radio controlled toy

----- Original Message ----- From: "Steven Cooke" Newsgroups: sci.electronics.design Sent: Sunday, December 06, 2009 9:44 PM Subject: radio controlled toy

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Progress report:

I bought an oscilloscope and measured some of the voltages at different points in the transmitter circuit.

What I think is the base of the transistor (1702L PM20) has 1.4V when read with my multimeter and 1.8V when read with the oscilloscope probe (1Mohm input resistance). What I think is the emitter of the resistor measures

1.71V with my multimeter but has an unusual shaped wave 1.16V(p-p) measured with the CRO.

(a bit like a chair if this can be read after transmission) /\_ /\_ \/ \/

The bottom of the graph is at 1.5VDC and the top is at about 2.66VDC. It has a frequency of about 27MHz.

What I presume is the collector has a 9.09MHz 8Vp-p sine wave (I presume that the oscilloscope probe is affecting it in some way - the probe has a capacitance of 23pF). I then measured after the decoupling 100pF capacitor and read a 27MHz signal of 2Vp-p and after the 23 turn winding as a 0.25Vp-p signal.

I have looked at a few books in my attempt to understand the transistor, the transmission line and the antenna of this circuit.

Reyner, J.H. (1942) Modern Radio Communication - Volume II (3rd Edition), Pitman Publishing Corporation, New York

page 26-30 Information on the size of the antenna

Boylestad, Robert L., Nashelsky, Louis (1982) Electronic Devices and Circuit Theory, Prentice/Hall International editions

page 674 - The pierce oscillator (seems to have an extra capacitor in series to the crystal)

Hibberd, R. G. (1973) Transistor Pocket Book, Butterworth & Co., London

page 179 - succests that a capacitor across the emitter and capacitor acts as a feedback capacitor (in this example a non-crystal oscillator)

My next step is to attempt to replace this transmitter with one I build myself, to replace components with easily accessable components.

My problem is in understanding what is happening in the circuit. I expect there to be a 0.6V drop across the base-emitter junction. Is this circuit running is class D or something? It doesn't look like it is biased as such. Has anyone in this newsgroup built a pierce oscillator or worked with one?

I have an Advanced Diploma of Electronics and a Bachelor of Science, but it is times like these I feel I shouldn't have been awarded either of them.

Once again, I would appreciate any insight that could be given to my project.

Thankyou,

Steven Cooke

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Neither link works as posted. Try these instead:

receiver:

transmitter:

An analysis of a similar circuit can be found in the following book under the section, "Three-transistor Toy CB Walkie-Talkie". I suspect that the green component in the receiver, which appears to be a single- transistor superregenerative detector, is a parallel inductor- capacitor combination, chosen for the intended operating frequency. The capacitor across the transistor is part of the capacitive voltage divider characteristic of a Colpitts oscillator, the other half of which is hidden inside the green component. The unused transformer looks like a tap-off point for a demodulated audio signal.

see

/dp/0521835399

-- Joe

Do you have a picture of the PCB itself somewhere?

I'm a little surprised your multimeter and scope disagree as to the DC voltage -- since generally a multimeter is just going to ignore some 27MHz RF riding around on top of it --, but I'd trust the scope over the meter.

23pF is pretty close to a short at 27MHz... you're probably benefitting from the inductance of your ground clip here in that it prevents you from completely loading down the collector.

If you don't have a FET probe around, you can try just inserting a 1k or

510ohm resistor in front of the probe tip and see if the measurements change much. (If you need accurate levels, set the 'scope to 50ohms and adjust for the voltage division.)

That 50pF capacitor shunting the antenna looks like it's meant to match a "too short" (...less than a quarter wavelength at 27MHz -- which is huge, >2.5m!) antenna.

Small capacitors in series with crystals will pull them off frequency a bit...

It does. It's not always readily obvious when looking at oscillators just how they work. Indeed, I suspect that historically many people managed to build oscillators without ever doing a formal analysis of them! :-) However, it can be done... most books on the topic start with a Colpitt's oscillator, which can be understood intuitively, and then proceed to move ground references around to demonstrate other commonly-seen oscillator topologies. Next there's some discussion of dealing with unwiedly inductor values using Clapp or Seiler modifications, and then it's usually on to crystal oscillators. By the time you're done, the circuit have been permuted enough times that someone who hasn't been following along will probably not be able to point out the finer points of specific components or the topology anymore.

Many college textbooks take what I think is the rather unfortunate approach of just grinding through the math and demonstrating that, yep, your small-signal model has zero (or slightly negative) resistance, so, sure it's gonna oscillate... but you haven't gained much insight into what's really going on.

From your measurements up there it sounds pretty close... 1.8V-1.16V=0.64V?

I don't think so.

Not a Pierce as such, no.

See rant above about college textbooks. Don't worry, most of your classmates who got their degrees would have just as much trouble here. :-)

Do you have access to a copy of Wes Hayward's "Introduction to Radio Frequency Design" by chance? It has pretty good coverage of this material, IMO. Another useful article is this one:

---Joel

Your 1702 transistor might be an ED1702. See the datasheet:

Looking at the pins, the pinout is:

** | E * | B * | C * **

Review the schematic drawn below to make sure it matches what you have. You didn't label the transistor with ebc, so be sure I've drawn it correctly. The component you identified as "red covered component" is undoubtedly an inductor - I identified it as L1 on the schematic below. I wasn't sure if one of the resistors was 3K7 or 5K7 so check that value, and change the schematic as needed.

View in Courier font:

V+ ---+ | o / o | +-----+--[L1]--+------+-----+-------+ | | | | | | | [10K] [Xtal] | [30pF] [100pF] | | | /c | | [10nF] +--------+----| | +-----[L2]---Antenna | | \e | | | [5K7] | [100R] [50pF] | | | | | Gnd---+-----+---------------+-----+-------+

Re: the measurements. You indicated you weren't sure which was collector, emitter and base. Look at the pinout on the datasheet and you can repost the measurements with any possible guesswork eliminated, and specify the value for Vcc

Ed

Thankyou to everyone who has responded to my post. I appreciate that a little bit of feedback makes a task seem less daunting 8). I am going to go and buy the parts and try to construct a replacement transmitter. I will let you know how well it goes.

(Current ASCII Schematic) View in Courier font:

V+ ---+ 6VDC ** | | E * o | B * / | C * o ** | +-----+--[L1]--+------+-----+-------+ | | | | | | | [10K] [Xtal] | [30pF] [100pF] | | | /c | | [10nF] +--------+----| | +-----[L2]---Antenna | | \e | | | | | | | | | +-----+ | | | | | | [5K7] [100R] [50pF] | | | | Gnd---+-----+---------------------+-------+

L1 is possibly an inductor (RF Choke) or a combination inductor capacitor. L2 is a set of windings to decrease the size of the antenna. Xtal is set at 27.145MHz

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

JK> Do you have a picture of the PCB itself somewhere?

Very difficult to get a picture that isn't blurry with my current equipment.

JK> I'd trust the scope over the meter.

I think I have been using it wrong 8). I'll make the readings again and resubmit.

JK> Wes Hayward (1994-2000) "Introduction to Radio Frequency Design"

3rd Printing, American Radio Relay League JK> JAL> "Three-transistor Toy CB Walkie-Talkie" JAL> single-transistor superregenerative detector JAL> Thomas H. Lee (2004) "The Design of CMOS Radio-Frequency Integrated Circuits 2nd Edition", Cambridge University

Homework

JAL> Neither link works as posted. Try these instead:

receiver:

transmitter:

My apologies for posting incorrect links.

Steven Cooke