- #1
Will this circuit work as I predict it to?
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Discussion Overview
The discussion revolves around the design and functionality of a basic oscillator circuit using an NPN transistor. Participants explore various configurations, predictions about circuit behavior, and the requirements for successful operation.
Discussion Character
- Exploratory
- Technical explanation
- Debate/contested
Main Points Raised
- One participant predicts that the circuit will function by charging a capacitor (C1) to 0.7 volts, which will activate the NPN transistor and allow current to flow to an LED, repeating the cycle.
- Another participant challenges this prediction, stating that the capacitor would not discharge as expected and that the voltage across the capacitor would rise to a level that keeps the LED lit, depending on the LED's voltage requirement.
- A suggestion is made for a simpler BJT oscillator using a negistor, noting that it requires a 12V DC supply and may not be compatible with all NPN BJTs.
- Another participant mentions the need for two transistors to create a basic multivibrator-type digital oscillator.
- A phase shift oscillator is introduced as an alternative, which can produce a sine wave output but requires a high gain transistor.
- Details are provided about a specific circuit that can drive a speaker, including a suggestion to make a resistor variable to adjust frequency.
Areas of Agreement / Disagreement
Participants express differing views on the functionality of the proposed oscillator circuit, with no consensus reached on the predictions or the necessary components for successful operation.
Contextual Notes
Participants reference specific voltage requirements for components and the necessity of certain configurations, but these assumptions remain unverified within the discussion.
- #2
vk6kro
Science Advisor
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No, sorry, it won't do that.
There is no reason the capacitor would discharge.
The voltage across the capacitor rises to about 0.6 volts above the LED voltage and then the transistor turns on and the LED lights up.
So, if the LED was a white one and it needed 3.5 volts to turn on, then the voltage across the capacitor would rise to about 3.5 volts + 0.6 volts or 4.1 volts.
This voltage would then stay on the capacitor and the LED would light up.
There is no reason the capacitor would discharge.
The voltage across the capacitor rises to about 0.6 volts above the LED voltage and then the transistor turns on and the LED lights up.
So, if the LED was a white one and it needed 3.5 volts to turn on, then the voltage across the capacitor would rise to about 3.5 volts + 0.6 volts or 4.1 volts.
This voltage would then stay on the capacitor and the LED would light up.
- #3
Jony130
- 626
- 137
If you want the simplest BJT Oscillator use the negistor
http://cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circuit.html
Keep in mind that this circuit need 12V DC and may not work with every NPN BJT.
http://cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circuit.html
Keep in mind that this circuit need 12V DC and may not work with every NPN BJT.
- #4
NascentOxygen
Staff Emeritus
Science Advisor
Homework Helper
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You need two transistors to make the basic multivibrator-type digital oscillator.Schmoopsiepoo said:
- #5
vk6kro
Science Advisor
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There is a phase shift oscillator which gives a sine wave out, but it requires a very high gain transistor:
There is this one (below) which gives enough output to drive a speaker.
http://dl.dropbox.com/u/4222062/NPN%20PNP%20OSC.PNG
The waveform shown is across the 8 ohm resistor which represents the speaker.
There is this one (below) which gives enough output to drive a speaker.
http://dl.dropbox.com/u/4222062/NPN%20PNP%20OSC.PNG
The waveform shown is across the 8 ohm resistor which represents the speaker.
Last edited by a moderator:
- #6
NascentOxygen
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Make the 100kΩ resistor variable to vary the frequency.vk6kro said: