i need help for circuit explanation plzz~

Thread Starter

pieces90

Joined Apr 2, 2011
9
im a student in electrical course. i cant understand the circuit given to me.. the function of the transistor, the NAND gate and why they placing the capacitor and resistor like that... im in an exam this monday.. and i cant understand it.. can someone help
 

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t_n_k

Joined Mar 6, 2009
5,455
They are operating as astable oscillators. Frequency of operation is determined by R & C values. U1C & U1D paired are a gated type (through U1D input), with the gating signal coming from the U1A & U1B pair oscillator.

Google CMOS oscillator ...
 
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Kermit2

Joined Feb 5, 2010
4,162
NAND gate 'c' and 'd' have their inputs tied together. That makes them into a NOT gate, or inverter.

The output of the combination NAND gates 'a' and 'b' is fed back into itself. This kind of closed loop is often seen in op amps and is known as 'feedback'. In this case the feed back is to reinforce oscillation. They are an 'oscillator'. NAND gates 'c' and 'd' are simply isolation between the oscillator and the low impedance input of the transistors.

The output is a speaker and a light in parallel. The transistors are between the oscillator and the speaker. So the transistors are acting as ........(this is the part where you are supposed to reply)

:)
 

Thread Starter

pieces90

Joined Apr 2, 2011
9
NAND gate 'c' and 'd' have their inputs tied together. That makes them into a NOT gate, or inverter.

The output of the combination NAND gates 'a' and 'b' is fed back into itself. This kind of closed loop is often seen in op amps and is known as 'feedback'. In this case the feed back is to reinforce oscillation. They are an 'oscillator'. NAND gates 'c' and 'd' are simply isolation between the oscillator and the low impedance input of the transistors.

The output is a speaker and a light in parallel. The transistors are between the oscillator and the speaker. So the transistors are acting as ........(this is the part where you are supposed to reply)

:)
ok, what i know, The design of a transistor allows it to function as an amplifier or a switch. This is accomplished by using a small amount of electricity to control a gate right? In this way, a very small amount of current may be used to control a large amount of current. but why so many transistor, why there are darlinton pair in this circuit?
 

Kermit2

Joined Feb 5, 2010
4,162
The darlington pair has higher gain and less 'loading' effect on the input signal source. (needs less current from input device)
 

t_n_k

Joined Mar 6, 2009
5,455
Hi pieces90,

Is the schematic you posted the one you were originally given or have you edited it somewhat for posting?

The placement of Q3 [BC548] for instance is most unusual. Whenever Q3 is turned on it will short the power supply - and probably self-destruct.

Also C3 at the connection to R7 should probably also connect to U1C's output pin 10.

Strange that you are being examined on a subject about which you seem to have limited understanding.
 

Kermit2

Joined Feb 5, 2010
4,162
It would make more sense to have Q3 driving the lamp alone, and the pair driving the speaker. Cut the parallel connection between the lamp and speaker.
 

Thread Starter

pieces90

Joined Apr 2, 2011
9
Hi pieces90,

Is the schematic you posted the one you were originally given or have you edited it somewhat for posting?

The placement of Q3 [BC548] for instance is most unusual. Whenever Q3 is turned on it will short the power supply - and probably self-destruct.

Also C3 at the connection to R7 should probably also connect to U1C's output pin 10.

Strange that you are being examined on a subject about which you seem to have limited understanding.
it was originally given.. the C3, R7 and the output pin 10 are connected actually.. i'm still new to this subject. when all those component were combine together, i mess up.. :'( .. if the Q3 placement is unusual, then what is the different when the Q3 is place or not?
 

Thread Starter

pieces90

Joined Apr 2, 2011
9
It would make more sense to have Q3 driving the lamp alone, and the pair driving the speaker. Cut the parallel connection between the lamp and speaker.
oo.. ok.. then all those resistor and capacitor at the gate, if it removed then the circuit will not function is it?
 

Thread Starter

pieces90

Joined Apr 2, 2011
9
It would make more sense to have Q3 driving the lamp alone, and the pair driving the speaker. Cut the parallel connection between the lamp and speaker.
ooo.. ok.. i get it... just want to ask your opinion, if we did not use the gate ic, do u think that this circuit still work?
 

Wendy

Joined Mar 24, 2008
22,581
Q3 is only good for smoke. It is connected directly between the power supply rails. There is definitely a wiring error on this schematic.
 

Wendy

Joined Mar 24, 2008
22,581
I think the other people analysis is accurate, but with Q3 shorting things out it needs removed. It does make me suspicious of the rest of the circuit. From what I can tell you have a fast oscillator (audio) and a slow oscillator (beep). Work from there.
 

Thread Starter

pieces90

Joined Apr 2, 2011
9
I think the other people analysis is accurate, but with Q3 shorting things out it needs removed. It does make me suspicious of the rest of the circuit. From what I can tell you have a fast oscillator (audio) and a slow oscillator (beep). Work from there.
ooo.. ok2... i starting to understand it.. so the gate function is for the oscillation of the sound and beep is it? erm why did it connect the capacitor and resistor like that to the NAND gate?
 

t_n_k

Joined Mar 6, 2009
5,455
As to the purpose and function of the R & C values in conjunction with the CMOS NAND gates you could do well to search for information on CMOS oscillators.

I've attached a National Semiconductor application note on that topic.

Essentially you can view the NAND gate as a substitute for the simple CMOS inverter function elaborated in the application note. The NAND function gives one the flexibility of being able to turn the oscillator on and off in a controlled manner.

Since this is akin to homework you should probably (in future) post in the homework forum and take note of the message that enquirers are expected to contribute by way of their own efforts rather than simply expecting forum members to provide the whole story.
 

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