Basic Transistor Switch Question

Thread Starter

boblob

Joined Jul 25, 2013
15
OK, here's my idea. The audio modulation will be dependent on the beta of Q2, so your results may vary.
Also, I didn't know what your audio input amplitude was. I chose 200mV p-p.
Absolutely superb - I'll get this up on the breadboard. The extra modulation control will be great - I had tried implementing that with just a 500K pot as a "level" control off the sawtooth, but it didn't work terribly well. I'm guessing the extra emitter-follower will make it much more stable.

200mV is about right for the input.

One (last, I promise) thing - what's the benefit of placing the LED load on the emitter here?

That's LTSpice you used for the simulation? I really need to get up to speed on that.

Thanks again - much appreciated.
 
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Thread Starter

boblob

Joined Jul 25, 2013
15
Not too comfortable with this alternate circuit. There are chances of burning out the EB junction on power up when the 4.7 uF behaves as a dead short.

Ramesh
So the 4u7 (very briefly) conducts to ground on power up? Another good reason not to use this one :D.
 

Ron H

Joined Apr 14, 2005
7,014
Absolutely superb - I'll get this up on the breadboard. The extra modulation control will be great - I had tried implementing that with just a 500K pot as a "level" control off the sawtooth, but it didn't work terribly well. I'm guessing the extra emitter-follower will make it much more stable.

200mV is about right for the input.

One (last, I promise) thing - what's the benefit of placing the LED load on the emitter here?

That's LTSpice you used for the simulation? I really need to get up to speed on that.

Thanks again - much appreciated.
It is LTspice.
I originally had the LED (without the series resistor) in the collector of the first emitter follower, removing the need for the second EF. The problem(?) with that was, while the LED current was modulated by the sawtooth, it never actually turned off. Another option is to eliminate the second emitter follower, but drive the LED and its resistor in parallel with the pot.
You could try those options and see if you like them.

BTW, if you don't put the LED in the collector of the 1st emitter follower, the pot could be 10k instead of 1k. If you do put the LED in the collector, the 1k pot is needed to allow adequate current for the LED. Or, you could use a 10k pot with a 1k resistor in parallel with it.
 

Thread Starter

boblob

Joined Jul 25, 2013
15
It is LTspice.
I originally had the LED (without the series resistor) in the collector of the first emitter follower, removing the need for the second EF. The problem(?) with that was, while the LED current was modulated by the sawtooth, it never actually turned off. Another option is to eliminate the second emitter follower, but drive the LED and its resistor in parallel with the pot.
You could try those options and see if you like them.

BTW, if you don't put the LED in the collector of the 1st emitter follower, the pot could be 10k instead of 1k. If you do put the LED in the collector, the 1k pot is needed to allow adequate current for the LED. Or, you could use a 10k pot with a 1k resistor in parallel with it.
OK, just given it a shot. The 1k mod control works very effectively - much better than my 500k pot alone. The LED and its resistor in parallel with the pot works fine, but of course flashes on the waveform peaks (remember I had the LED in parallel with the 2N2222 in my first design so the flashes happen in the troughs - i.e. in time with the audio peaks). More importantly adding the emitter-follower Q1 also adds quite a lot of oscillator feedthrough ticking. I've tried adding 100n to ground after R2 to form a low pass filter and take some of the sharp edge off, but it's only partly successful. Adding Q4 to switch the LED just adds more feedthrough, so I'm inclined to go parallel with the 1k pot.
 
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Thread Starter

boblob

Joined Jul 25, 2013
15
I made some improvements(?).
See attachment.

EDIT: I changed the schematic slightly.
Excellent. Don't follow what Q4 and Q5 - looks like a rectifier to me. Be very interesting to see how effective the click filtering will be - that's always a problem in this type of circuit. Itching to get this up on the breadboard, but it'll have to wait until the morning...
 

Ron H

Joined Apr 14, 2005
7,014
Excellent. Don't follow what Q4 and Q5 - looks like a rectifier to me. Be very interesting to see how effective the click filtering will be - that's always a problem in this type of circuit. Itching to get this up on the breadboard, but it'll have to wait until the morning...
Q4 and Q5 basically make a cheap comparator. Current flows through the transistor with the higher base voltage.
Another way of looking at it:
Q5 is a constant current sink when its base voltage is higher than the sawtooth voltage on the base of Q4. When the sawtooth is higher than the base of Q5, Q5 is cut off. I set the voltage on the base of Q5 to approximately the mid-point of the sawtooth, so you will get a ≈50% duty cycle in the LED.
 

Thread Starter

boblob

Joined Jul 25, 2013
15
Q4 and Q5 basically make a cheap comparator. Current flows through the transistor with the higher base voltage.
Another way of looking at it:
Q5 is a constant current sink when its base voltage is higher than the sawtooth voltage on the base of Q4. When the sawtooth is higher than the base of Q5, Q5 is cut off. I set the voltage on the base of Q5 to approximately the mid-point of the sawtooth, so you will get a ≈50% duty cycle in the LED.
Just boarded this up - sounding pretty good! I started by adding the click filter, which didn't make a huge difference, then swapped the 1k mod pot out for a 10K - no ticking at all. Adding the comparator + LED the tick is back, but reducing the LED current by using a 4k7 for R10 helps a great deal. Very promising!

One modification I need to try... the modulation control is pretty bunched up towards the end of the pot's range (it goes from 0-full within the last half turn), so I think a 4k7 pot in series with 5k6 will be better.
 
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Thread Starter

boblob

Joined Jul 25, 2013
15
Can you post a picture of your breadboard?
Here you go. You can see the audio amp + click filter top right, unijunction top left, LED flasher bottom left and modulating transistor bottom right.

The extra pot with the red leads is just a level control tacked on at the end.

It's not the tidiest, so maybe some shielded cable on the input + output and a decent layout will help reduce ticking. It's actually pretty good on the breadboard as long as the LED current is kept low...
 

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