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transistor keeps burning out in the circuit! Answered

hi everyone! i followed this 'ible (https://www.instructables.com/id/Laser-Perimeter-Alarm/) to make a laser burglar alarm for an electronics project. i soldered everything together properly, followed the schematic and all, but every time i connect the battery, the transistor just explodes, like, there's a small spark and next thing you know, its damaged. what can be the reason for this and what can i do to solve this problem? i've already used 4 transistors and all burned out! please help!




Here a diagram of an LM 317 from the 1980 from wiki. A four amp Op could have 300 or more transistors doing all kinds of different jobs. A One amp Op from the 160 in 1 only has 3. When i built IC radio from the 130 in 1 Elenco I could not hear anything (too much power loss in the IC) but with the 160 in 1 IC radio I could.


555 timer radio fm Jammer that I opperated with bright LEDs,

Dual flasher FM radio jammer 002.jpg

I know that transistors even if used properly can fail,,,,, that is what happened to my 60 in 1 and eventually the 130 in 1 twice from elenco. 555 timers, Amp Ops and NAND gates are just a whole bunch of transistors. Once with the Power amplifier I reversered the polarity of one Project with the dual amp Op. It got so hot that smoke came out of it and luckly for all the other projects it stilled worked.

Ok... I actually looked at the schematic of the circuit you are talking about. It looks to me his schematic will work, but if you turn the little trimmer-pot all the way to one side, then FULL 9volts goes to the BASE of the transistor. That is too much current for the base. You need to either turn the trimmer resistor all the way to the other side and adjust slowly, or you need to add a 10,000 (ten K) resistor to the base of the transistor. This will limit the base current to a small amount. This still leaves the cadmium cell vulnerable to the same fate. I am unsure if the cadmium cell can take full 9volts on it??? so I would add a 1k resistor in series with the cadmium cell also. This might get your circuit going without "letting out the smoke."


i could eliminate the transistor problem by trying the suggestions made by you and others on this question.. so, thanks for that. but, now, there's another problem. the system doesnt work. i mean, when i connect a battery, the buzzer starts ringing. but then, when i focus the laser beam on the ldr, it just doesnt stop buzzing like it should. it keeps on ringing and ringing. any idea why this may be happening?

It could be several things. You will have to perform several tiny experiments to determine which it is. It could be the transistor is shorted. Test the transistor using a Digital voltmeter on the DIODE TEST function, not the ohms function. Between E and B it should measure about 500 to 900 aprox ohms ONE WAY and OPEN the other way. Do the same test from B to C. The final test is between E and C should be OPEN both ways (reverse your probes). This test is done with the transistor OUT OF THE CIRCUIT.

Another possibility, assuming you wired it correctly, is the laser is not triggering the CDS cadmium photo cell. Try shining a flashlight onto it to test that theory. Maybe the CDS is not sensitive enough for a small dot of a laser?

Another possibility is you may need larger resistor for the 5K. Try something like 100K. Maybe 5K is too much current going to the Base, and is swamping the transistor "ON". A circuit like this is simple, and there are Equations to calculate transistor base current and collector current, but that is beyond the scope of JUST GETTING IT TO WORK. The method we are using here is "trial-and error" not "Design then Build". If you have a Voltmeter, watch the voltage on the Base while you turn the variable resistor.

Try the experiment of connecting a jumper wire from transistor BASE to GROUND. This should turn the transistor OFF and the buzzer should stop. If that works... then the problem is not the buzzer and not the transistor... it is the circuit driving the BASE causing you problems.,

This is a very good circuit for you to learn on. Don't give up --- have patience and you will get it going. The thing to learn here is HOW TO TROUBLESHOOT. I hope you catch on to my EXPERIMENTAL way of thinking .... don't just robotically follow my ideas. Think about "what could be
the CAUSE..... and how can I prove THIS or THAT is true or false??? That
will get you to SOLVE IT. Let's say for example, you don't understand something I wrote... like "E, B, and C" What is THAT??? You will have to research that so you can continue. (Others are reading this, so I am writing to them also.... I am pretty sure YOU know "E, B, and C."

What the circuit is trying to accomplish is this :

The Transistor is operating as a SWITCH. It is just performing an ON and OFF function. If the BASE voltage is close to zero, then the transistor turns OFF... and the buzzer stops. This is accomplished by the cadmium cell receiving LIGHT into its surface and this HOPEFULLY grounds the BASE of the transistor to shut off COLLECTOR current. The little variable pot is used to adjust the voltage to the base to ALMOST turn on the transistor. Then when light hits the CDS... it provides a GROUND connection that should turn off the base voltage.

Another thing you may try is move the CDS cell directly to the transistor base. The other side of the CDS directly to ground. You are using a Cadmium photocell... not a Solar cell... right?

There are many things could be causing the problem... but I am showing you here how to think and do experiments to PROVE what is the problem for yourself. Try measuring the CDS cell out of circuit using an OHM meter. Determine how many ohms it measures with NO LIGHT shining on it and with the LASER shining on it. Write down your results. What does it do when a FLASHLIGHT is shined on it? If it does not react, you have proven the CDS cell is not working.

+1, though 10K might limit the maximum current too much, this transistor doesn't have a great Hfe

Check your supply voltage, check your transistor connections, check that the load is acceptable for the transistor. What are you using for the buzzer ?

Can you put a picture of the buzzer here ? I have a feeling this MIGHT not be a current problem, but I need to see a picture to check.

That looks like an electromagnetic buzzer. It will take quite a lot of current, but try putting a DIODE across the connections, put the end of the diode with the strip to the positive supply, and on end of the buzzer, and put the other end of the diode to the other end

1N4001 or 1N4007, for preference.
I'm NOT guaranteeing anything, and we STILL don't knwo what current that buzzer draws.

that's true.. but thanks for helping anyway... :) i'll let you know what happens!

If you built the circuit as in the Instructable I’m not surprised.

It is the buzzer if you are not using the exact same one you are drawing to much current.

Add a 50 ohm resistor to the collector of your transistor.

Yea a 2N3904 is limited to if I remember right 200 ma so at 9 volts you want at minimum 45 ohm resistor to limit the current.

Check the datasheet here and look at collector current:


By the way you can get datasheets free there.

If a part OVERHEATS and burns out, that indicates too much current going through the device. If it EXPLODES, then you have WAY too much current going through the device. This usually indicates your load is a SHORT...or close to it. What are you using as a buzzer, a CAR HORN? That would explain it.

I'm using a 9 V buzzer, if that helps. I still cannot comprehend what exactly is going wrong. :(