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I found some old Radio Shack Electronic books from my childhood (authored by Forrest M. Mims III) and have been trying to construct a few of the circuits for fun. I've been stumped by the operation of the Code Transmitter he describes on page 44 of the Engineers Mini Notebook - Communications Projects. See attached schematic.

I get the 555 timer is being used to generate a tone. I'm not sure why they connect it to the emitter? I see a pot being used to adjust the base voltage of a 2N2222. What I'm not really sure about is how this thing oscillates. It kind of looks like a Hartley oscillator but I've not seen the tap connected to +Vcc before. I assume C4 is some sort of AC filter, C3 is used for the tank circuit, and C5 maybe as a ground reference for the antenna?

I'd really appreciate someone more experienced explaining the oscillator portion of this. I can get it to oscillate but not anywhere near the right frequency. I also have an Eagle file if that is helpful.

enter image description here

JYelton
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Ciphergeek
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4 Answers4

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Yes, C3 being the smallest capacitor associated with the coil, it influences the resonant frequency the most.

C4 is a coupling capacitor. It blocks the +3V DC from reaching the base. But it passes feedback AC current from the tank unattenuated back to transistor base. DC biasing from the 50k pot allows the transistor to start-up as a linear amplifier. With the wiper near ground, DC base voltage is less than 0.6V and the transistor is starved - it won't oscillate.

Don't get caught up in oscillator naming convention. There are so very many variations of a basic theme. The basic theme is amplifying some signal and feeding a portion back from amplifier output so that it reinforces the alternating signal at its input.
In this case, the amplifier is an inverting type: when its input (at transistor base) swings +ve, its output (at transistor collector) swings negative.
The resonant tank, mostly consisting of coil and C3, feeds amplifier output back to amplifier input. But here's the trick...it feeds back the AC signal inverted so that input signal is reinforced. We consider this a 180 degree phase shift at the frequency of oscillation.
This tank works like a kid's see-saw. It is pinned to ground at the middle by the tap, so that either end can swing up and down. Ground is considered the same as +3V for alternating signals by virtue of C1's low impedance at RF frequency. When one end of the resonator goes up, the other end goes down. That's a 180 degree phase shift in engineering terms.

The term "ground" is a misnomer. Since the +3V is likely a battery, this whole apparatus is floating. But where the battery's negative end is connected (to the bottom-most point on your circuit) we use as a voltage reference that we consider unvarying, similar to earth.
The circuit still oscillates if you connect its "ground" to true earth, and it's antenna likely radiates more efficiently as well.

The 555 pin 3 is a digital-type signal that is high or low. When it is high, it robs DC current from the transistor RF amplifier, starving RF oscillations. When pin 3 goes low, the amplifier amplifies vigorously.
I'd think pin 3 could drive the top of R3's 50k pot. The pot may have to be re-adjusted to achieve oscillation. Expect less RF signal this way, but battery may last longer.

The circuit is arranged so that you can build it in two stages...the RF oscillator should work on its own without the 555 pin 3 connected.
Then you build up the 555 audio oscillator and get that producing an audible tone. Then you connect them together and tune in the AM radio.

AM modulated transmitter

glen_geek
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  • Thanks glen_geek. I have a few follow on questions. 1) You say C4 blocks +3V DC from reaching the base which I agree. However the POT is providing say 1.5v to 3v DC to the base continuously right? 2) Also you say the TAP is pinned in the middle to ground? Not sure I follow. The tap is pinned to +3V. 3) Why does the 555 connect to the emitter? why no to the collector or somewhere else? – Ciphergeek Mar 19 '20 at 15:00
  • 4) What starts the oscilation of the tank circuit? I see it receiving +3v but its not obvious to me how it connects to "ground" to get the current flowing? – Ciphergeek Mar 19 '20 at 15:07
  • *edits added to main text*. I notice that 555 is not spec'd below 4.5V DC supply...a 3V supply is surprising. Its logic high voltage at pin 3 *might* supply enough voltage to power the oscillator...it is perhaps 1.5V lower than 3V, which makes it +1.5V above gnd. So I'd not suggest connecting pin 3 to the oscillator's tap instead of transistor emitter - but it'd be a neat experiment...how much weaker will oscillations be? Oscillations likely start from the huge transient when pin 3 switches low. Otherwise, they start like any other oscillator: from tiny noise. – glen_geek Mar 19 '20 at 15:47
  • Why connect the tap on the coil to the +3v and C1 net? Why not just connect it to -vcc ("ground") directly? What does C5 do? – Ciphergeek Mar 19 '20 at 21:48
  • The tap feeds +3V to transistor collector which must be the most +ve of the three transistor pins. C5 perhaps stabilizes RF frequency somewhat when antenna is attached...adding the antenna tends to move it to a lower frequency. In SPICE simulation, it is a squirrelly oscillator when driven by the 555. That large 0.1uf capacitor interferes with the 555 frequency. It can do unexpected things. – glen_geek Mar 20 '20 at 00:32
  • Does it need to be driven by the 555 to operate. I've been trying to build the parts separately to debug them. The 555 piece is easy and works fine. I have so far not been able to get the oscillator section to oscillate. Could you share the SPICE simulation? I've been looking for a good way to experiment with this. – Ciphergeek Mar 20 '20 at 01:10
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What is the frequency you are trying for, and what frequency are you getting? You can change the frequency by changing the tank coil or tank capacitor. Remove a turn from each end to raise the frequency. Or replace C3 with a smaller value. The percent change in frequency is half the percent change in C3. (With larger C3 lowering the frequency.) Add turns or increase C3 to lower the frequency. You can put smaller capacitors across C3 to lower the frequency.

Be sure to find a good Elmer and learn the code after you get this thing running. Beware of the terrible Wouff Hong!

richard1941
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  • I was trying to get somewhere in the 700khz AM band but at the moment I can't get it to oscillate. I've got it built on a PCB and breadboard and can't get either to work. The 555 piece is straightforward its the oscillator I was trying to better understand. I assume it should oscillate without the 555 connected but it isn't. I've used an LCR meter to measure the coil I built and basically get 7uH between the tap and each end. I've also tried those inductors that look like resistors using values I could find that are approx 6.8uH. – Ciphergeek Mar 21 '20 at 13:37
  • It is possible to wind the straw coil improperly. For example, if you fold the 8' wire in half, and wind both wires as a pair - that's wrong. This coil should be wound starting at one end and finishing at the other end. If you have an LCR meter, you should get a result **larger** than (7uh + 7uh) from end to end. If you measure anything **less than** 7 uH end-to-end then the coil is suspect. Also, be sure to properly strip the enamel insulation from wires ends, and tap too. – glen_geek Mar 21 '20 at 19:20
  • So for my coil, I folded the 8' of 30awg wire in half. Fed that through a hole in the center of the straw (actually a plastic ball point pen housing) I pulled that through the straw to serve as the tap. Then I wound the remaining wire in opposite directions from the center hole without overlapping toward each end of the straw. Enamel was stripped. – Ciphergeek Mar 23 '20 at 16:14
  • If you are oscillating on 1.95 MHz, you need a new coil. Go to an asian boba joint. Enjoy the drink there and save the fat straw for a new coil form. The greater diameter will give you more inductance and a lower frequency. The original coil is ill-specified, as the diameter is not stated. It sounds like you did the coil OK, one continuous wind with a center tap. Check for good insulation in the coil; a shorted turn can kill it. I wish I could be there to help you. I love the smell of a soldering iron in the morning! – richard1941 Mar 25 '20 at 17:24
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Thanks to everyone that assisted me.

So here's what I learned. The .005uf capacitor (C5) was dampening the waveform by grounding it. The reason I believe is that the coil I built, when used with C3 at 470pf, is giving me approx 1.95mHz not 700kHz as advertised. This is probably my fault but as of now I can't figure out what I've done wrong with the coil. In any event, I have gotten the circuit to work on both a bread board and custom PCB at this higher frequency. As glen_geek says above, this oscillator is a bit squirrelly.

Ciphergeek
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0

The 555 modulates the RF carrier signal with a single audio tone, so that it can be heard on a domestic AM radio that doesn't have a beat frequency oscillator (BFO).

Chu
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