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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Off the top it looks like you are using the 741 as a comparator. R2 & R3 form a divider leaving about 3.4 Volts as a voltage reference on the 741. The old 741 is really a poor choice as a comparator. A better choice would likely be a comparator chip. This is a very good read on comparators and for a single comparator I would likely choose the LM311 which is a single comparator. Read the link. Then VR2 looks to be used for hysteresis and if that is the case you want hysteresis tied back to the + (non-inverting) input. If you scroll down the link you will see hysteresis applied in comparator circuits. I would think about moving the LED up into the collector circuit of T1 the BC 548 NPN. That is as far as I looked.

This would go better if you explained your circuit operation so those viewing it know what you had in mind and your expectations of the circuit.

Ron

 

Where are you going to get the music chip from ???

 

... possible audio feedback from speaker to mic?

 

The UM3561 IC appears to be obsolete. Neither Farnell nor Digikey list it.

 

Hello,

When Farnell/newark, Digikey and Mouser do not sell them anymore, you can try sites as ebay and aliexpress:
https://www.aliexpress.com/price/ic-um3561_price.html
http://www.ebay.com/sch/i.html?_nkw=UM3561

Bertus

 

The entire circuit uses old and or obsolete parts. While the LM741 continues in production it is what? Pushing maybe 50 years old?

Ron

 

Hello,

You also will not find T2 (AC 187) anymore.
The AC 187 is an old germanium transistor.
http://www.radiomuseum.org/tubes/tube_ac187.html
(the picture shows the K version with an extra heatsink).

Bertus

 

Guys, I have the UM3651, and alternative parts for the rest. I tried this circuit and it doesn't work. Someone from Electronicspoint told me this circuit is full of errors. I would love to learn what is wrong with this circuit and fix it.

 

Guys, I have the UM3651, and alternative parts for the rest. I tried this circuit and it doesn't work. Someone from Electronicspoint told me this circuit is full of errors. I would love to learn what is wrong with this circuit and fix it.

What exactly are you wanting this circuit to do? As it looks the 741 which I mentioned is used as a comparator and is a poor choice. R2 and R3 form a voltage divider. Here is an example of using the 741 as a comparator. The problem is in your circuit the divider network is going to the non-inverting input (pin 3). A simple Google of 741 as a comparator will yield other examples. The inverting and non inverting inputs are drawn wrong! When the input exceeds 3.4 volts the output will go low when it needs to go high. I suggested earlier you read and understand comparators, I suggest again you read about comparators. You are trying to build a circuit which is a poor design and don't understand how it should work. Next the idea is for the 741 to swing its output and turn on T1 an NPN transistor. When T1 turns on, even momentarily it pulls (or should pull) pin 2 of the 555 timer IC 2 low triggering the 555. The 555 Timer (IC2) is configured as a One Shot so I suggest you read using the 555 as a one shot timer. R6 and C2 determine the timing of the output pulse at pin 3 of the 555. Read the 555 data sheet to understand timing. Your drawing would have a pulse out of about 110 seconds.

The 555 output pin 3 is being used to power your sound generator chip the output of which is amplified by T2. Since this is your project shouldn't you know how it works or should work? The Internet is loaded with poorly designed circuits which do not work. This is pretty much the same response you got in other forums you placed this circuit in. If you want one that works you need to understand what is going on and why.

Ron

 

Thank you Ron. You are right. I really don't understand and I am in the process of learning. Yes this circuit is full of errors. I am going to take the time to understand why it DOESN'T work, which would benefit me greatly. I would love more help understandin why I doesn't work. BTW this circuit is suppose to trigger the alarm when the mic picks up sound.

http://www.electroschematics.com/5346/panic-alarm/

Here is the link to the web page. Thanks again, this is interesting.

 

Yes, it is sound activated, when a sound passes a preset threshold the process begins. The 741 inputs are backwards for starters. The non inverting input is the + and the inverting input is the -. I would switch those for starters. Again, I see the LM741 as a poor choice for a comparator but it should work with the inputs switched.

Ron

 

Yes, it is sound activated, when a sound passes a preset threshold the process begins. The 741 inputs are backwards for starters. The non inverting input is the + and the inverting input is the -. I would switch those for starters. Again, I see the LM741 as a poor choice for a comparator but it should work with the inputs switched.

Ron

So all I have to do is switch the inverting inputs and it will work?

 

Also, as soon as I power on, the alarm sounds right away. Something is triggering the 555 besides the mic

 

Off the top it looks like you are using the 741 as a comparator.

The 741 inputs are backwards for starters.

I don't think so.

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, T1 turns off and R5 pulls 555 pin 2 up to 12 V. When that audio half-cycle goes high, T1 turns on and pulls 555 pin 2 below 4 V to trigger the monostable.

R4 should be 2.2K.

Also, as soon as I power on, the alarm sounds right away. Something is triggering the 555 besides the mic

The 555 powers up with pin 2 low, the trigger condition. That is a flaw of the design.
ak

 

I don't think so.

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, T1 turns off and R5 pulls 555 pin 2 up to 12 V. When that audio half-cycle goes high, T1 turns on and pulls 555 pin 2 below 4 V to trigger the monostable.

R4 should be 2.2K.

The 555 powers up with pin 2 low, the trigger condition. That is a flaw of the design.
ak

Do you have any idea how to fix this circuit?

 

On further thought, there might be other problems with the circuit. Other, as in it might never turn off the audio. Hmmm ...

Is your assignment to blindly assemble something you don't understand, or do a little circuit design work?

ak

 

I don't think so.

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, T1 turns off and R5 pulls 555 pin 2 up to 12 V. When that audio half-cycle goes high, T1 turns on and pulls 555 pin 2 below 4 V to trigger the monostable.

R4 should be 2.2K.

The 555 powers up with pin 2 low, the trigger condition. That is a flaw of the design.
ak

You may want to heed what AnalogKid has to contribute as he is much better than I at diagnosing this circuit. I was seeing the 741 as a
comparator which I had wrong.

Ron

 

On further thought, there might be other problems with the circuit. Other, as in it might never turn off the audio. Hmmm ...

Is your assignment to blindly assemble something you don't understand, or do a little circuit design work?
It was my fault for picking up something I don't understand for my assignment. However, I'm really keen on figuring out how to fix it.
ak