First of all, just a caveat: I am a complete newbie with electronics, computer software is more my thing!

I have bought a (used) car this morning and it came with an old TrafficMaster Satnav installed which I have now removed. It seems that a power jack has been installed in the dashboard to power it.

I usually have my phone in a dock and run a long micro usb charging cable to a cigarette lighter USB charger so I can charge my phone while driving.

It would be nice to utilise the power jack that has been installed, to charge the phone as it will be a neater solution than running the long USB cable.

The old Trafficmaster is powered through a 2.5mm audio jack, the other end is moulded to the unit. Therefore the power jack installed in the car is the female end. This all works and the Trafficmaster recieves power.

The TrafficMaster is pictured here

What I want to do if possible is make a cable that has the 2.5mm jack at one end and a micro USB on the other.

Is it possible to take a spare USB charging cable cut the micro USB off, cut the 2.5mm off the trafficmaster and solder the ground USB wire to the ground audio jack wire and the 5V USB wire to the 5V wire in the 2.5mm connector? (Is there one!?)


closed as off-topic by uint128_t, DoxyLover, Wesley Lee, Dmitry Grigoryev, Voltage Spike Apr 3 '17 at 15:50

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – uint128_t, DoxyLover, Wesley Lee, Dmitry Grigoryev, Voltage Spike
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  • \$\begingroup\$ Of course it's possible to make (and even buy) such cables. Was that your question? \$\endgroup\$ – Dmitry Grigoryev Apr 3 '17 at 10:19
  • \$\begingroup\$ I couldn't find any which is why I was looking to make one. Do you have a link please? \$\endgroup\$ – SEarle1986 Apr 3 '17 at 11:21
  • \$\begingroup\$ Simply googling for "jack to micro USB" shows a lot of results in amazon. No idea which one you need for your TrafficMaster though. \$\endgroup\$ – Dmitry Grigoryev Apr 3 '17 at 11:47

"2.5 mm jack" is a mechanical spec. You also need the electrical spec. What voltage does this jack provide? At what maximum current? Is it a regulated and de-glitched 5 V? A direct connection to the car 12 V system? Something else?

Measure what power comes out. Only then can you decide what needs to be electrically between this jack and a USB charger output. If the jack provides 5 V, then maybe you can just connect them directly. If 12 V, then you have to assume all the nasty spikes on the car 12 V system come along with it. You could then hack up the same charger you now plug into a cigarette lighter outlet to plug into this jack instead.

  • \$\begingroup\$ I have a multimeter with a red and black probe. Can I use the car's body work for the black probe as there is no bare metal inside the car \$\endgroup\$ – SEarle1986 Apr 3 '17 at 16:15
  • 1
    \$\begingroup\$ @stev: Only if it's metal and connected to the car's ground. \$\endgroup\$ – Olin Lathrop Apr 3 '17 at 16:27

You'll have to figure out the pinout of that 2.5mm connector, it's impossible to guess for us here. Measure the voltages between the different rings? Does one give something like a stable 12V, 5V or so?

Note that you probably do not want to use some old car nav supply to charge your devices. Modern smart phones can draw multiple Amperes, and there's so far no reason to assume that

  • there's a DC/DC converter built into your dashboard and that instead of the car-typical 12V, you get 5V on that connector,
  • that if there is such a DC/DC converter built into the dashboard, it will give you exactly 5V,
  • that if it exists and gives you 5V, it will be powerful enough to charge a modern phone (which will probably draw a lot more current when charging than some old-school car nav).

So, while if you're already measuring, sure, look for 5V between any of the rings on the converter, then connect a 5V/2A = 2.5Ω resistor and see how much that voltage drops if you draw a 2A charging current. But: If you'd start by walking into a store, buying a multimeter, trying to grab a lot of resistors to put in parallel so that you can even have such a dummy load: forget about it.


Not likely directly possible. The jack for the traffic master most likely puts out +12V to match the vehicle power system. The micro USB to your phone is 5V.

Note also that cigarette lighter phone chargers have an internal DC-DC converter that converts vehicle +12V to 5V.


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