Toyota OEM microphone adapter for aftermarket receiver

When I upgraded the stereo of my 2013 Toyota Sienna (although I don't own the car anymore), I found that most of aftermarket receivers are not compatible with the stock hands-free microphone. So, it is usually recommended to give up the stock microphone and install a new one. However, I really wished to re-use the stock microphone and did some research myself. Actually, doing the research, I found that I was not the first who thought about re-using the OEM Mic for the aftermarket receiver. However, to my surprise, none of the precedents seemed to get successful outcomes.

The issue I identified was that, the OEM Mic system uses balanced output, which is generated by an integrated amplifier within the Mic module. On the other hand, the aftermarket Mic system uses unbalanced input. Also, since the aftermarket receiver uses a capacitor Mic, there is phantom power supply to the Mic.

The wiring diagram of Toyota stereo is shown below. We can see the two balanced output terminals MCO+ and MCO- from the Mic module.

Pin	Description	Test condition	Test value
MACC	Microphone power supply	Ignition switch off	Below 1V
		Ignition switch ACC	4 to 6V
SNS2 (SGND)	Microphone connection detection	Always	Below 1V
MIN+	Microphone voice signal	-	-
MIN-	Microphone voice signal	-	-

Another issue I noticed was that the Mic module requires 5V supply, not 12V. 

Okay, so to summarize, I needed the following functionalities:

• DC 5V supply
• Balanced to unbalanced conversion
• Phantom power blocking

So, I designed a converter circuit (schematic is shown below). It worked greatly for my aftermarket (Sony) receiver, so I was so happy with that.

Parts:

• 1:1 audio transformer

• Note: the transform ratio may need to be adjusted depending on the receivers. In my case, 1:1 transformer turned out working fine. However, for some receivers, it may saturate the AGC amp of the receiver, and distort the sound. Alternatively, a potentiometer can be connected in parallel at the output stage (Mic jack) for volume control.
• LM7805
• Note: if the receiver has a USB port, the 5V supply could be tapped out from the USB port (I haven't tried this though).
• 50uF x 1
• 10uF x 1
• 1uF x 1
• 50k potentiometer x 1

Schematic:

LED headlight - Radio noise filter

After installing a pair of retrofit LED headlight bulbs, I started getting noise when listening FM radio using my care stereo. The RF noise is from the driver circuits of the LED bulbs, which use high frequency switching similar to the RF radio frequency.

So I decided to design a passive filter, which I found very effective to block the noise. Now, after installing the filters, the radio noise is completely gone.

2s Li-ion battery (bicycle light) charger for cars

I made this for charging my bicycle light battery pack in the car.

1. A TP5100 charging module (ebay) has been used

2. Cut the cord and solder the wire on the charger module.

3. Packaging in a plastic enclosure

Because the charging indicator LED would not be seen with the case closed, I drilled a hole on the case lid and plugged a clear acrylic screw.

3. The test was successful and it charged my battery pack like a charm.

Multi-purpose variable voltage DC power supply for cars

Intended use: charging laptops and smartphones in cars, for example, at a campsite without an electricity hook-up.

Features:

USB PD 18W

QC 3.0

Low input voltage (car battery) alarm

Variable DC output

Left slot: USB PD and QC 3.0 charging ports
Middle slot: Voltage meter for the DC port on the right slot
Right slot: DC output port

For the DC output, I used a SEPIC converter module, which purchased on ebay. This converter module provides 5A output currents with 1V-30V voltage range.

To avoid over-draining the car battery, a low battery alarm module from aliexpress has also been added.

On the back, there are vent holes for cooling, power switch, and voltage adjusting knob.

LED turn signal relay MOD

1. Remove turn-signal relay from the car (TOYOTA 81980-02030)

2. The stock relay comes with a 0.03 Ohm current-sensing resistor, which detects over or under-load conditions. The most typical case for the under-load condition is that a turn signal bulb is out. Upon detection of this condition, the blinking frequency is doubled (a.k.a., hyper flash). On the other hand, when the over-load condition is detected, the blinking stops after a single blinking.

In my case, the replacement LED bulbs consumes approximately one third the power of the original Halogen bulbs. Thus, I decided to replace the current sensing resistor with a 0.1 Ohm resistor.

 3. Resistor replacement is done!

Solid-state turn-signal flasher relay; it's completely noise-free.

Mechanical flasher relay has been replaced by a solid-state relay.

Left - solid-state relay
Right - OEM flasher relay

It's completely silent! So, I found it was sometimes dangerous because I kept forgetting to turn off the turn signal. So I decided to add artificial sound. A door chime unit is used to this end.

Rather than irritating mechanical ticks, it makes clear ringing when the turn signal is on. The sound is so refreshing.