8550\8050 v's 2N3904/2N3906

[john monks]

If you use the negative side of the battery as a reference or zero volts then the emitter of the 2n3906 will be sitting at 1.5 volts. The base of that transistor is connected to the base of the other transistor. Assuming that both transistors are similar except for the polarity the pn junction of the 2N3906 and the np junction of the 2N3904 are effectively connected in series with the battery and therefor should be dropping or have the same voltage across both of them. Or looking at this another way the voltage on the NPN transistor emitter is zero volts, that is it is connected to the reference, the negative side of the battery. The base or np junction is connected to the base of the other transistor has 0.75 volts on it relative to your reference. The base of the PNP transistor therefore has 0.75 volts on it relative to your reference and the emitter has 1.5 volts because of the battery. The normal voltage of a np junction just as it starts to turn on is around 0.75 volts and the normal voltage of a pn junction is about -0.75 volts, the same but opposite polarity. You must keep in mind that in a normal pn junction you must apply about 0.75 volts on the p-channel of a silicone device to get it to turn. A NPN and PNP transistor work the same except the polarities are the opposite.
I know this is a long answer but the concepts are important.
So let's see what happens.

 

[BanksySan]

OK... so... more questions.

Firstly, why do I need to transistors at all? What do they add that the 555 couldn't do alone?

Assuming that there is a good reason for them, the emitter for Q2 (the PNP) is tied to GND. Both the bases are connected to the OUT pin, which will flick between 1.5 volts & 0 volts (I think, rather than between 1/3 & 2/3 of Vcc?)

Given that, I can see that if we assume equal resistance between the two transistors then the drop over each would be Vcc/2 = 0.75V.

When you say the "NP" junction, is that the Base-Emitter junction?

I'll try to get this drawn on a simulator and see if I can make more sense of it in my head.

The second attempt at soldering it didn't work out either, maybe I fried the 555, I could try de-soldering it and plugging it in a breadboard to see if that's it.

 

[john monks]

The transistors are necessary to drive the speaker. The 555 will only drive a few milliamps and the transistors will drive about 100 times that.
The bases of the transistors will not go between 0 and 1.5 volts because the NP and PN junctions will limit it to much less swing. The design is relying on the fact that a 555 will not put out to much current with a 1.5 volt supply so as to overdrive or damage the transistors.
Then NP junction I am referring to is the Base-Emitter junction.
From my experience it is very difficult to fry a 555 with a soldering iron so you maybe ok there.

 

[TedA]

BanksySan,

The IC is an NE555.

This part will not work at 1.5V. It is specified for a nominal 5V to 15V power supply.

If you are working from some sort of existing design that used a 1.5V power supply, it is likely that a CMOS timer chip was used, rather than the NE555. Some CMOS 555 devices will not work well, or at all, on a 1.5V battery.

You should note that even the bipolar parts, such as the NE555, vary a bit from one manufacturer to another, but all have very similar specifications. This is not the case with the CMOS versions. Each manufacturer tends to make his own version. The minimum power supply voltage and the output drive current are two parameters that vary.

I'm driving a Peizo speaker (http://www.coolcomponents.co.uk/cata...ker-p-733.html)

The link takes us to a page describing a permanent magnet electro dynamic speaker. This sort of speaker appears in one of the photos, as well. This is quite different from a Piezo speaker.

It is likely that the original circuit used the transistors as emitter followers. The bases would connect together to the 555 output, the emitters together to the output coupling capacitor, and each collector to one supply. The NPN collector to Vcc ( positive ), the PNP collector to GND ( negative ). The transistors serve to boost the output current. No extra resistors are required. The drawback is that about 1.4Vpp is lost in the BE drops. So this will not be ideal for a single cell supply.

In any case, an NE555 will drive either sort of speaker quite well without the extra transistors. If it is a 64 ohm device, the peak current will be under 70mA, with a 9V supply. This is well within the NE555 output capability.

So I suggest tossing the transistors and using the 9V battery.

The quality of the help you get from forum members depends on the quality and completeness of the information you provide. It's a good idea to give us a link to the original design information, if possible. And post the full device part numbers. What is an "8050", for instance. Not likely a 2N8050. Perhaps an MPS8050? 2SC8050? And an NE555 is not the same as a TLC555.

Other posters are asking for a schematic diagram. Having a link to one would be very nice. It can be quickly sketched, given the layout pictures, but why should each responder have to do this? Anyway, the circuit appears to be a normal 555 astable multivibrator ( oscillator ), with an external timing resistor. The problem is with the added transistors on the 555 output, which do not make sense, as drawn.

Ted

 

[KrisBlueNZ]

A 555 won't operate at 1.5V. Its specified minimum operating voltage is 4.5V. It will run at less than that, but not as low as 1.5V.

Besides, there's very little point adding a two-transistor buffer on the output of the 555 because it already has quite a high drive capability. In the low state, it pulls to ground pretty firmly, and in the high state it pulls to within about 1.5V of VCC.

If you want to get every ounce of power out of a limited supply voltage, you can use a half H-bridge or a full H-bridge. Or you could just increase the supply voltage.

BTW I suspect the original circuit was supposed to be using the transistors as emitter followers, with their bases tied together and driven from the 555, their emitters tied together feeding the transducer, and the collectors connected to VCC (for the NPN) and 0V (for the PNP). This will increase the drive current, but will reduce the drive voltage somewhat.

 

[KrisBlueNZ]

TedA: Snap!

 

[john monks]

Ted make a good point. The regular NE555 may not work.
I saw two specs for the TLC555. One from 1.5 to 12 volts and one from 2 to 12 volts.
The TLC551 goes from 1 to 15 volts for the supplies.
Are you using a TLC555?

 

[BanksySan]

Hi all,

Thanks all for chipping in, apologies for not getting back to you sooner.

Here's the links to the original design, circuit layout and finished product:

http://www.2ne1.com/index.php/diy-hacks/drawdio//
http://www.2ne1.com/files/3913/0968/4001/tracks.jpg
http://www.2ne1.com/files/6713/0968/3989/built.jpg

8050 & 8055 were just transistor numbers I got from these diagrams, I assumed that the 2N3904 & 06 would work as substitutes.

I like that I could use a 9V without the transistors, it makes the circuit much easier for me to understand. (Transistors still scare me).

The 555 IS is a NE555P, Texas Instruments. There's a code "18AER3M" on the IC too.

Cheers all!

 

[TedA]

BanksySan,

Thanks for posting the links.

I haven't had time to make an exhaustive search through all the links, but we seem to have mostly photographs and videos, with few construction details.

It is a bit hard to tell, but it appears that there are several designs.

One uses a bipolar NE555, 9V battery, and no transistors.

Another looks to have transistors and a single cell for power. This is the one you appear to have tried to duplicate. I suspect that this version utilizes a CMOS 555. I would guess that the transistors are connected as push-pull emitter followers to drive the speaker. I think that this is the only way that what we see in the photos could work.

I hope it is clear that there are different versions of the "555", that are not always interchangeable. The part numbers have different prefixes or suffixes to differentiate the different ones. If the prefix ends with the letter "C", that is an indication that the part is CMOS. Viz, "TLC555".

"NE555" is an original, Signetics, designation for their bipolar part. The one that started it all. It is interesting that TI is still using this prefix, while NXP, the successor firm to Signetics, has gone on to some other prefix.

If you have an NE555P, and a 9V battery, the obvious first choice is to revert to the design that used these elements, instead of the low voltage version. You can take the assembly you have, delete the transistors, and connect the 555 to the speaker through the existing capacitor. This should work with the 9V battery without smoking.

Ted

 

[BanksySan]

BanksySan,
If you have an NE555P, and a 9V battery, the obvious first choice is to revert to the design that used these elements, instead of the low voltage version. You can take the assembly you have, delete the transistors, and connect the 555 to the speaker through the existing capacitor. This should work with the 9V battery without smoking.

Ted

Thanks Ted,

Do I even need the two caps between the electrodes on the speaker? Can I drive it from the number 3 pin directly?

 

[TedA]

BanksySan,

You can try connecting pin 3 of the 555 directly to the speaker. The circuit will draw more current from the battery that way.

I'm not sure what you mean by "two capacitors". Only the single 10uF capacitor has to do with the speaker in the circuit of your early post. You would connect the positive end of this capacitor to pin 3, instead of to the transistors.

Ted

 

[BanksySan]

There are two 100uF caps, on the left of this, which then connect to the speaker (marked GND & SP).

http://www.2ne1.com/files/3913/0968/4001/tracks.jpg

 

[john monks]

The speaker connects between GND & SP.
I'm not sure I understand your question.

 

[BanksySan]

The speaker connects between GND & SP.
I'm not sure I understand your question.

Aye, the speaker goes to GND & SP, each of which are in series with the 100uF caps.

 

[TedA]

> Aye, the speaker goes to GND & SP, each of which are in series with the 100uF caps.

I'm looking at this:

http://www.2ne1.com/index.php/diy-hacks/drawdio//

No. The SP connection is in series with a 100uF cap. Ground connects directly to the 555 pins, etc. Not in series with any cap.

A cap in series with your ground connection would block all DC current and prevent power from reaching the rest of your circuit. It would prevent anything from getting hot, too. So presumably your actual circuit has no cap in series with the ground connection.

Ted

 

[BanksySan]

> Aye, the speaker goes to GND & SP, each of which are in series with the 100uF caps.

I'm looking at this:

http://www.2ne1.com/index.php/diy-hacks/drawdio//

No. The SP connection is in series with a 100uF cap. Ground connects directly to the 555 pins, etc. Not in series with any cap.

A cap in series with your ground connection would block all DC current and prevent power from reaching the rest of your circuit. It would prevent anything from getting hot, too. So presumably your actual circuit has no cap in series with the ground connection.

Ted

Sorry, looked again. Yes, the SP is in series with the cap, the GND is just to ground on the battery and is in series with a cap to the reset pin on the 555.

Do I need these caps though? shouldn't reset just be held low?

 

[john monks]

The caps are necessary.
One keeps the DC component off the speaker.
The other keeps the AC component off the 555.

 

[BanksySan]

The caps are necessary.
One keeps the DC component off the speaker.
The other keeps the AC component off the 555.

OK...

By "DC Component", do you mean the square wave coming from the 555? I guess the sudden change would damage the speaker and the cap will soften that?

The cap connected from the RST pin to GND though, Wouldn't GND be ground all the time? Where's AC component as it's powered from a battery?

 

[john monks]

One cap I am referring to is the one that is connected in series with the speaker. Yes, this cap charges up to the average DC level that is being outlived by the 555. So the average direct current being passed through the speaker is 0 amps. This prevents the speaker from being damaged by a constant current coming from the 555 and passes a square wave or any other AC signal to the speaker. This way the current is passes one way through the speaker and then another. Speakers are built for this type of operation that is speakers are designed for AC operation.
The other capacitor I was referring to was the capacitor across the power pins of the 555. This helps supress the AC voltage that will be generated by the 555 and the transistors on the battery.
A capacitor is typically placed in parallel with a capacitor to prevent AC from being impressed across it by the circuit. Batteries have an internal resistance or impedance that can become problematic.
The cap connected to the RST line is probably to keep excessive noise off the line. It is a small value so this let's up to moderately fast signals on the RST but not super high rates of change. It is a filter. And the ground or GND is the reference point for all your signals.