Introduction: 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:

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.
An other 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 ( ).
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 ( ). There is at least one filter mounted on the input, near the antenna. While using 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=N2*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*N2 . 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.


eel_dahc made it!(author)2016-09-06

I found your instructable helpfull. I bought about 17 various rc cars from a flea market, some had controllers, some didnt. I couldnt figure out how to make a 27 remote work with a 27 car. Well after finding this instructable i learned what that ferite core was for and bypassed it on the transmitter and now a few of the 27 cars that didnt do anything now work!

Andr%C3%A1sL made it!(author)2016-01-09

Thanks for the awesome guide. May I ask what is the minimum gap for the frequency separation? I have two 27.145Mhz RC cars (gifts for the boys from my mother) but I only found 27Mhz crystal's on ebay so far. Will that be sufficient difference?

Daniel%C3%98 made it!(author)2015-06-27

I have a old nikko car that runs on 40.685mhz and the controller is broken in the plastic steering-pins, so I tried using a new 40mhz controller I had laying around, but car would only drive backwards. No matter if I "drove" back, forward or to the sides.

So I switched the crystals from the two remotes, even though the crystal in the new one says, 40B685E7AC.. But it didn't help.. car only runs backwards, no matter how I steer

ksh2 made it!(author)2014-04-07

Please help me

How can I increase the range of the transmitter؟؟

ksh2 made it!(author)2014-04-07

Thank you very nice topic

Cybersaber made it!(author)2014-01-04

Hello I'm very new in rc, i'm starting with my 9 years old boy. And I found that it could be very interesting and fun to have project with my boy.

My boy had an old Fisher Price 4x4 but we lost the RC, is it possible to change just the old coil by a new one that we took from a working car since this new car has a working receipter??

sliderguy82 made it!(author)2012-02-29

On the transmitter side i have located the transmitter but it is welded and does not look like a quartz crystal. Do i need to take that of and get a quartz crystal and weld that on?

Damp+Cuttlefish made it!(author)2010-08-26

omg Fernlenkflitzer is the coolest word ever!, except maybe platitudinous

zack247 made it!(author)2010-06-24

can i use a quartz crystal on the car as well? or would that not work?

frickelkram made it!(author)2010-06-25

Hi, you ca't use "any" quartz crystal. This modification only works in a small ranke of several kHz. On the receiver side you need to change the coil on the input. On the side of the car, there is no quarz crystal for this simple receiver. There is a resonator in the IF frequency part of the receiver. The selectivity of the receiver is mainly controlled there. Changing the IF frequency is not an easy task because you will not find cheap pieces on the market to do it. Working on the IF frequence also changes other paramters, like sensitivity and bandwith, that could lead to a non working receiver. But, changing the coil is not as difficult as it seems ...

zack247 made it!(author)2010-06-25

i know, but i had so many 27mhz crystals that i wondered if it was possible

zack247 made it!(author)2010-02-13

AWESOME! Now i can brag to my friend that i can coose the frequency I want, and he cant! BWAHAHAHAHA!

joker12 made it!(author)2009-09-15

is it the same for a controler?

nrdesign made it!(author)2009-03-19

NEIN! Ich hatte den als Kind :(

omkar_hummer made it!(author)2008-12-22

ok thanks

deweywsu made it!(author)2008-12-21

WOW!! I LOVE projects that show simple steps you can take to mod radio-based circuits. Not too much theory, not too much modification, just simple steps and simple items, most of which you can find around the house. This Instructable was just awesome. Thank you for your work on this! It is very well explained and at a skill level that I think most people will benefit from a great deal. You are a good teacher. Please consider doing something like this again! -Josh Seattle, WA

stonefisk made it!(author)2008-12-19

Isn't a coil and capacitor in parallel a blocking filter? (i.e blocks the resonant frequency) and allows all others. Where as coil and inductor in series will pass only the desired resonant frequency and block others which in this case is the characteristic that is wanted.

frickelkram made it!(author)2008-12-20

Thank you for the comment. You are right.
The schematic I added is wrong. It is correct that the filter used is a parallel resonant filter. But the rf amplifier can't be connected to the other end of the filter.
I have to draw a correct schematic. This one shows the same input stage that is used in the rc car.

stonefisk made it!(author)2008-12-19

Never ever use a normal screwdriver to adjust ferrite cores, they will surely always break! That's a cowboy mistake. Use proper ferrite core trimmer tool for the slot size, usually made of plastic. Vintage radio repair publications state that age hardened locking wax can be softened using ear wax busting products like "Earex" worth a try before going to the extent of winding own coils.

frickelkram made it!(author)2008-12-20

Right, there are special tools available. Using standard screw drivers is a bad idea because the blades are to thin. They twist and put to much pressure on the edges of the ferrit core. I made some tools on my own, using plastic rods. I filed a tip that was formed a little bit smaller than the ferrite cores hole. But even with the correct tools the wax is still a problem. Never heared of "Earex" beeing used for removing the wax. Nice Idea, I'll give it a try. Thank you for the comment!

Hands+Without+Shadows made it!(author)2008-12-19

Could you use this to change hobby grade RC electronics from 72 mhz to 75 mhz?

frickelkram made it!(author)2008-12-20

Yes, that should work too. The difference between the two frequencies seems to be small enough.

poppniete made it!(author)2008-12-20

Modifying a transmitter voids the operating license in some countries.

frickelkram made it!(author)2008-12-20

Definitly YES! Respect the law of your country while experimenting.

omkar_hummer made it!(author)2008-12-20

what will happen if a higher frequency oscillator is used?

frickelkram made it!(author)2008-12-20

I don't understand your question ... but if you mean to change the frequency from lets say 27.005Mhz to 27.415Mhz, it's basically the same. The only difference here is that you need less windings on the new coil, because the inductance need to be lower on higher frequencies.

nofunclub made it!(author)2008-12-20

got a RC car, but no controller with the right frequency. thx 4 da info, gives me room for experimenting. Dr.No.

gedden made it!(author)2008-12-19

Thank you I've been looking for somthing like this for a while!

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