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Can you check over this Alarm Circuit?

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EdgyNinja

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Hey guys I'm new here and my class is having a practice test project. Our teacher instructed us to pick a circuit of our liking to make in a weeks time. But before I go out and buy the components, I want to make sure that this circuit would work. Thanks for your help.
 

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Reloadron

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I left a few suggestions in the other forum you posted this schematic in. :)

Ron
 

audioguru

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You found a circuit that has some very old parts. The website and designer are in India. Maybe the old parts are still available there.
IC3 is very old.
The DC-coupled opamp feeding the transistor that feeds pin 2 of the 555 might not work because the transistor might be turned on all the time. The base of the transistor should have a resistor to ground and a capacitor should couple the audio signal from the opamp to R4.

HEE, hee. The 741 opamp was designed 49 years ago, works poorly but is still available today. Hee, hee again. T2 is an AC187 germanium transistor that is 60 years old! It must be replaced by a power transistor that is available today. Which power transistor will you use?
 

dr pepper

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So its a circuit that makes a funny noise if it 'hears' something, looks as though it oghta work, make sure the mic doesnt listen to its own sound from the speaker.
 

ci139

Active Member
as far as i get it if you tap onto the mike it generates a short pulse train burst !if the opamp has enough gain! that multiple re-triggers the 555 that will power the um3561 until the cap charges to apx. 2/3 Vcc or Vdd e.g. 8V incase of 12V supply

everything to the left from pin-2 of 555 ic must be tuned experimentally - or in other words - might fail to work as the designer of this circuit intended e.g. you need to match C1,R1,R2,R3,R4,VR1,VR2,R5 with your actual mic. opamp and NPN -- nothing impossible -- but i guess you do need to do this under supervision and guidance of some more experienced person - - - or simply put if you see your led blinking when tapping at mic. it's doing what it should
 

audioguru

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The very old circuit says it uses a "condensor mic". But a condenser mic must be biased with 48V, not just 12V, and it must feed a preamp with a very high input impedance that this circuit does not have. So it won't work.
The mic must be an "electret mic" which is a condenser mic with 48V permanently stored in its electret material and it has a Jfet inside the mic with a high input impedance that can be biased with 12V.

I agree that the left side of the circuit is a DC-coupled nightmare needing everything to be tested to match, instead of using proper DC biasing and a coupling capacitor in series with the base of T1 for the signal.
 

ci139

Active Member
there are actually some principal shortcomings - from the actual use point of view
  • it quite much resembles to a past millennium's not so advanced "Quiet!" circuit e.g. it's set to respond to a noise level in classroom for example so . . .
  • if the mic. gets an atmospheric feedback from alarm it keeps re-triggering - (A) a simple solution is diode (+in series attenuation resistor feedbak from 555's pin-3 to the base of T1) . . .
  • however such disables a possible cancel alarm at dropped input noise level - (B) a more complicated fix to that would be anti-phase feedback from pin-3 of the UM3561 to summer formed abou-around C1 , R1 . . .
  • +
  • other than that as the (( SP(eaker) )) and NE555 and even T1 draw signifficant power ...
  • the power to IC1 (µA741) and MIC should be (at least) AC decoupled from supplie's positive rail (+12V) - (C) a possibly sufficient would be diode in series with 47Ω (or grater) resistor then - an AC shunted - with capacitor 10µF or grater
  • +
  • if "your" circuit initiates - it will probably start to charge the C2 so the alarm would play at power up -- if such is not desired - (D) something must be set to keep pin-5 of 555 down (at 1/3 of Vdd 4V) OR the high ohmic impedance in between pin-3 of 555 and pin-5 of UM3561 OR a high ohmic impedance in between pin-3 of UM3561 and the base of T2 -- at startup while the C2 is reaching the 2/3 voltage level of the supply . . . OR a bridge to the R6 from lower resistor (240Ω) and PNP -- conducting at power up -- so there will be a short "click" at starup)
  • ... perhaps there's more
 
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audioguru

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This old circuit has too many errors for a student to use as a school project.
 

ci139

Active Member
the Ti provided spice model operates at ±2.5V (5V total) supply ??? quite excellent ???

-- i have the russian v. of the chip (i'm not rushing to test this time)
... as studied from spice !!! there are two limits for total supply for simple BJT dif-input op-Amps -- 6.8V (input voltagess near the supply median) & 9.6V (near med. + performance appearing also at higher frequencies) ???


-- this likely requires 8V total if fed from 12V and opAmp's powered through an "AC guard" it'd be better to go with LM321 or similar

-- also the electret is more likely option google returned Need help for electret mic preamp that is better so opAmp biasing won't eat off the INP signal
 
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EdgyNinja

New Member
Yeah guys thank you a lot. This circuit looks like it has too much errors for my school project. I am willing to correct it but what you guys said is kind of confusing.
 

audioguru

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The schematic says "electroschematic" which is a website in India. most of its projects are designed by Mr.Kumar but this circuit with its very old parts is not there anymore.
 

AnalogKid

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The 741 is an adjustable-gain inverting linear amplifier. T1 with the LED in the emitter is the comparator. When the 741 output goes below approx. 2.4 V (LED + silicon Vbe), T1 turns off and R5 pulls 555 pin 2 up to 12 V. When that audio half-cycle goes high or the input audio goes quiet, T1 turns on and pulls 555 pin 2 below 4 V to trigger the monostable.

When the input audio is quiet, T1 is on all the time. This is normal. The DC coupling between IC1 and T1 is required for the circuit to work as designed. There is no need for a resistor from T1 base to GND. R4 should be 2.2K.

In a quiet environment, the 555 will power up in the triggered state (pin 2 below 4 V) and the audio output will be active for one timing cycle.

Hmmm ... Thinking about this, it is possible that the 555 never is out of the triggered state long enough to time out. Not surprising, since almost everything I've seen on that site has issues. It's late; I'll think about it tomorrow.

ak
 
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dr pepper

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The thing with noob type circuits is that those with a little more knowledge will pull them to bits, as there are often several shortcommings done to make it simple enough to build.
If we made the thing out of a 32 bit digital signal processor you wouldnt be able to build it without a PhD.

Get yourself a breadboard and lash it up, I wouldnt be surprised if it works well enough to get your class marks.
 

ci139

Active Member
much errors
actually there are none - B-coz - we donno what the circuit was initally used for , how the speciffic NE555 used in circuit behaved , how did the other parts "fit together" , what was the 3D relative spacing of the components e.g. the mic.+OpAmp might have been in a garage/shop-floor and the alarm at the room/office . . . e.c.

http://microphone-data.com/media/filestore/articles/Mic impedance-10.pdf -- quote -- "For dynamic and condenser microphones, the preferred pre- amp input impedance is generally about ten times that of the microphone output; normally around1.2kΩ or 2kΩ."

??? it seems the electret is condenser not piezo (10MΩ) ??? http://www.es.co.th/schemetic/pdf/kuc.pdf
-- lists output impedances 1kΩ (@ 4.5V , 800µA bias ?5k6 ESR?) or 2.2kΩ (@ 2.0V , 500µA bias , ?4kΩ ESR? e.g. 20kΩ series biasing resistor for 12V) . . . + i got 1 of theirs' -- it seems it can be directly measured by multimeter connecting it's CAT to case and AN to output (showed 1.9 smth. kΩ !!!) -- notice that even the simple stuff such as getting the polarities correct (e.g. no sign errors) might turn out to be tricky -- as in following the same brand having a differences , also different electrets may utilize different polarity Fet-s (so the case being neg. or CAT(hode) may not always hold and must be traced down from datasheet)
IMG_9803_cc2rs1.gif IMG_9806_cc2rs1.gif
 
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audioguru

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A 741 opamp has a range of allowed common mode input voltage range that is not near the negative supply voltage that is caused by the bias voltage divider of R2 and R3.
A red LED has a range of voltage. If its voltage is low then it and Q1 are turned on all the time then the 555 will not be triggered unless the audio level is very high. If the voltage of the LED is too high then the 555 trigger input will not go low enough to work.

I have never studied languages in India so I do not know if they know what is a modern electret mic. Maybe they wrongly call it a condensor mic. Some people call it an electret condenser mic. This circuit will not work with a condenser mic that must have a 48V supply. The electret material in an electret mic "stores" the required 48V and it also has a Jfet impedance converter (that needs the voltage from VR1). Every electret mic datasheet I have seen say the Jfet draws 0.5mA. I think the metal case of an electret mic is always connected to 0V or a negative voltage.

This circuit should have the opamp biased at half the supply voltage and use a capacitor to feed the base of the transistor that is biased at 0V with a resistor. The transistor should not have an LED in series with its emitter, its collector should swing close to 0V when there is audio.

I guess the sensitivity can be turned down to prevent normal noises from setting off the alarm but allow the alarm to be activated when a burglar is smashing the house down.
 
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