Radio Controlled Toy


The following is a transcript of the audio that appears in my video on youtube

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The video contains a reasonable summary of the stage I am up to on my project. It is taking quite a long time. The video is about 60MB if one intends to download it. I have tried to understand the circuit but I am still unable. Any input would be appreciated.


I am attempting to replicate some of the functions of a radio control car using electronic components which are locally available.

This car seems to be the simplest type that I can find. The car moves forward when the button is pressed the first time and then in a circle when pressed the second time. The motion in a circle is achieved mechanically so the direction of the motor is all that changes.

I have reverse engineered this toy. First I disassembled the car and the transmitter and copied the copper track side of the boards onto pieces of paper. I flipped over the board and worked out the positions of the components in relation to the tracks, recording the values of the components wherever possible.

Upon consultation with people from the newsgroup, I made the assumption that the transistor which is labelled 1702L PM20 was a Philips ED1702L after obtaining the datasheet.

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The transmitter and receiver circuit both work on 6V input so I soldered voltage regulators to the circuits which were able to be connected to plugpacks. I later bought a variable power supply for easier experimentation. I bought an oscilloscope to study the signal and voltage levels at different parts of the transmitter circuit. I also used multimeters to measure DC voltages.

I could not identify a red component in the transmitter. I assumed that it was an inductor based on class A power amplifiers that I have seen which use an inductor in the collector output stage to create a voltage swing centered around the value of the DC supply voltage. The component could also be an inductor/capacitor combination as is present in tank circuits to select a required frequency. In order to measure an inductance I needed an inductance measuring device. I bought a multimeter that had such a function but it did not have a low enough setting to measure this inductance. I found a circuit that purported to measure values between 3 microhenries and 7 milli henries in the ARRL handbook for the radio amateur. It may also be found at

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I constructed this circuit on some stripboard and despite my use of ceramic capacitors rather than monolithic capacitors as stated, the circuit did not seem to work as advertised. The circuit oscillates and then passes created signals through a 74HC132 integrated circuit until a signal may be read by a digital voltmeter.

Instead I bought a kit from Altronics that was able to measure inductance easily. I desoldered the red component and soldered some longer leads to enable the connection to the connection posts on the meter. The meter gave a value of 2.56 microhenries for the inductor.

The final component that required description was a ferrite cored inductor near the antenna. The dimensions that I measured (using a ruler with millimeter gradings) were a radius of 4mm and a coil length of 5.5mm. There were 24 and three quarter turns of enamelled copper wire. Using a version of Wheeler's formula that I found on the internet L=(0.394*r^2*n^2)/(9r+10b) microHenries (where r is radius, n is number of turns and b is length of the coil), I found the inductance to be 4.244 microHenries. Perhaps I should also have used the altronics meter to confirm this result, but I haven't. I bought a ferrite core and plastic holder that have been discontinued from Dick Smith Electronics (Australia's answer to Radio Shack). This plastic holder was thinner than the original so I performed calculations to try to match this inductance with 0.125mm thickness enamelled copper wire. I settled on 35.5 winds after performing the necessary algebra and spreadsheet calculations. I created the winding by using a glue gun to hold the wire in place then I wrapped some sticky tape around the windings.


V+ ---+ 6VDC ** | | E * o | B * / | C * o ** | +-----+--[L1]--+------+-----+-------+ | | | | | | | [10K] [Xtal] | [30pF] [100pF] | | | /c | | [10nF] +--------+----| | +-----[L2]---Antenna | | \e | | | | | | | | | +-----+ | | | | | | [5K1] [100R] [50pF] | | | | Gnd---+-----+---------------------+-------+

L1 is 2.56 microHenrys. L2 is a set of windings to decrease the size of the antenna 24.75 windings. Xtal is set at 27.145MHz


V+ ---+ 6VDC ** | | C * o | B * / | E * o ** | +-----+--[L1]--+------+-----+-------+ | | | | | | | [10K] [Xtal] | [33pF] [120pF] | | | /c | | [10nF] +--------+----| | +-----[L2]---Antenna | | \e | | | | | | | | | +-----+ | | | | | | [5K1] [100R] [56pF] | | | | Gnd---+-----+---------------------+-------+

I was unable to find exact matches for the capacitors so I altered them to be higher than the original values. I replaced the 2.56 microHenry red component with a fixed value 2.2 microHenry inductor. I created an antenna similar to the one on the original toy by wrapping some 0.9mm picture hanging wire around a coathanger wire until 20cm length was achieved.

I tried to find a similar transistor from a list within the catalogue based on the maximum power dissipation, the maximum voltage across the collector emitter, maximum collector current and similar Hfe. I decided that the BC338 had the closest values for each of these.

+------+------+----+-----+-------+-+--------+ | |PD@25C|VCEO|IC |Hfe |@|IC(cont)| +------+------+----+-----+-------+-+--------+ |ED1702|625mW |25V |500mA|132-189|@|100mA | Original +------+------+----+-----+-------+-+--------+ |BC338 |625mW |30V |800mA|100-630|@|100mA | Replacement +------+------+----+-----+-------+-+--------+

Understanding the circuit operation is difficult. I tried to find a similar circuit with a good description in the literature. For a time I believed the circuit to be a pierce oscillator minus the capacitor in the feedback path. Oscillators are classed by the connection of the feedback network at the output and the way the feedback network is connected to the input. In order to understand the circuit I need to know where the input is coming from and where the output is. It seems to me that the output is at the collector, where the crystal sits providing a kick to the circuit. The 30pF capacitor directs the output to the input at the emitter.

I have measured the signals at both input and output and found that there is a phase shift. In an oscillator circuit there is supposed to be a total phase shift of 0. The gain also needs to be equal to 1 at this point. Because this circuit has input at the emitter and output at the collector, it should be a common base configuration, but I do not see a capacitor to ground the AC signal attached to the base.

Books about oscillators typically only describe the wien bridge, phase-shift, twin-T, Colpitts, Clapp, Hartley, Armstrong and then a few crystal oscillators, none of which is similar to this.

I found some books from the Australian Department of Civil Aviation to be enlightening but it suggests that there are practicals which may be performed of which I do not have a copy. Instead a book by Patrick and Fardo called "Electricity and Electronics" provided a circuit I could use to measure the input and output characteristics of the transistor. I attempted this but I think I may have had the transistor around the wrong way for some of the tests. The notion in these books is that the transistor may be described mathematically as a 4 terminal device. If I could describe the crystal also, perhaps I could understand how this circuit works.

Regardless of my lack of understanding I connected the circuit together, turned it on and found that it was capable of changing the direction of the motor within the car. However something smells hot on the system and I am reluctant to keep it turned on for very long in case something catches on fire. My next step is to work out what is getting hot (my thoughts are the transistor or the output inductor).

I have taken apart a few other radio controlled toys to see how similar they are to this toy. More elaborate toys have directional controls for the wheels which are controlled by a second motor. A walkie talkie also makes use of a crystal oscillator, but I have not studied these in depth yet.

I was unable to find copies in libraries of any of the books mentioned in the newsgroup I found a book by an author of one of these books but it was not particularly well written so it has not persuaded me to buy the book sight unseen.


Steven Cooke

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