Circuit Description
The circuit consists of two separate free running multivibrator and an oscillator.A free running or astable multivibrator is one which has two quasi-stable states and the output of one stag is connected to the input of the other through a coupling capacitor.Since both the states are quasi-stable, the output attained is continuously varying in nature i.e. high, low high low.
The output is in the form low pulses, the frequency of which depends on the base biasing resistor and the coupling capacitor, When these resistances and condensers for both the stages are of different values, the output wave form is rectangular; this is because the time constant of the two quasi-stable states becomes different.
If this time constant of the two, states is made the same, the output obtained then is square wave. Two states of the multivibrator are made identical by the use of the same values of components.
The components used in this dual tone siren circuit result in a square wave output and the time constant selected is so as to give a fairly good rise and fall of the siren.
However, one may change the value of coupling capacitors to get any other desired time constant.
The second unit is an oscillator section. The condenser connected at the output is the feed back condenser. It determines the tone of the siren.
Higher the value of the condenser the lower is the pitch. for high pitch sound (generally used in siren) feed-back condenser ranging from 0.047 ufd to 0.1 mfd should be selected. The speaker may be metallic case (horn type) or small paper cone. The metallic cone horn gives better results.
Parts List for dual tone siren circuit
(1) 'Transistors ;—BC177 2 Nos.
BC 107 1No. T 1
SK100 1No.
·2) Coudensers :—16 mfd 16 volts 1 No.
0.1 mfd 3 Nos.
(3) Resistances (1/4watt) 2.2 K 2 Nos.
22 K 2 Nos.
27 K 2 Nos.
10ohm1No.
(4) Speaker 8+16 ohm.
surya smart says
Hi Abu,
I have gone through the link you have provided. I want to know, how did you connected the speaker. I can't find the speaker in the software. so, I connected a 8 ohm resistor instead. With this, I simulated the circuit. But, I am not getting the sound.
Abu-Hafss says
Hi Surya
I have simulated the circuit for you in LTSpice.
Here is the output:
https://dl.dropboxusercontent.com/u/20969135/dual-tone.wav
If you want the LTSpice data file, here it is too:
https://dl.dropboxusercontent.com/u/20969135/Two-tone%20Car%20Horn.asc
https://dl.dropboxusercontent.com/u/20969135/Dual%20Tone.png
surya smart says
Hi Abu,
Thank you for simulating circuit for me. i have an doubt, in posted diagram c1 and c2 is .1uf but in your circuit c1 and c2 was 47nf . is there any problem using capacitor .1uf.
Can you send me a link for LT spice software to download.
Abu-Hafss says
Hi Surya
I think you have read the article carefully. In the last para it is clearly mentioned that "feed-back condenser ranging from 0.047 uf to 0.1 mfd should be selected".
0.047µF = 47nF and 0.1µF = 100nF
With 0.1µF, the pitch of the sound would be low.
You could have googled for LT Spice to get the link. Anyway, here it is:
http://www.linear.com/designtools/software/
You will find the option for downloading for Windows 7, 8 and 10.
surya smart says
Hi Swagatam
I tried the transistor as per the datasheet. But, it is not working. Can this circuit be simulated . If so, please provide me the link.
Swagatam says
Surya, a transistor astable is one of the easiest circuits and has hardly any parts in it….please build the astable section again separately and keep testing it until it works….make sure the transistors are of good quality and are not faulty.
you can connect a small speaker in between the collector and positive of any one of the transistors, and test the sound with a 3V supply, if the speaker generates a buzzing sound then the astable may be working fine.
surya smart says
Swagatam, Thanks for your valuable reply. I tried as per reply and get back to you soon.
surya smart says
Hi Swagatam,
I tried this circuit but it is not working. I trace out this circuit and found the problem is multi vibrator circuit is not working. Only one transistor giving continuous output other transistor didn't give any output.
T1 Gives continuous output T2 didn't give any output. But T2 base getting 12 volt. Why it happens, any modification required in this circuit. Please do the needful.
Swagatam says
Hi Surya, if your astable is not working then you will have to locate the fault by yourself, I cannot troubleshoot from here. you can check the transistor polarity or simply change them completely,
remember the supply here is opposite compared to the normal circuit diagrams, the positive is applied to the bottom rail and negative to the upper rail….you can disconnect R5 and first verify the astable working….and then proceed.
surya smart says
Hi Swagatam
Thanks for your reply.. How to check the transistor polarity? i google it some image show it as a 1st pin as Collector another image show it as 1st pin as Emitter. How can i confirm the transistor polarity? Please do the needful
Swagatam says
Surya, you can easily verify the pinouts by checking the datasheet of the particular transistor, for example if it's BC557, then you can search for "BC557 datsheet" and confirm the pinouts there.
Abu-Hafss says
Hi Swagatam
I had discussed about a car horn circuit earlier, based on 2 x 555. I failed to locate that post. I tried that circuit, it produces sound which has digital signal and hence cannot be used in cars.
Is it possible to convert those digital signals into analog to have a real sound like electrical car horns? Any simple approach?
Swagatam says
Hi Abu-Hafss,
A tried a lot too, but couldn't find it, I don't think a relevant article was published, we may have only discussed it through comments.
I do not have much idea regarding a DAC, rather an ADC is much easier.
may be this post could throw some light:
www(dot)circuitsgallery(dot)com/2012/04/digital-to-analog-converter-using-r-2r(dot)html
Abu-Hafss says
Hi Swagatam
Yes, we discussed it through comments of some article. But I couldn't find the article.
Where the digital input be fed into the DAC circuit shown in the link you provided?
Swagatam says
Hi Abu-Hafss,
You can conversation from your email, but again it could be quite tedious to find them from so many posts:(
the digital music should be fed to the "clock" input
Anonymous says
how i connect the buzzer coil across the piezo?
Swagatam says
by soldering the leads….
Anonymous says
can i use piezo transducer to this circuit…………if it possible wat will i do for that?
Swagatam says
replace the speaker with a two terminal piezo, connect a buzzer coil across the piezo.
Anonymous says
how we calculate the resistor and capacitor value?
Swagatam says
it depends on many factors like curret, frequency, voltage etc. ….resistors adjust these parameters so that the circuit appropriately functions.
Anonymous says
if i given 3 volt to circuit ?what i will modified in the circuit?
Swagatam says
no mods are required. will work with 3V also.
Anonymous says
why we use transistor BC557.BC547 and 2N2907?
can u explain me?
Swagatam says
It's calculated By the amount of current that would pass at the particular positions in the circuit where the transistors are posted.
example, the speaker would consume max current, so 2N2907 is employed which is more powerful than the other transistors comparatively….
Anonymous says
what is the resistor R3 and R4 = 2K2?
Swagatam says
2.2 K resistors….
Anonymous says
may i see the schematic pls. I would like to use it for my PIR. I did one, almost finished from my breadboard but when I transferred everything to the PCB, the circuit messed up! really a disappointment. I did checked for loose contacts in the pcb. could the difference of distance in the breadboard also affect the output?
Swagatam says
It's surprising how I missed the schematic, or may be it just disappeared lolz….never mind I'll try to trace it out from my older files and post it soon.
Thanks!!