Modding Fisher-Price 72825 Formel Junior Fernlenkflitzer

I'll show you how to modify a rc toy car to work on a different frequency. Why should you do that?

A colleague of me bought two identical rc cars for christmas as gifts for his sons. The problem was that the remote control of both cars work on the same frequency. The boys will not be able to use both cars at the same time.

The cars are sold in Germany only in one product version. There is only on 27Mhz version.
I think the car is not sold worldwide. To get an Idea what I am talking about have a look here:
http://www.amazon.de/Fisher-Price-72825-0-Formel-Junior-Fernlenkflitzer/dp/B0002HB0IM/ref=pd_cp_toy_1
http://spielzeug.search-desc.ebay.de/Fisher-Price-Fernlenkflitzer_Spielzeug_W0QQfclZ3QQftsZ2QQsacatZ220QQsalisZ77

However, the method I describe here should work with nearly every rc car. I tore different cars apart and always found nearly the same technology inside.
https://www.instructables.com/id/Using-RC-car-parts-as-remote-control/
https://www.instructables.com/id/Power-to-the-Super-Rebound/
An other Teardown
https://www.instructables.com/id/Micro-RC-Car-Teardown/

Step 1: How Does It Work?

RC remote controlled toys work relatively easy. They need to be cheap. The manufacturers keep costs as low as possible.
On the transmitter side a quarz chrystal is used to keep the transmitter on the defined frequency. In our case it is 27.415Mhz. The transmitter needs to maintain a stable frequency, that's why the quartz crystal is used ( http://en.wikipedia.org/wiki/Crystal_oscillator ).
On the receiver side no quartz crystal is used because it costs to much money. The stability of modern receiver circuits is good enough for a simple toy. To make the receiver work it needs to filter the radio frequencies to detect only the signal sent by the transmitter ( http://en.wikipedia.org/wiki/Receiver_(radio) ). There is at least one filter mounted on the input, near the antenna. While using superhet ( http://en.wikipedia.org/wiki/Superhet ) technology there is also a if band filter needed. Most of the time this is a small ceramic resonator.
Looking at the Wikipeda article you will find a picture under "Design and its evolution". The block named "RF Amplifier" is the place where the first filter, designed with a coil and a capacitor, sits. The second filter, the resonator, is marked with "Filter".
I added a "partly re engineered" schematic of the input filter. "Partly" means, that only the important components are drawn.

Step 2: Modifying the Transmitter

We want to change the working frequency from 27.415Mhz to something that is as far away from that frequency we can get but still inside the 27Mhz band. Looking into frequency tables we find a standard quartz with 27.005Mhz, beeing channel 5 here in Europe.

On the transmitter side only the old quartz need to be replaced by the new one. Dismantle the remote control, locate the quartz, desolder it and solder the new in the same location.

Now it's time for the first testing. When using the remote control the car should NOT respond to it. That's the way we like it to be because the original (not modified) car should not be controlled by a modified remote control. If that does not work for you, the difference between the two frequencies is not big enough.

Step 3: Modifying the Receiver

The receiver should not respond to the commands of the modified remote control because the input filter is still tuned to a different frequency. There are two ways to make the receiver detect the signal.

1. You take a very small screw driver and turn the ferrite inside the original coil to adjust the filter to the new frequency.
2. You build a new filter using a filter set and some small insulated copper wire.

The first option fails most of the time, because trying to turn the ferrite core, breaks it. The ferrite core is fixed in place with wax. It is very, very unlikely thet you manage to get the wax out of the core and then be able to turn the ferrite core. You can try to get the wax out by carefully warming the coil with a hairdryer. You have to be very careful not to melt the plastics of the core. I tried to adjust filters this way some times and nearly every time I failed. Once the ferrite core is broken it is stuck in the plastic core and you can't turn it in any direction anymore.

If the first method fails you need to take the second because you filter is broken. You have to do a little bit math to calculate the ammount fo windings needed for the coil. In this case I did the following math.

Find out the inductance of the original coil. The diameter is 5mm. The wire is 0.3mm thick. There are 7 turns on the coil. The operating frequency is 27.415Mhz. The length of the coil is 3mm.
Using the formula L=N^2*D2/l (L=inductance, N=number of windings, D=diameter of coil, l=length of coil) I get an inductance of 0.408uH. I assume that we do not change the value of the filter capacitor. I got an AL value of the new filter from the data sheet. The inductance can be calculated with L=AL*N^{2 . Using this I get: N=sqrt(L/AL), leading to N=9.}
Our new coil also uses a core with 5mm diameter. We also use the same wire (taken from a wire wrapping tool).

I also simply could have taken the 7 turns from the original coil. The two cores are very similar. The values should not differ too much. The calculation is not exact, but gives a good estimate how to wind the coil. Using 9 windings gives us the possibility to take some windings back if we are not able to adjust the new coil to the new frequency. Even if you try to use the old coild it could be necessary to rewind it in case the adjustment does not work. We lowered the frequency, therefore you have to add windings to the coil to make it work.

In my case the 9 turns worked brilliant! ;-)

Step 4: Getting It to Work

Do some testing. Switch on the remote control. If you are very, very lucky, the car might respond ... but I don't think that will happen ;-)

Very, very carefully turn the ferrite core into the coil. Use a small screw driver and very, very carefully turn the ferrite into the core while keeping the remote control switched on. After a while of turning the car should respond. If not, you are not lucky ... If the ferrite is already at the base of the core you need to remove some windings from the coil. Remove a maximum of two windings at once. Repeat the turning of the ferrite. If there is less than 4 windngs left on the coil ... something is terribly wrong. Either you made a mistake while replacing the coil or the starting coil hat not enough windings. In that case you have to start over from the beginnig winding more turns at the start.
If the car responds, make the distance between the remote control and the car bigger. It is a good idea to make someone help you with that. Take the remote as far from the car as the car stops responding. Turn the ferrite then and see if the car starts responding again. At the maximum possible distance you get sharp point while turning where the car responds. This is the best adjustment for you.

In my case 9 windings where perfect. On the picture you can see that the ferrite core is not fully turned into the coil, that's good.

When everything works you need to apply some glue to the coil to make it mechanically stable. Don't forget to fix the ferrite core! You need not to use wax, glue will do too (how poetic ...). If the car is running and the windings on the coil are jumping or the ferrite is vibrating you may loose you adjustment, and the car may stop working ... ;-)

One more thing ... You can change the frequency only in a small frequency band. Changing the frequency from the 27Mhz band to the 40Mhz band requires more work. It might work, but it need not work. There are rc chips that can handle both frequency bands, but they use different filter sets then. Only changing the coil will not work in most cases.