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555 overheating

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at0mbxmb

New Member
Hello!

I have some questions about a project I'm working on. I'm trying to build the pocket theramin featured at popsci a while back (Build a Pocket Theremin on the Cheap | Popular Science) and I'm having some troubles. The link above shows a schematic, and you can see my board here:
IMG_4784 on Flickr - Photo Sharing!
and
IMG_4785 on Flickr - Photo Sharing! (closeup)

Supposedly, the sound will vary depending on how much light it is exposed to. I'm getting very little sound out of it (running at 9V), and only when the pot is turned to the lowest possible resistance.

Also, my second 555 (the lower one) is getting VERY hot.

Any suggestions for what my problem might be?
 

tom_pay

Member
Hi

I was making a different circuit once and the same thing happened, 70°+. What happened was I got the battery polarity mixed up. I then replaced the 555 and hey presto, it worked.

So try replacing it, that's what I did

Hope it works :)
 

MrDEB

Well-Known Member
Never herd of one of these

kinda cool sounding.
Now I want to build one but using a PIC.
I just started using PIC and want to get into all its apps.
using say 6-8 cds etc?
different caps or ??
 

MrDEB

Well-Known Member
just getting into them

just thinking how it could be done.
what with so many inputs etc.
 

at0mbxmb

New Member
Battery polarity, you say?

I'm running off of wall power (9V adapter), and I checked the board with a multimeter to make sure the +'s and -'s are all straight... and I checked the 555 chips and they're all right-side up. hmm...


Also: I was getting readings as high as 12V (with a 9V adapter??) from the multimeter. Any suggestions why that's happening?
 

confounded

New Member
its hard to see if you've wired your citcuit correctly from your photos.

Have you ensured that all your grounds and supplies are connected, by this i mean i see you have used both sides of the breadboard as a +ve rail and -ve rail are all these connected to the power supply +ve and -ve

actually i notice lower 555 has no connection to +ve supply on pin 8 (top right pin) and it should do
 
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at0mbxmb

New Member
confounded:

Man, thanks! I wired the second 8 pin to +. Big mistake on my part.

I checked all around the board, too, and yeah--power is everywhere.

I put in a new 555 just in case the old one was fried and tried again. Still getting the same problem with heat and no sound, though. :confused:

I cleaned up the board and re-checked everything. Here's a new picture that's hopefully easier to see:
http://farm4.static.flickr.com/3298/3641474157_3698c16a1b_o.jpg
 

Willbe

New Member
I guess you could look up the thermal resistance of the 555 chip, measure the temp. with your "digital probe"

no burn ever at 42C
burned in 30 sec at 54C
5 sec at 60C
1 sec at 71C

and then figure out why this chip is dissipating so much power.
 
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Boncuk

New Member
Hi,

did you really use an 8Ω speaker?

A speaker is an inductive load and will cause spikes much higher than the supply voltage hitting directly into the 555 output.

You might try to add an electrolytic cap between output and speaker. (Use 100 to 200µF/35V and connect the positive terminal to the output and the negative to the speaker.)

Boncuk
 

audioguru

Well-Known Member
Most Helpful Member
Simply use ohm's Law to calculate that the 555 is severely overloaded when it directly drives an 8 ohm speaker if the 9V battery is new.

The circuit is missing an important supply bypass capacitor. Use 100uF.
 

at0mbxmb

New Member
Thanks so much for all of the help so far!

I got a 100µF electrolytic capacitor and I put it between the pot and the speaker, but I'm still getting the same problem. You said "between the output and the speaker", so I interpreted that as between the pot and the speaker. Did I misinterpret where it should go?
 

audioguru

Well-Known Member
Most Helpful Member
The 555 is not a power amplifier and cannot directly drive an 8 ohm speaker without getting too hot. Add a 6.8 ohm/0.5W resistor in series with the 8 ohm speaker to reduce the current into the speaker. The 6.8 ohm resistor will also keep the pot from burning up.
 

nike6

Banned
I'd suggest 47 Ohm to 220 Ohm though you get a hudge loss,
or to use audio amplifier IC such as TBA820
TBA820 Amplifier

futurlec has them for less than 50 cents!
they can work at 5 volts.

if you breadboard the circuit, you can try to remove a few of the components, and still have the circuit working without too much distortion.
 

audioguru

Well-Known Member
Most Helpful Member
The 555 produces a square-wave output like a buzzer so of course it will sound distorted.
 

Boncuk

New Member
The 555 is not a power amplifier and cannot directly drive an 8 ohm speaker without getting too hot. Add a 6.8 ohm/0.5W resistor in series with the 8 ohm speaker to reduce the current into the speaker. The 6.8 ohm resistor will also keep the pot from burning up.
That's still too high a load. (about 400mA too much!)

Better use a telephone speaker of 32Ω impedance and a 15Ω resistor in series.
 

the eagle

New Member
hi

Thanks so much for all of the help so far!

I got a 100µF electrolytic capacitor and I put it between the pot and the speaker, but I'm still getting the same problem. You said "between the output and the speaker", so I interpreted that as between the pot and the speaker. Did I misinterpret where it should go?
mr.at0mbxmb if you put th tr with out the capacitor that is wrong
you must put it too on the c of the tr to take the true signal
successful
 

confounded

New Member
I just built the circuit from the schematic you attached (with a 82ohm resistor in series with 5k pot) and found i needed to add a small resistance (500ohms) in series with the LDR's and i had to increase the capacitors to 0.1uf to get good variance in sound when i dimmed my lights.

I did not experience an over heating 555.
 
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