im shure that this screen has amplifer, but can not find where solder wires. Who can help me?
Feb 04, 2016, 05:12 AM
Registered User
Tom_1971's Avatar
I also have a similar display and the same issue: no audio at all!

However, as the last user wrote one year ago, it seems that firmware is able to manage the audio input signal (there are volume adjust keys with feedback in the OSD), so I'm also wondering if the audio section is present somewhere in the original circuit and what is missing are just the speakers. But to be connected where?

The solution proposed in this thread (i.e. add an audio amplifier circuit) is also the first I thought about (and maybe the one I'm going to implement since it is very straightforward and very cheap!) but I'm wondering if there is some hardware already in the board, with gain adjusted by the front panel buttons, from where one could pick just the signal to be sent to the loudspeakers...

If someone knows or tried more, he is very welcome to report to the community. Thanks a lot!
Feb 06, 2016, 05:22 PM
Registered User
Tom_1971's Avatar
I didn't find if and where to connect just the speakers, so I went ahead with the full mod (audio amp based on LM386 + speakers).
The upgrade went smooth and costed about 10 USD (half of which were the two speakers) in a local electronics shop. We don't have Radio Shack here in Europe It could have been cheaper on line, but since I had a free time slot to do the mod, I went to a local shop to save some time.
The audio works fine, although it is quite noisy. I don't think I need HI-FI, just to detect if the motor is still running in case I should have troubles with the car...
Frankly I didn't remove the white pin from the connector inside the monitor, jut soldered a wire after the connector to pick up the audio signal. I thought I made a test by placing the wire directly at the output of the AV RX, but later realised (when everything was already closed!) that the connector were still in place to power the amplifier through the monitor, so even in this case if there is additional impedance in parallel to the LM386 input, that was still there. I don't know if this may decrease the input signal level. It's a long time since I studied Ohm's law at university I should make another test in the future, for the moment I'm satisfied with the upgrade as it is now.
I post some pictures, for those which may be interested.
Bye.
Feb 06, 2016, 06:36 PM
kad
kad
Quad Crasher
kad's Avatar
Quote:
Originally Posted by Tom_1971
We don't have Radio Shack here in Europe
Don't feel too bad. We really don't have Radio Shack here any more either. The store that calls itself Radio Shack sells almost nothing but mobile phones now. They have a tiny rack in the back with some resistors, capacitors and generic LEDs.

Quote:
Frankly I didn't remove the white pin from the connector inside the monitor, jut soldered a wire after the connector to pick up the audio signal. I
That might account for the nosiness.

-K
Feb 06, 2016, 09:30 PM
AMA 1033652
mnemennth's Avatar
A much simpler solution would be to use one of the cheap PAM8403-based Class D amplifier PC boards avaialble on fleaBay.

http://www.ebay.com/sch/i.html?_from...AM8403&_sop=15

There are basic versions with just the amplifier and required discrete components as cheap as 70 shipped (I use one of these powered from a USB port on my PC to feed a second set of speakers for when I'm playing videos or games on the big TV ); there are also versions with a volume pot for like 90.

All they need is 5V at 500mA (less if you're only running at 500mw-1w) to operate; you can either find 5V on the Monitor PCB or add a 7805 / A1117-5 regulator at the power input pins to power it.

Good luck,


mnem
*Amped*
Feb 07, 2016, 08:24 AM
Wallop!
Martin7182's Avatar
Quote:
Originally Posted by kad
Don't feel too bad. We really don't have Radio Shack here any more either. The store that calls itself Radio Shack sells almost nothing but mobile phones now. They have a tiny rack in the back with some resistors, capacitors and generic LEDs.



That might account for the nosiness.

-K
I doubt that. V2 is a video input which doesn't add any noise. In my case it only lowered the impedance which knocks down the audio volume.

Quote:
Originally Posted by Martin7182
I used the V2 video input i.e. the white wire as audio input, but kept the wire in the socket. So I can still use a second video signal (but then without audio of course) and pulled the wire out of the socket. The video input has an impedance of 75 ohm which attenuates the signal too much.
Martin


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