I don't care for that one because the Hfe affects timing, once the initial turn on through the coupling capacitor is charged up, the ON transistor starts to go linear, and this affects the timing in an Hfe dependent and unpredictable way. The diode method is predictable. Speaking of which the OP needs to increase his timing caps to 22u for
1Hz on each LED.
If you add up all the voltage drops, I think there is no breakdown problem at 9V. Also, the VBE reverse breakdown is in series with the reverse breakdown voltage of the LED.
Tam
F
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You've wired it up wrong
Check your emitter and collector against the Fairchild datasheet.
Some other datasheets make determining which lead is which difficult, and lead to the common error of getting c & e reversed. With everything wired right except c&e reversed, you'll get both leds glowing, but more dimly than normal.
With the values (10 uF and 39K in the .gif) and a working circuit, the blink will be easily seen - no need to change the valuse as has been speculated in the thread.
Ed
My guess ? it's working but just flashing to fast for you to see it? try increasing the caps size by 10 times..
or. Both Transistors are shorted?
who knows.. are you sure you have the correct size caps?
I got bored and ran an SWCad simulation on this. Oddly, with the two diodes, it takes the circuit about 20 seconds to start up. The 150 Ohms has no effect on this. If you just connect the 39Ks to the supply, startup is about
200 ms.With no diodes, and no 150 Ohms, the 3905 VBE reverse is 5.9V. About 4V with the 150R
Tam
You need to add a noise source of some kind. The transistors in SWCad are magical, they have no noise. Any oscillator that is self starting starts on noise. In SWCad, the only noise is the rounding errors in the math.
You can't go by that, the circuit will fire the LEDs up immediately and start the alternation in one or two seconds. Try imbalancing the circuit a bit by changing component values within say 1% on either side and then see what happens.
Even at 9V, which I assume means an alkaline battery that will spend most of its time in the 7.2-8.4V range, you have the caps charging to
9-2.25( the 150 ohm v-drop)-2.2 ( the Vbe + LED drop of the ON transistor) for 9-4.45=4.65V. Then the OFF base sees this minus the Vbe+Vled (2.2V) of the ON transistor for 2.2-4.65=-2.45V, and this is what my SPICE sim shows the reverse peak to be. Another feature, which is more of an accident than anything else, is the oscillation period is dead on within a few milliseconds for battery variation in the 9->6V range. The LED current goes from 15mA to 8mA though, so it does dim slightly. The diode ORing is nothing new, people have been using it for over half century. And placing the LEDs in the emitter allows you to increase the ac-loop switching gain due to their relatively large capacitance rather than decreasing it with the collector placement, the drawback is that 39K is just about the maximum base resistance you can use because the base junction requires about 1.8V to fire things off.
That appears to be right. If I change one of the 330 ohm resistors to 329, or add a 1PF capacitor from collector to ground, it starts up in 24 ms.
Tam
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