problem???

[pooja1110]

Hi....this is pooja....lost in the world of electronics??made a project on Electronic Railway Signalling in which two red LEDs should flash alternately...but only one of them is doing it!!!!got it first time but lost again...don't know where am i wrong!!!!!somebody please help...i need it desperately!!!! the circuit i am enclosing.....

 

[mixos]

If you got it... and now you lost it... check for damage in connection wires or on the breadboard. Sometimes the wires are cutted and you can't easily find them!

 

[Nigel Goodwin]

If only one is flashing, there's very, very, little that can be wrong - due to the way the circuit works. It can only be the non-working LED, it's series resistor, or the connections to them - nothing else.

 

[andy257]

Hi there,

like they say if you already had it working it must be a fault in a component or wire. Check over all your connections again.

Do you need to use a 555 timer? your circuit seems rather complex for what it does. Ive built something similar and i used flip flops. Reduces the component number by half.

**broken link removed**

hope it helps

andy

 

[Nigel Goodwin]

Do you need to use a 555 timer? your circuit seems rather complex for what it does. Ive built something similar and i used flip flops. Reduces the component number by half.

Nice little circuit, but I'm not happy about it's reliability. The maximum Vbe voltage on the transistors is listed at 6V - when the transistor switches off the capacitor turning it off applies '9V-0.7-LED drop' to the Vbe junction. With new batteries this is getting a bit close, and may tend to blow the transistors, you should add 'catching diodes' to prevent it. There are various ways, an easy one is to insert a forward biased diode in the emitter of each transistor.

 

[pooja1110]

hi

Its me again.....Pooja...checked the whole circuit,connections and the components too!!!!!but this is really bothering me...I can't even sleep at night because i am working for a project at my college which is on 21st of this month....Please help me

 

[Nigel Goodwin]

Re: hi

Its me again.....Pooja...checked the whole circuit,connections and the components too!!!!!but this is really bothering me...I can't even sleep at night because i am working for a project at my college which is on 21st of this month....Please help me

As I posted above, there's hardly anything that can be wrong!.

First thing, which LED is flashing? - top or bottom in the circuit?.

Measure the voltage on the output pin (pin 3) to the LED's, you should see it going up and down

Then try measuring all four pins directly on the LED's, one on each should be varying exactly the same as the previous test - as they connect directly to the same point. The other two connections should be slightly different, the top one higher, and the bottom one lower, but varying just the same.

Try that and report back!.

 

[elec_lover]

If one LED is flashing, that's a good sign, the last 555 is working.
Maybe, the led not flash is connected in the revers, or there is a bad connection, look close around these LEDS.

 

[mozikluv]

555timer

hi,

isn't your voltage supply to the series red led quite low? your current supply to these leds is somewhere between 6 - 7 ma using your 680R resistor. remember that the 555 timer drops the voltage between 2 - 2.2v at its output due to the internal totem pole connection. since your 3rd timer gets its supply from the 2nd timer, the supply is only 6.8v, so its output will be 4.6v

 

[jem]

Try these mods...