Modding Fisher-Price 72825 Formel Junior Fernlenkflitzer

Picture of 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
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Step 1: How does it work?

Picture of 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.
ksh211 months ago

Please help me

How can I increase the range of the transmitter؟؟

ksh211 months ago

Thank you very nice topic

Cybersaber1 year ago
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??
sliderguy823 years ago
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?

omg Fernlenkflitzer is the coolest word ever!, except maybe platitudinous
zack2474 years ago
can i use a quartz crystal on the car as well? or would that not work?
frickelkram (author)  zack2474 years ago
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 ...
i know, but i had so many 27mhz crystals that i wondered if it was possible
zack2475 years ago

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

joker125 years ago
is it the same for a controler?
nrdesign6 years ago
NEIN! Ich hatte den als Kind :(
ok thanks
deweywsu6 years ago
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
stonefisk6 years ago
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 (author)  stonefisk6 years ago
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.
stonefisk6 years ago
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 (author)  stonefisk6 years ago
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!
Could you use this to change hobby grade RC electronics from 72 mhz to 75 mhz?
Yes, that should work too. The difference between the two frequencies seems to be small enough.
poppniete6 years ago
Modifying a transmitter voids the operating license in some countries.
frickelkram (author)  poppniete6 years ago
Definitly YES! Respect the law of your country while experimenting.
what will happen if a higher frequency oscillator is used?
frickelkram (author)  omkar_hummer6 years ago
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.
nofunclub6 years ago
got a RC car, but no controller with the right frequency. thx 4 da info, gives me room for experimenting. Dr.No.
gedden6 years ago
Thank you I've been looking for somthing like this for a while!