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Old 12-03-2010, 01:10 AM
solder_pat solder_pat is offline
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Default Keying transistor blues

Hello gang, I'm kind of stumped here. I put together a tiny little one transistor crystal oscillator. It's keyed by grounding the emitter and it works. So I decided I'd key it using an arduino micro controller and send some pulses to it via a 2N3906 transistor.

I would then tie the arduino LED pin via a 1K resistor to the base of the 2N3906. It works, my system voltage is 7.2 volts and the arduino is clearly sending turning the transistor on grounding the emitter of the oscillator I get morse code.

But when I up the entire system voltage to 12 volts, immediately the circuit begins oscillating. It's not drawing full current mind you only 10mA as opposed to the saturated 50mA, but it's turning on. I can't seem to figure out why, but it's as if the collector emitter junction is allowing current through regardless if the base is tied to ground or biased.

Thanks,

Pat
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Old 12-03-2010, 01:20 AM
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Can you post the schematic as an attachment in .PNG format?
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Old 12-03-2010, 01:33 AM
solder_pat solder_pat is offline
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Can you post the schematic as an attachment in .PNG format?

Yeah sorry, my bad.

And to answer anyone who has any legal questions, yes I am a licensed Amateur radio operator, yes the output is run into a 1/2w 50 ohm load.
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File Type: jpg tinyoscilator.JPG (45.4 KB, 18 views)
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Old 12-03-2010, 01:45 AM
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What frequency is it oscillating at?

How close is the keying transistor to the RF circuit? If the wires are very long, it could be an antenna.

What is the physical location of the 4.7uF cap in relation to the Transistor/Arduino?

Also, your schematic shows an NPN transistor, but it is labeled 2N3096 which is a PNP?
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Old 12-03-2010, 02:04 AM
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Quote:
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What frequency is it oscillating at?
7.110 MHz

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How close is the keying transistor to the RF circuit? If the wires are very long, it could be an antenna.
The keying transistor is on the same board as the oscillator. It's on a double sided clad board, I'm building the circuit manhattan style, the leads of the switching transistor are less than .3 inches from the ground plane and other pads. I also grounded both sides of the copper clad board to get rid of any stray capacitance the double sided board might be introducing.

Right now the arduino is disconnected as I'm trying to figure out why the circuit is oscillating without any of the arduino connected. I originally thought that maybe stray RF might be coupled into the base of the transistor turning the thing on.

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What is the physical location of the 4.7uF cap in relation to the Transistor/Arduino?
It's only about 1/2" away from the switching transistor, I had to reduce the value of the 4.7uf cap, because keying was very slow and wooopy, 1uf made the keying much more crisp minus the clicks.

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Also, your schematic shows an NPN transistor, but it is labeled 2N3096 which is a PNP?
Yes, I'm sorry I'm using a 2N3904, not a 3906, It's been one of those days!
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Old 12-03-2010, 02:09 AM
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If they transistor is electrically removed (short C to E w/no connection on base), does the oscillation still occur when at 12V and not at 7V?

If not, try moving the (now) 1uF cap to be from collector of 2N3904 to GND/Emitter, it will electrically still be in the same position as shown on the schematic.
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Old 12-03-2010, 02:15 AM
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If they transistor is electrically removed (short C to E w/no connection on base), does the oscillation still occur when at 12V and not at 7V?
Yes, oscillation occurs regardless of voltage once C to E is shorted.

Hang on, it appears my link between the 1uF cap (formerly the 4.7 uF cap) to the bottom the emitter resistor is not there. Ugh.

Let me warm up the iron, and fix this.
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Old 12-03-2010, 02:38 AM
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Okay fixed the link, yet it exhibits the same results, 7.2 v battery no oscillation. 12 v battery it starts oscillating.

I thought for a second, how could stray RF get into the base of the circuit if the transistor is not switched on for oscillation to occur? if I ground the base, and connect the 12 volts it will still oscillate. If I forward bias it, it starts drawing normal current.
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Old 12-03-2010, 02:40 AM
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If you remove the transistor from the circuit, and use the standard key, does the circuit exhibit the same problem?

We could be looking at the wrong thing here.
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Old 12-03-2010, 02:44 AM
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Quote:
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If you remove the transistor from the circuit, and use the standard key, does the circuit exhibit the same problem?

We could be looking at the wrong thing here.
If I remove the switching transistor, and ground the emitter resistor (of the oscillator) it keys wonderfully under both voltages.
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