Kenwood RC700A Remote
The RC700A remote control is part of Kenwood-Electronics accessory range for the car audio headunits they also sell. This version is meant to be attached to the steering wheel but there are other versions which are free standing.
Having looked at the internal connections for the IR receiver on my headunit it is apparent that the output from the IR receiver is connected in parallel to the external input that is available for connection to car OEM controls. These OEM controls are found on many modern cars and they are often built in to the steering wheel. For this reasons it is useful to know the protocol used by the remotes, as it seems that any OEM converter would also use the same protocol.
Case & Electronic Design
As you can see in the picture above, the actual IR diode is in the little external plastic case connected to the main remote by cable. This wire is a simple two core cable directly connected to the two terminals of the IR diode. The main electronics are in the larger casing. Getting into the case is quite easy; all that is needed is a small screw driver to unclip the case sections as the following shows. (Click the images for a larger version.)
As you can see from the image to the left, the back of the remote flips up. (The screwdriver is being used here to hold the flap up for the picture) This is meant to just give access to the battery, but the clip holding the two case parts together can be seen just below the edge of the battery marked with an A in the larger version. This can also be seen in the second image to the left for which the battery has been removed.
To open the case, you need to put a small screwdriver into the slot and push the bottom part out and down. It must be done with care as to not damage the internal circuit.
Once the case is open the top part can be removed and the switches can be seen as in Fig 3. The circuit board can be removed, just pull it gently and it will come out. The under side of the circuit board can be seen in Fig 4.
The circuit is quite simple, most of the board is used for the battery connection which is performed by the spring and the metal plate to the right.
The 3.6MHz crystal to the bottom right and the two capacitors
provide the clock for the circuit. The actual part that does all the
work is in the top left.
It is a small microprocessor or encoder. (I am not sure which as I have
not found who makes it.) The chip takes the inputs from the switches
and converts it into a set of pulses on the IR output. The structure
of which is discussed in the next section.
The last thing to note in this section is how to put the unit back together. It is quite simple, just the reverse of taking it apart, but it is necessary to get the battery spring in the correct place. The spring coil should be located on a peg on the back cover. This can be done by putting the circuit board (crystal end of the circuit) back into the cover and use a small screwdriver to push the spring into the correct place. The end of the spring also needs to be held back behind the plastic arm on the case. (Difficult to explain but trust me it is easy!) The case should then simply clip back together.
Transmission Protocol
As with all IR remotes each key has a unique transmission profile, defined by the method of encoding used by the remote. In this case, the profiles are defined by the sets of pulses used and not like some remotes on the width of the pulses. (The latter is Pulse Width Modulation.)
There are three parts to the profiles; lead in, device data and key data. Before I explain these parts I should first explain the general properties of each profile. Each on pulse has a width of 0.5ms, the off pulse has the same unless it is one of the gaps between pulse sets which have a width of 1ms. What this actually means will become more apparent. (In the following pulse means on then off and gap indicates the longer off period.)
The first section to describe is the lead in. The first on pulse is 9ms wide and followed by a 4.5ms wide off period. The device data then follows. The device data, which is the same for all the profiles, is defined by the first 8 pulse sets. In this case the pulse sets are 1 3 1 1 2 2 1 4, meaning pulse, gap, pulse, pulse, pulse, gap, pulse, gap, pulse etc. The last part, the key data, for which there are 9 sets, changes depending on the key. The last 3 sets in this case are always 1 1 1. It should be noted that the largest pulse set found was 7. There is an inbuilt error check because the number of on pulses (ignoring the lead in) should always add up to 33 for every key profile.
The keys on the remote have the following pulse sets: (placeing the mouse pointer over the name will give an image of the obtained waveform)
| Key | Device Data |
Key Data |
Total |
|---|---|---|---|
Volume Up |
1 3 1 1 2 2 1 4 |
4 2 4 1 2 2 1 1 1 |
33 |
Volume Down |
1 3 1 1 2 2 1 4 |
2 2 2 5 2 2 1 1 1 |
33 |
ATT |
1 3 1 1 2 2 1 4 |
3 1 2 4 3 2 1 1 1 |
33 |
Forward |
1 3 1 1 2 2 1 4 |
2 1 2 7 2 1 1 1 1 |
33 |
Back |
1 3 1 1 2 2 1 4 |
3 2 5 2 2 1 1 1 1 |
33 |
SRC |
1 3 1 1 2 2 1 4 |
2 1 3 6 1 2 1 1 1 |
33 |
Disc+ |
1 3 1 1 2 2 1 4 |
2 2 1 6 3 1 1 1 1 |
33 |
Disc- |
1 3 1 1 2 2 1 4 |
4 1 5 1 3 1 1 1 1 |
33 |
