In this project, we will show how to build a relaxation oscillator with a single transistor and a few other components such as a resistor and capacitor.
Components Needed
2N4401 NPN Transistor
3300μF electrolytic capacitor
1KΩ resistor
LED
The 2N4401 transistor is an NPN transistor. The datasheet for this transistor can be found at the following link: 2N4401 NPN Transistor Datasheet.
You really could use any NPN transistor. Other popular transistors are the 2N3904 and 2N2222 NPN transistors.
The relaxation oscillator circuit we will build with a transistor and resistor and capacitor is shown below.
First and foremost, we use 12VDC of power for this circuit. Actually any voltage from 12V-15V will work. But voltages much below 12V will not allow the circuit to work.
Next, in parallel to this voltage source, we place a 1KΩ resistor and a 3300μF electrolytic capacitor in parallel. Then in parallel to the capacitor, we place an LED and 2N3904 NPN transistor in series.
but the LED is in short. What is the problem ?
Edit:
I do not have much confidence in circuits where transistors are “abused” like this. You’re using it outside of it’s specifications and that may bring you into an area where it may or may not work, or you may be plagued by a high variability of the untested parameters.
It’s quite possible that your transistor just works as a zener diode with no oscillation at all for example.
I would not design a PCB for this.
It can be fun to experiment with such circuits on a breadboard though.
Also the voltage required depends on the reverse breakdown voltage of the EB junction of the transistor and this varies according to model, so this is not a very good design. And since you are relying on the resistance of the transistor to limit the current, you may or may not destroy the transistor and/or LED by discharging the 3300μF capacitor.
If you want a blinky, use a 555, they are cheap. If you want to spend more money on a blinky, then use an Arduino or RPi.
This circuit needs some negative resistance and a Vbe breakdown does not really give that.
Long ago we used unijunctions in this circuit or at higher voltages a neon bulb
Funny I was struggling with a simple oscillator design last night. But this design has a couple of aspects which are abuse or non-rigorous use of the transistor.
If I assume you have drawn it correctly, any conduction will be via reverse breakdown of the base-emitter junction. Current flow direction is reverse from normal for an NPN. This could destroy the transistor.
It is normally bad practice to leave the base of a transistor floating. There can be some leakage current and no base-emitter resistor causes a lot of variability.
Also destructive: If the transistor avalanches somehow, there is not much to limit peak current through the LED and the transistor. This could damage either. Putting 470 ohms in series with the LED might help this.
In spite of what I have said, this might be interesting to play with. I have made avalanche circuits for test purposes; these similarly tend to overstress the components and is likely to result in component damage. Also fun is to make an LC oscillator with a single TTL inverter. All of these are toys or experimental but not recommended for serious design. The LC oscillator is not to be confused with LC the cow. https://en.wikipedia.org/wiki/Elsie_the_Cow
BTW there are some pcb fabricators which are cheap enough so that doing a board to play with may not be unreasonable. The main cost is shipping from China, but if the order is combined with another (a serious board design) then I think the added cost for a small 2 layer board can be negligible.
I am only barely familiar with either of those; probably because I lost interest after checking them out. If you are serious about a bread oven…I am dubious as to what could be done with it creatively. Perhaps some thyristor based enhancement of the thermostat?
Anyway a relaxation oscillator sounds like my way to relax.
Don’t blame ML9104 for changing topics. I have been as guilty as anyone for taking discussions off topic. I think none of this discussion is very serious. “no harm no foul”?
Yeah, I understand that, and my post was sort of tongue-in-cheek.
But this forum is really about design software, not about dodgy cicuit design. That was my point.
The OP’s design would have been ripped apart on a real electronics site. Then it’s much warmer and fuzzier here.
Yeah, the original blinkys. I once made a darkroom timer using a neon bulb triggering a valve controlling a relay. Naturally the valve (octal base even) came from an old radio. We used what was on hand in those days.
Was the neon bulb shielded from light? I am trying to remember something I had heard of involving a neon bulb. I think it was a printer. When the final product was buttoned up it quit working. The designers eventually figured out what was going on and put a fluorescent lamp inside the printer.
