Newbie needs help with blinking light circuit.

Hi All,

I hope someone can help me with my circuit dilemma. I recently built a simple blinking light with two electrolytic capacitors and two bipolar junction transistors (along with two LEDs and a bunch of resistors). It was working fine, but I took it apart, and when I put it back together, the LEDs would just stay on rather than blink. I have made sure that the capacitors are in the right way, and I put in the correct values for the resistors. I also swapped the LEDs and capacitors for known working ones. Here is exactly what I am using:

2 x 10 Ω resistors
2 x 4.7K Ω resistors
2 x 100 µf capacitors
2 x 2N4393 BPJ transistors
2 x 1.5 v AA batteries
2 x LEDs

The schematic is attached.

I substituted the 9 v battery for two AA batteries, and because of that, swapped the 470 Ω resistors from the schematic with 10 Ω resistors. I also swapped the 47 µf capacitors with 100 µf capacitors because I only had one 47 µf capacitor.

I think it has something to do with the transistors. I am not sure whether I am using NPN or PNP transistors. I am using different transistors than I had when it was working, but I tried those and they gave the same result, so I am pretty sure it is not the fact that I have changed them.

I have a bunch of parts at home, so if I need to substitute something to troubleshoot, I will probably have the parts to do it. Thanks in advance for any help.

Hi

Have you replaced the 9V battery for 2 1.5V batteries??? Does it mean your circuit is powered by a 3v battery????
If that is your case, that is the problem, put the 9V battery back and it should be ok, Another thing, the 10 ohm resistors may give you brighter illumination but it consumes a lot of energy.

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Sal

This circuit does not work as 2N4393 is a JFET, not BJT.

You should use any low power npn BJT such as 2N2222 or PN2222, 2SC1815, ... instead.

nguyennam

see the waveform at junction of R2 C1and R3 C1

BigFoot13,
With the battery polarity shown, you must use NPNs. To use PNPs, you must reverse the battery connection, the LED connections and the Capacitor polarities to ensure that you do not damage the electrolytic capacitors.
.
The circuit is locked up in a state where both transistors are on. Try momentarily shorting one of the transistor bases to ground. This should start the multivibrator oscillating. You may need to add a start-up circuit to ensure that the oscillation starts when power is applied.
.
If this doesn't work, you may have destroyed one or both of the transistors for the following reason:
.
Suppose that TR1 is turned off. C1 charges to approximately 9V throught L1, R1, and the base-emitter diode of TR2. Now, when TR1 turns on, a negative pulse of approximately 9V is applied to the base of TR2. This -9V pulse exceeds the base-emitter breakdown voltage rating of most transistors, with the potential of destroying the transistor. You can avoid this problem by connecting diodes from the bases to ground (cathode connected to base).
Regards,
Kral

nguyennam said:

This circuit does not work as 2N4393 is a JFET, not BJT.

You should use any low power npn BJT such as 2N2222 or PN2222, 2SC1815, ... instead.

nguyennam

Click to expand...

I agree. Nguyennam has stated the problem and the solution.

Thanks. I switched in a NPN (2N3904) transistor, and now it works. I guess I got thrown off by the other ones that didn't work. But now it works fine with 3 v and 10 Ω of resistance. Thanks again.