Sena SR10 to Motorola TLK100 - A guide to making an adaptor cable

The reader is expected to be familiar with the Sena SR10 and have basic wiring and soldering capabilities. While all care has been taken to ensure this guide is correct, it is entirely your responsibility to accept any and all consequences if you chose to follow these instructions. Also note that the author of this guide is barely competent at electronics and soldering, so make of this what you will...

If you are a pro and have no need for this step-by-step guide, then the Sena Hirose plug pin-outs, the Motorola plug pin-outs and the wiring schematic is probably all you need. Note that the main complexity is that the PTT switch has to be wired in to both the SR10 PTT circuit and the TLK100 PTT circuit.

If you're not a pro, perhaps read on...

Background

A feature of the Sena SR10 is the ability to bridge a two-way radio to Bluetooth speaker and microphone systems embedded in motorcycle helmets. Most two-radio comes with auxiliary ports to allow cabling to an external microphone and speaker and the SR10 physically cables into these auxiliary ports allowing the motorcycle rider to safely operate a two-way radio while riding.

Unfortunately, many two-way radio manufacturers design unique auxiliary ports such that custom adaptor cables are required for different models. While Sena provides a few adaptor cables for some popular two-way radios, there are a large number of two-way radios which lack a standard adaptor cable. The Motorola TLK100 falls into this latter category.

To complicate matters, the auxiliary port of the TLK100 includes a Push-To-Talk signal (PTT) which activates the microphone and transmission circuitry. This is uncommon as most PTT buttons simply complete the microphone circuit (otherwise known as make/break circuits). This complication necessitates special wiring using the Sena PTT cable to activate both the SR10 PTT circuit and the TLK100 PTT circuit.

Dealing with compatibility issues and adaptor cables is a way of life for us SR10 owners, even so, working out the details for the TLK100 and testing prototype cables would not have been possible without the generous support and assistance of Motorola Solutions Australia. Thanks J.

Getting Parts

While it's possible to make an adaptor cable completely from scratch with basic components - assuming such components can be acquired - the practicality is that it is vastly easier to start with a number of pre-made cables and combined them. Specifically you will need the following parts:

TLK100 Auxiliary plug and cable - to connect to the TLK100
SR10 2-Way Radio Cable with Open End - to connect to the SR10
Extension Cable for Wired PTT Button for SR10 - to share the PTT across the SR10 and the TLK100
Small container/hobby box/jiffy box to package the wiring

Each of these parts are discussed in the following sections.

The approximate cost to purchase these components is about $70AUD in 2022 dollars. Most SR10 owners will already have an unused extension cable which comes with the SR10 package.

TLK100 Auxiliary plug and cable

The TLK100 uses a custom Motorola four-pole TRRS (Tip, Ring1, Ring2, Sleeve) auxiliary plug. It's a variant of an industry standard plug - with the tip being slightly longer than usual. Unfortunately sourcing this plug is difficult, if not impossible. Typically the easiest way to acquire one is to amputate it from an accessory designed for the TLK100. The PMLN7156A is a suitable donor and can be acquired at modest cost in most markets.

[PMLN7156A donor accessory]

You need the plug and lead up to the PTT button and microphone. The best way to do this is pry open the PTT button and desolder the leads. You can simply snip off the cable at some suitable length, but you will find that re-tinning the leads is very tedious and time-consuming as they are coated in some form of acrylic insulation which is hard to remove. Not recommended.

While the color-coding is usually pretty consistent, it is important that you verify the correct leads by testing continuity to the pin-outs as shown in this image:

[Pin-outs of Motorola plug]

Always trust the pin-outs before trusting the color-coding referred to in the schematic.

SR10 2-Way Radio Cable with Open End

The Sena Open Ended cable (aka the SC-A0116) is specifically designed to make it easy to create adaptor cables. That funny looking connector is known as a "Hirose 6-pin connector" in Sena terminology. This is a bit ambiguous because Hirose is a Japanese company which makes many 100s of different sockets and connectors. Nonetheless, for our purposes we'll simply refer to it as a Hirose connector.

[SC-A0116 2-way Radio Cable]

The following diagram extracted from the Sena web site is a reminder of the "pin-out" and color coding.

[SC-A0116 2-way Radio Cable pin outs]

SR10 Extension Cable for Wired PTT

The TLK100 is somewhat unique in that it has a separate PTT circuit rather than relying on the more typical make/break of the mic circuit as used by many other two-way radios. Since the SR10 assumes all two-way radios are make/break, this extension cable (SR10-A0203) is needed to work around this limitation to operated both PTT circuits simultaneously.

[SR10-A0203 SR10 Extension Cable for Wired PTT]

At first glance, this cable appears to be a standard mono 2.5mm extension cable, but it's not.

First, the 2.5mm PTT socket on the SR10 has a very small entry hole with a diameter of just 7mms - presumably to reduce dust and water ingress. This entry hole is much smaller than standard 2.5mm sockets and thus an off-the-shelf 2.5mm socket is very unlikely to fit the SR10.

Second, the socket end of this cable has a small collar which mimics the socket on the SR10. This means it too does not fully accept an off-the-shelf 2.5mm plug.

For these reasons, this guide strongly suggests using this extension cable rather than purchasing off-the-shelf 2.5mm mono sockets and plugs.

The only caveat with this cable is that there are actually three leads inside this cable so you'll need to work out which two are relevant by checking for connectivity with the contacts on the plug. I have found that the green and blue leads are generally the active leads.

Packaging

Once all components have been acquired, the next step is to assemble the cabling according to the schematic. Before doing so, you need to decide how to package and protect the end product. Viable choices include some form of jiffy box or a heat-shrink solution. The former being more physically robust and easier to construct with the latter presenting a much lower profile which can be more readily stowed in a jacket pocket or similar.

Here are examples of different packaging for the adaptor cable. The first is a heat-shrink of the cable bundle and the second is an ABS container used to house and protect the joints.

[Adaptor cable packaged in ABS water-proof box]

Step 1 - Gather Required Cables

Assuming you have all of the following donor cables: [Donor cables]

and a container of some sort, you are ready to proceed.

Step 2 - Prepare TLK Auxiliary Cable

Prepare the TLK auxiliary cable by prying off the PTT/Mic covers, de-solder the leads and remove the strain-relief jacket.

The end result should look like this:

Step 3 - Prepare SR10 Extension Cable

The extension cable needs to be cut in half with all four of the PTT leads tinned, like this:

As mentioned earlier, there are actually three leads in this extension cable, so be sure to select the correct two. Normally it is the green and blue leads, but it's easy enough to determine with a continuity test as the third lead appears not to be connected to anything.

Step 4 - Prepare Container

Assuming you're using a jiffy box or similar container, now is the time to prepare it prior to starting soldering. The reason for this is that it's a lot easier to feed the cables into the container prior to soldering and heat-shrinking. This image shows a small ABS package with a prepared grommet hole:

Step 5 - Feed cables prior to soldering

[Container with cables fed thru grommet]

Step 6 - Solder according to schematic

For those experienced in soldering, this step should be pretty straightforward. For those less experienced, it can be quite tricky as the leads are very fine and require a delicate touch to ensure they don't get vaporised. Fortunately, there is plenty of cable length available for you to make a few mistakes before you get it right.

You should ensure there is enough lead material so that you can put a good length of heat-strink over each joint which means separating the leads somewhat.

It is very important to verify that the correct leads are being soldered together. In other words, make sure that the color-coded cables match the pin-outs according to the earlier images. If not, believe the pin-outs and ignore the colors.

If you need to tin, or re-tin the leads, note well that they are coated in an acrylic-like insulator which is very hard to remove without damaging the underlying copper lead. I have had success by brushing with a very hot (400C) soldering iron, or delicately burning off with a butane gas torch. Even then, the prepping and tinning is a slow process and gentle process. The flame of a cigarette lighter is insufficiently hot to remove this insulator - it merely soots-up the cable and make matters worse.

It is recommended that you check continuity of each join prior to heat-shrinking. In fact, if you're unsure of your interpretation of the schematic, I recommend deferring all heat-shrinking until after you've successfully tested the cable.

The most important suggestion I have is to be patient and use a good soldering iron with a fine tip. Clean the tip often due to the sooting acrylic deposits which accumulate.

[Schematic Diagram showing leads to solder]

Testing - the final frontier

You'll obviously need two TLK100 radios to perform testing - this is where a friendly Motorola Solutions rep could come in handy. One thing to note about these radios is that they take a while to establish a channel when first powered on, so make sure they are working as just radio-to-radio prior to testing with your adaptor cable.

Hooking up your adaptor cable should be obvious as each connector is unique and thus can only be connected to the appropriate socket on the appropriate device.

In all cases the fault should be either a poorly soldered joint or a joint which does not conform to the schematics. Now it could be that these instructions are deficient and a step has been overlooked or poorly described. Unfortunately you'll have to work out which is which.

The easiest part to test first is audio-out to the SR10 and onward to your Bluetooth speakers. If that doesn't work, check continuity from the soldered joints back to the the TLK plug pin-outs. As mentioned earlier, the pin-outs are the source-of-truth which the color-coded cables may and should follow. You should also be able to check continuity from the Hirose socket of the SR10 cable all the way to the TLK plug.

Audio-in is trickier. Recall that the PTT button closes the PTT circuit on the SR10 and the TLK100, thus testing audio-in (aka the microphone), involves checking three things.

First confirm that the SR10 is activated when the PTT is pressed. As someone experienced with the SR10 you should hear the unmistakeable (albeit slight) hiss from the speakers which provides audio feedback of the audio-in. Second confirm that the TLK100 is activated when the PTT is pressed. This is indicated on the TLK100's LED display which should show the current channel. You will hopefully have observed this behavour when you first tested the two TLK100s independently of your adaptor cable. Third, confirm that when you speak into your microphone, the audio is heard on the receiving radio.

In all three cases, a separate pair of joints and leads are involved. As with the audio-out side of things, it means that either the joint is no good or that incorrect leads have been joined together. The solution is as before: check that the pin-outs from the Hirose socket of the SR10 cable have matching continuity to the pin-outs of the TLK plug.

It shouldn't be necessary, but it's worth also confirming that both PTT buttons on the SR10 function properly. That is the external button and the one mounted on top of the SR10.

Step 7 - Sealing the package

Even though your joints are correctly soldered and heat-shrinked, the cables are still not yet anchored in any way and will vibrate a fair amount as a natural part of being on a motorcycle. Furthermore, the grommet hole used for cable entry is not weatherproof, so over time dirt and moisture ingress can prove detrimental to the joints. To solve all of these problems, I choose to immerse the cables in an epoxy, such as this:

You should be able to fine similar in any good hardware or hobby store.

I have found the resin to be non-conductive and it creates a bullet-proof encasement of the cables and the container. It is of course a one-way exercise. Once cured, there is no removal or repair possible short of cutting off the cables and starting again. So make very sure your joints are well-soldered and that the cable performs as expected prior to entombing your handiwork!

Giving feedback to the authors

We're always looking for feedback which can improve this guide. Any errors of course are of particular interest to us as are suggestions on how to make the guide better. To do so, please send mail to (this hopefully spam-scraper resistant address) nnu@india.emu.ST and we'll try to reply promptly. Keep a watch on the change history at the top of this document to see when your feedback is incorporated. Note that the above email address may change without notice if it gets too much spam, so please don't bother to add it to your address book. Always come back here for the current address.