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Making simple relay oscillator: stuck...

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Beamin:
So I'm copying things I saw online but this one doesn't work in the simulator nor does it seem like it would:



Its just a 470 cap across but that's not going to oscillate because once its charged it going to stay charged. I was thinking a diode and resistor: the cap charges as its doing it the coil turns on. Cap gets fully charged then discharges through the diode. The resistor is there to slow it down. Once discharged the current source flows into the cap repeating.

My other thought would be to have the relay turn on charging another cap as the first cap gets charged the coil closes and the second cap powers up the coil again restarting the sequence. Sounds simple but can't find an example that works. As a kid I did this to make a spark gap transmitter that made watching
TV impossible.






https://www.falstad.com/circuit/




EDIT image didn't post.  is there a way to get image locations in internet explorer? In firefox you just right clicked or at most clicked view image then copied the url. Or does Microsoft not want you to do that? They already took away the view image option in google images. This makes posting pictures on the forum a pain in the ass.

Beamin:
Can you blink an Led with one transistor in a similar fashion? all blinking led circuits are multivibrators I found.

Zero999:
No, it won't work. The relay coil will turn on for a short length of time, after the power is applied, then turn off, when the capacitor has fully charged.

To make an oscillator, wire the normally closed contact in series with the coil. It will oscillate at an audio frequency. To slow it down, put a resistor in series and a capacitor in parallel with the coil.

janoc:

--- Quote from: Beamin on September 23, 2018, 03:44:53 pm ---Can you blink an Led with one transistor in a similar fashion? all blinking led circuits are multivibrators I found.

--- End quote ---

That won't work, as Hero999 said. That simulator is rather primitive and simulates ideal components with no losses. So you will have oscillating current there. Real components are lossy, so unless you add something to add energy to the system, the oscillation will rapidly die down because the energy is being expended as heat.

Think of it like a child's swing - if you don't keep pushing the swing it will stop. If you push it at a wrong moment (= out of phase), the amplitude of the swings will become smaller and it will stop faster. Only if you push the swing at the right moment (phase) will it keep going.

This holds whether the oscillator is mechanical or electronic.

For blinking a LED a multivibrator is typically easier than an LC oscillator. LC oscillator will require large inductance and capacitance to blink a LED at a visible frequency, plus a second transistor to isolate the oscillating transistor from the load - that's just not practical. If you are going to use two transistors anyway then you can build a multivibrator and without the inductor hassle.

alsetalokin4017:

--- Quote from: Beamin on September 23, 2018, 03:44:53 pm ---Can you blink an Led with one transistor in a similar fashion? all blinking led circuits are multivibrators I found.

--- End quote ---

Here, LMGTFY:






etc. etc.

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