Clock Radio Upgrade - Fitting Bluetooth Unit

There are two parts to this modification, one is the existing Clock Radio and the other is the "Integrated Bluetooth Hands-free MP3 Decoder Board ZTV-M01BT Shell & Remote Control" which will need an additional Audio Amplifier.

The Clock Radio had always been a disappointment as the radio audio was poor, weak and distorted so I had been using it as a clock in the kitchen but the setting buttons became intermittent and annoying to use so I was thinking of throwing it away. Whilst wondering what I could salvage, after all there is always a power supply, I guessed that the clock was a separate module and then I remembered that I had the ZTV-M01BT Radio module.

After measuring the width of the front panel to confirm that the ZTV-M01BT module would fit an upgrade was born!

I had bought the ZTV-M01BT module for another project but as the information was less useful than your typical self-assembly furniture instructions it was never used. Whilst being a wonder of modern electronics and manufacturing, once the gibberish was translated it was found not to be what I wanted and, like all these cheap imports, no useful information or traceability.

Here is an example of the limited instructions: "Can memory songs and volume before Memory blackout, Breakpoint memory(it will off after taking out of U disk or SD card after 3 seconds under the state of playing, it can play memory breakpoint when power on)"

and the specification showed the lo-fi compromise: "Support MP3/WMA/WAV format music. MP3 Songs: 2-320kbps, WAV/WMA: 1411kbps and below"

and the total technical information: Input voltage: 7-12V Product size: approx. 85x40x8mm/3.34x1.57x0.31" **

Opening the Clock Radio proved that the Clock and Radio were separate items and, after removing the Radio board, the entire lower space is available for other components.
Extra items required are an Audio Amplifier, two coupling capacitors and two mixer resistors.

The ZTV-M01BT module output needs an amplifier to drive anything other than "bud" headphones but there is no real information so I fit a coupling capacitor of 470uF in each output and, because it provides stereo output, the two outputs are combined by resistors for mono amplification.

The one-chip Radio removed was the typical 1 CHIP AM/FM RADIO IC KA22427 which operates on "Wide operating voltage : 3 - 13V" or "ABSOLUTE MAXIMUM RATING(Ta=25°C) SUPPLY VOLTAGE VCC 11v SUPPLY CURRENT ICC 44mA" or "Internal Regulated Voltage 12.5min. 13.2typ. 14.0max." and "Power Output: POUT 8O,VCC=5.5V,f=1KHZ THD=10% 0.28W"
so, usual nonsense but likely that the existing power supply will give 12V at 0.25W or more ...to be confirmed.

After measuring, marking and carefully making a cut-out in the front panel, the ZTV-M01BT module can be fitted.

**The size quoted would appear to be the PCB, the cut-out required is 84 x 20mm and the screw holes are on the centre-line at 94mm apart i.e 5mm each side of the cut-out.

et voila!

I found this Audio Amplifier on ebay "TDA2822m 1w×2 Stéréo Mini 2.0 Channel Audio Power Amplifier Board 5V-12 V DC" but there are many others suitable and cheap!
A mono amplifier is all that is needed but I chose this as it can be "bridged" to increase the output power and so also makes up for the attenuation of the mixer resistors as well as being less sensitive to the supply voltage.

It is not important what amplifier is used though this diagram shows you the steps required on how I adapted the TDA2822 module to connect the ZTV-M01BT module and mix the two channels in "bridge" mode. Firstly, coupling capacitors of 470uF (5-12volt or so) are needed for left and right channel, positive terminal to the output as the output audio will always be positive with respect to the amplifier input resistor. Secondly, the "mixer" resistors are connected in series and the other end is joined at the amplifier input.

So we have positive lead of the capacitor connected to the left hand output with the negative connected to a 4K7ohm resistor and the same for the right hand channel with the free ends of the resistors connected together at the input of the amplifier.

The way to chose the resistor value is to take the existing input resistor value, in this case it is 10Kohm, and use half the value for each of the resistors. This does mean that the signal levels are halved which is why I chose the "bridge" configuration as it amplifies in opposing polarities i.e x2 so restoring the original levels.

If you look at the diagram you will see that in the "bridge" mode the output coupling capacitors are not needed so I can use them as input coupling capacitors, heh! sometimes you win!

I have to add the extra capacitors for the "bridge" signal and now it is time to connect everything to check it works.

The ZTV-M01BT module has an onboard 7805 regulator and the amplifier module has a supply smoothing capacitor so it is a matter of connecting the supply to each board: the negative supply leads should be connected seperately to the source to avoid interference but the positive can go first to the amplifier module and then on to the ZTV-M01BT module to take advantage of the on-board smoothing capacitor; the loudspeaker is connected to the output pins and it is ready to test.

How cool is that?

Not only can I remotely change channels while I am sat having my tea but, thanks to the Hands-free Bluetooth, I can also chat to my Mum on the telephone when I am cooking!

I hope that information was instructive and apologize for the poor photos.