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Connecting a 8 ohm buzzer with a 555 timer

Audioguru

Sep 24, 2016
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Get rid of the tangled mess of wires all over the place and instead use a neat and tidy pcb.
The buzzer is probably an 8 ohm speaker that will quickly burn out the 555 oscillator.
 

laurencelovesphysics

Nov 26, 2021
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whats a pcb ? yes the speaker is a 8ohm, what does it mean that it burns out the oscillator? What can I do to fix it ? Thank you for responding !
 

Audioguru

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A pcb is a Printed Circuit board with the parts soldered on to it.
The maximum allowed output current for a 555 is 200mA. The circuit with a 12V supply feeds 5V pulses to the 8 ohms speaker. 5V/8 ohms= 625mA that burns out the 555.
I said how to fix it in your other forum about it.
 

Audioguru

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Simply do the math. A 555 will be overloaded when powered with 9V and is feeding an 8 ohm speaker.
Also, the rest of the circuit might not work with only 9V.
 

laurencelovesphysics

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A pcb is a Printed Circuit board with the parts soldered on to it.
The maximum allowed output current for a 555 is 200mA. The circuit with a 12V supply feeds 5V pulses to the 8 ohms speaker. 5V/8 ohms= 625mA that burns out the 555.
I said how to fix it in your other forum about it.
hi, why does the 12V source only pulses 5 V in the timer ? How many volts would the 9V source pulse into it ? thank you
 

Audioguru

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The circuit has capacitor C4 in series with the speaker to avoid the speaker having DC in it.
Without the capacitor, the 555 output tries to go from almost 0V to almost 10V which tries to produce a current of 10V/8 ohms= 1250mA which is way too high for a 200mA 555.

With the capacitor, the capacitor has an average of 6V across it then the voltage to the speaker is AC with plus and minus 5V peak.

A brand new little 9V battery cannot produce more than about 200mA for a few minutes. After the few minutes it is nearly dead.
 

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laurencelovesphysics

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so if it used to work and its not working anymore i might just be because there is not enough voltage in the battery ? thank you so much for responding once again
 

Harald Kapp

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This project is meant to be used with a piezo buzzer, not a speaker.
Such buzzers have a high impedance and will not overload the 555.
so if it used to work and its not working anymore i might just be because there is not enough voltage in the battery ?
More likely you burnt the 555 by overloading it as has been stated before. Get a new 555, a fresh battery and a true buzzer. Get rid of the speaker.
 

Audioguru

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The original circuit shows photos of a piezo transducer, not a speaker and not a piezo beeper that has an oscillator in it..
 

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roughshawd

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I think the 8 ohm thing got you. If I remember my electronic study, most electronic circuits use 126 ohms, thats because after you add a few components to the signal you experience signal loss, and putting source in there just makes noise. Upping the ante can give you a little bit of amperage that you wouldn't normally have in a standard power circuit.
 

AnalogKid

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If I remember my electronic study, most electronic circuits use 126 ohms
What ?
thats because after you add a few components to the signal you experience signal loss, and putting source in there just makes noise.
What - what - ?
Upping the ante can give you a little bit of amperage that you wouldn't normally have in a standard power circuit.
What in the world does this mean?

ak
 

hevans1944

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I am wondering, what is the purpose of C1 (22μF, 50V)?

If the rain sensor becomes sufficiently conductive, because a rain-drop hits it, then Q2 (2N2222), will conduct through R3 (10kΩ), the base-emitter junction of Q1 (BC548), LED1, and R2 (330Ω). Thus, Q1 conducts and LED1 illuminates if the rain sensor is sufficiently conductive.

Q2 is held nonconducting by R1 (220kΩ), connected between its base and circuit common, but only if the rain sensor is non-conductive. So... one drop of rain turns on LED1 until the drop evaporates or another drop of rain comes along...

Meanwhile, a positive pulse is conveyed from the emitter of Q2 to the base of Q3 (2N2222), via C1, which momentarily turns on Q3 until C1 charges. With Q3 conducting through D1 (1N4007), and R5 (3.3kΩ), a positive pulse is applied to the RST input of the 555 timer. After a fairly long interval, determined mainly by the base current charging C1 through the base-emitter junction of Q3 in series with D1 and R5, C1 will be charged and no further base current will flow. That means the base-to-common voltage developed across R4 (470kΩ), falls to zero and Q3 ceases to conduct, allowing the RST input to 555 timer to be pulled to zero (circuit common) by R5.

Note that if the rain sensor continues to conduct, then C1 will eventually be charged and Q3 will turn off, even though it may continue to rain.

As for the low-impedance speaker connection, instead of the connection of a high-impedance piezoelectric sounding device, it should be easy to insert a suitable resistance in series with the speaker voice coil that will limit the current supplied by the output of the 555 timer. Or add another output stage to drive the speaker, meanwhile replacing the (probably) bricked timer. You can test for that by replacing the speaker with an LED and replacing C4 (100μF, 25V) with a suitable current-limiting resistor for the LED, say something in the neighborhood of 600Ω.

As a side note, we are planning to replace our 14-year old asphalt-shingled roof with a steel roof and solar photovoltaic panels. I think we will enjoy hearing the patter of rain drops on the new roof. Maybe I could rig a small microphone in the attic to pipe the "music" to some other area of the house, or trigger an iPhone message that says it is raining. This is an IoT project if I ever saw one! Hmmm. What about snow sensors for you folks living north of sunny Florida? Maybe a heated sensor, so the snow melts to provide conductivity?
 

Audioguru

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If a speaker is used instead of a piezo beeper that has its own audio oscillator then the speaker will only "click".
 

hevans1944

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If a speaker is used instead of a piezo beeper that has its own audio oscillator then the speaker will only "click".
Why would you think that? For the timing values given (Ra = 3.3kΩ, Rb = 68kΩ, C = 10 x 10-9 F), The Texas Instruments datasheet says the 555 timer, when enabled (RST high), will oscillate at roughly 1000 Hz according to this formula:

freq = 1.44 / [(Ra+2Rb) C]

Well, it oscillates briefly, how brief depending on how fast C1 charges toward 12V when Q2 is conducting. Problem, as I see it, is that once Q2 is conducting (it's raining!), Q3 only conducts until C1 is charged, and then Q3 shuts off. This pulses the RST input on the 555 timer high while C1 is charging, enabling the oscillator, but allowing only a brief period of time for 1kHz oscillation until C1 charges to about 12V, cutting off the base voltage that was causing Q3 to conduct. I suppose, if this interval is short enough, it could sound like a "click" from the speaker...

If the rain stops, and Q2 becomes non-conducting, then C1 slowly discharges through R4, R2, LED1, the emitter-base junction of Q1, and R3. R4 = 470kΩ and C1 = 22 x 10-6 F provide the major portion of the time-constant that's slightly more than 10 seconds! Of course C1 doesn't have to fully discharge to pass another positive pulse to the base of Q3, but the rain sensor must definitely become a high enough impedance to force Q2 into non-conductance again to allow C1 to begin to discharge.

Perhaps all this will occur between successive rain drops, perhaps resulting in a brief "chirp" for each rain drop that lands on the rain sensor. More likely, the rain sensor will become soaked with rain, remain conductive and keep Q2 turned on so C1 remains charged. So, the first drop of rain that causes the rain sensor to trigger the 555 timer into oscillation, causes only a brief "beep." I doubt this is the intended mode of operation.

Hmmm. Maybe someone will breadboard this circuit to see if the analysis above is correct. Make sure the 555 timer is a new one, not freshly baked, and maybe just put an LED and a 600Ω current-limiting resistor on the 555 output pin. Does the LED light up (briefly) when a drop of water falls on the rain sensor?.
 

Audioguru

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I wrongly thought the 555 was used as a timer instead of as a power oscillator.
Even if a piezo transducer is used as a speaker it will not produce 1kHz.

The original article had the fuzzy schematic with its parts too far apart and the 10nF looked like 10uF.
The text said when the reset pin went high then the output of the 555 also went high. Later (I did not read it before) it said the 555 began oscillating.
 

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hevans1944

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There is a lot wrong with the original article. The parts list specifies "1 x Small Rain Sensor" but provides semi-useless instructions on how to make one using interleaved aluminum strips, presumably aluminum foil, cut out and glued to a "Bakelite or Mica board." The picture shown on the website features a soldered right-angled pair of connector pins attached to the Small Rain Sensor board. Clearly, this is an etched and tin-plated circuit board, conventionally manufactured, with no indication of how to obtain one. If one is gluing interleaved wires to a bare circuit board substrate, the obvious choice is to use bare copper wires, which can be easily soldered, instead of aluminum foil, which cannot be soldered... at least not as well-soldered as shown in the picture.

It doesn't end there. The website goes on to try to describe Working of the Circuit: "The 555 timer is configured in Astable Mode. As the Reset pin of the 555 Timer IC is given positive voltage, it becomes active and we will get a Pulse signal at the output pin 3 of the 555 Timer IC. This will turn ON the buzzer and the alarm is activated." There is No indication in the parts list of what the "1 x Buzzer (or Speaker – 8Ω)" might be, but as @Audioguru pointed out in his post #18, as well as earlier posts, a typical piezo sounder resonates at somewhat less than 5kHz and will produce NO output at the roughly 1kHz frequency produced by timing components R6 (Ra), R7 (Rb), and C3 (C) as described in the Texas Instruments datasheet and my post #17. Of course an 8Ω loudspeaker, even (or especially) one as puny as the speaker shown by the original poster, would reproduce a 1kHz square wave... except its low impedance would overwhelm a 555 timer, as @Audioguru pointed out in his post #8.

Conclusions? This circuit does not work as described. It was probably never built and tested as described. The author does mention that C1 eventually charges up, turning off Q3 and stopping the oscillations from the 555 timer, but if the "Small Rain Sensor" is still conducting, Q2 remains conducting, and C1 has no discharge path. Also, C1 DOES NOT charge (mainly) through the huge resistance of 470kΩ presented by R4. It mainly charges through R5 (1.3kΩ), the forward-biased diode D1 (1N4007), and the emitter-base junction of transistor Q3.

Recommendations: Add a current-limiting resistor in series with the 8Ω speaker. Try about 1kΩ and work down from there until sufficient audio volume is achieved or the 555 timer burns out, whichever comes first. Or maybe just add another transistor to drive the loudspeaker... Fugeddabout using a piezo buzzer without circuit modifications, which I am sure someone here on Electronics Point would be willing to provide. So, all you EP gurus, jump right in here and help @laurencelovesphysics get his rain detector working. I would breadboard a working version of this, but my wife's HoneyDew jar is full of work slips that have nothing at all do do with electronics. <sigh>
 
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