# i need help for circuit explanation plzz~

Discussion in 'General Electronics Chat' started by pieces90, Apr 2, 2011.

1. ### pieces90 Thread Starter New Member

Apr 2, 2011
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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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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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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 ...

Last edited: Apr 2, 2011
3. ### pieces90 Thread Starter New Member

Apr 2, 2011
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meaning? erm, may i know why there are darlington pair use in this circuit?

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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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)

5. ### pieces90 Thread Starter New Member

Apr 2, 2011
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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?

6. ### Kermit2 AAC Fanatic!

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

7. ### pieces90 Thread Starter New Member

Apr 2, 2011
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i know the function of resistor and the capacitor.. but may i know why it put it that way to the gate?

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
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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.

9. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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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.

10. ### pieces90 Thread Starter New Member

Apr 2, 2011
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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?

11. ### pieces90 Thread Starter New Member

Apr 2, 2011
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0
oo.. ok.. then all those resistor and capacitor at the gate, if it removed then the circuit will not function is it?

12. ### pieces90 Thread Starter New Member

Apr 2, 2011
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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?

13. ### Wendy Moderator

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

14. ### Audioguru New Member

Dec 20, 2007
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The circuit will not work because the CD4011 is not powered and the battery has no voltage (it is dead?).

15. ### pieces90 Thread Starter New Member

Apr 2, 2011
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erm, do you think that if i replace the Q1 n Q2 with other type of transistor wilit still working?

16. ### Wendy Moderator

Mar 24, 2008
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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.

17. ### pieces90 Thread Starter New Member

Apr 2, 2011
9
0
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?

18. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
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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