Alternating blinking leds, pnp transistor not fully off

Thread Starter

Sergio34

Joined Nov 10, 2018
52
Hello, it is another of these days that I am trying to resolve the obvious. Something that should work immediately it does not. I am trying to blink the two leds alternatively using a pnp transistor for one of them. The pnp does not fully turn off. When one of leds (the one connected straight to the 555 output) is on the other is also on but a little less, it fades a little but it does not turn off completely. I used both configurations as you can see, one is on the emitter side and the other on the collector. Both of them the same, I tried lower resistor for the pnp base down to 820Ω with no luck. Transistor is 2N2907. If I use the 2N2907 and led alone though (without the other led connected) it works, it switches off, and with a resistor from its base to ground it switches on, perfect. Why when it is connected to another load (like the led without pnp) it does not switch off? It looks like the led without the pnp "steals" all the current from the pnp and does not let it switch off completely but this does not make any sense since they are just two components connected in parallel.
 

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Thread Starter

Sergio34

Joined Nov 10, 2018
52
The output of a 555 IC does not go high enough to turn off the PNP transistor. Add two series diodes in series with the PNP emitter.
I knew that somebody was going to mention that. I've tried that too, however I did not try it with one of the configurations, I will let you know.
 

rphare

Joined Nov 20, 2015
9
I think you can get rid of the transistor circuit. Connect the "off" (left-hand) resistor and LED from pin 3 to Vcc (watch LED polarity). When pin 3 goes low, it will sink current from Vcc through the LED/resistor to ground.
 

Thread Starter

Sergio34

Joined Nov 10, 2018
52
Thank you for your replies, the only thing that worked so far is all this together: two diodes in series with the "off" led plus a voltage divider (a 3.9K resistor from led to ground), it needs the voltage divider as even three diodes in series were not enough. Too many components... I will post a schematic soon for reference. I am yet to try rphare's suggestion.
 

ci139

Joined Jul 11, 2016
1,696
? you put only 10mA through each LED (12-3.12-0.7)/820≈10mA , drive CE 2N2907 (h.FE≈200) with 3.4mA sufficient for 680mA collector current

+ your PNP is ON even without 555 coz it gets current through the led - should use diodes 1N4004 or alike like . . . not helping your schematic

http://tinyurl.com/ukrynyg a Falstad simulation
 

Thread Starter

Sergio34

Joined Nov 10, 2018
52
So here is what worked only with this, it is the only one that the pnp led goes off completely. Of course if I investigate more other solutions could work too, but I will leave this research for another time... (Photo did not come out very well, Vcc for 555 is 12V)
 

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Thread Starter

Sergio34

Joined Nov 10, 2018
52
I think you can get rid of the transistor circuit. Connect the "off" (left-hand) resistor and LED from pin 3 to Vcc (watch LED polarity). When pin 3 goes low, it will sink current from Vcc through the LED/resistor to ground.
I was very excited to do that when I wake up, I was thinking "how I did not think about that?" However it did not turn off completely, I guess if I work a little around that it should work but life is short and I would have to add components again anyway.
 

ci139

Joined Jul 11, 2016
1,696
playing is creation - such fuzzy experiments actually teach you a lot more than over sophisticated text books using too technical language (not always)
 

Reloadron

Joined Jan 15, 2015
5,735
If all the objective is would be have two LEDs alternate on a 555 output I don't see the need for the transistor.

LED Alternate CKT.png

When the 555 output is low D1 is on, when the 555 output is high D2 is on. The standard TTL 555 can operate from a supply voltage between 4.5 volts and 18 volts, with its output voltage approximately 2 volts lower than its supply voltage VCC as was pointed out. The 555 can source or sink a maximum output current of 200mA, (but it may get hot at this level), so the circuit variations are unlimited but driving a few LEDs is no problem. Now if you have some reason for using the transistor then carry on. :)

Ron
 
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