***
>
> 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 sci.electronics.design 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.
>
> Original
>
> 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
>
> Altered
>
> 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 sci.electronics.design. 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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>