All About Circuits Forum Keying transistor blues
 Register Blogs FAQ Members List Today's Posts Search Today's Posts Mark Forums Read

 General Electronics Chat Discussion forum for general chat about anything electronics related, including asking questions about material in the All About Circuits E-book, Worksheets, and Videos.

#1
12-03-2010, 01:10 AM
 solder_pat New Member Join Date: Dec 2010 Posts: 9
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
#2
12-03-2010, 01:20 AM
 thatoneguy Senior Member Join Date: Feb 2009 Location: Midwest USA Posts: 6,356 Blog Entries: 4

Can you post the schematic as an attachment in .PNG format?
__________________
$\small{\fra{1}{\fra{1}{R_1}+\fra{1}{R_2}+{\dots}+{ \fra{1}{R_N}}}$ Nifty? -Click Here to see how to make your formulas Look GOOD!
Also-Impedance Graph paper.
#3
12-03-2010, 01:33 AM
 solder_pat New Member Join Date: Dec 2010 Posts: 9

Quote:
 Originally Posted by thatoneguy Can you post the schematic as an attachment in .PNG format?

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.
Attached Images
 tinyoscilator.JPG (45.4 KB, 18 views)
#4
12-03-2010, 01:45 AM
 thatoneguy Senior Member Join Date: Feb 2009 Location: Midwest USA Posts: 6,356 Blog Entries: 4

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?
__________________
$\small{\fra{1}{\fra{1}{R_1}+\fra{1}{R_2}+{\dots}+{ \fra{1}{R_N}}}$ Nifty? -Click Here to see how to make your formulas Look GOOD!
Also-Impedance Graph paper.
#5
12-03-2010, 02:04 AM
 solder_pat New Member Join Date: Dec 2010 Posts: 9

Quote:
 Originally Posted by thatoneguy What frequency is it oscillating at?
7.110 MHz

Quote:
 Originally Posted by thatoneguy 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.

Quote:
 Originally Posted by thatoneguy 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.

Quote:
 Originally Posted by thatoneguy 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!
#6
12-03-2010, 02:09 AM
 thatoneguy Senior Member Join Date: Feb 2009 Location: Midwest USA Posts: 6,356 Blog Entries: 4

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.
__________________
$\small{\fra{1}{\fra{1}{R_1}+\fra{1}{R_2}+{\dots}+{ \fra{1}{R_N}}}$ Nifty? -Click Here to see how to make your formulas Look GOOD!
Also-Impedance Graph paper.
#7
12-03-2010, 02:15 AM
 solder_pat New Member Join Date: Dec 2010 Posts: 9

Quote:
 Originally Posted by thatoneguy 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.
#8
12-03-2010, 02:38 AM
 solder_pat New Member Join Date: Dec 2010 Posts: 9

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.
#9
12-03-2010, 02:40 AM
 thatoneguy Senior Member Join Date: Feb 2009 Location: Midwest USA Posts: 6,356 Blog Entries: 4

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.
__________________
$\small{\fra{1}{\fra{1}{R_1}+\fra{1}{R_2}+{\dots}+{ \fra{1}{R_N}}}$ Nifty? -Click Here to see how to make your formulas Look GOOD!
Also-Impedance Graph paper.
#10
12-03-2010, 02:44 AM
 solder_pat New Member Join Date: Dec 2010 Posts: 9

Quote:
 Originally Posted by thatoneguy 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.

 Tags blues, keying, transistor

 Related Site Pages Section Title Textbook The cascode amplifier : Bipolar Junction Transistors Textbook Bipolar junction transistors : Solid-state Device Theory

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Electronics Forums     General Electronics Chat     The Projects Forum     Homework Help     Electronics Resources Software, Microcomputing, and Communications Forums     Programmer's Corner     Embedded Systems and Microcontrollers     Computing and Networks     Radio and Communications Circuits and Projects     The Completed Projects Collection Abstract Forums     Math     Physics     General Science All About Circuits Commmunity Forums     Off-Topic     The Flea Market     Feedback and Suggestions

All times are GMT. The time now is 12:15 AM.