Making simple relay oscillator: stuck... - Page 1
Forum section: electronics / beginners

author: Beamin
author_rank: Super Contributor
timestamp: September 23, 2018, 03:38:19 pm
content: 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 watchingTV 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.

author: Beamin
author_rank: Super Contributor
timestamp: September 23, 2018, 03:44:53 pm
content: Can you blink an Led with one transistor in a similar fashion? all blinking led circuits are multivibrators I found.

author: Zero999
author_rank: Super Contributor
timestamp: September 23, 2018, 06:22:41 pm
content: 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.

author: janoc
author_rank: Super Contributor
timestamp: September 23, 2018, 06:48:39 pm
content: Quote from: Beamin on September 23, 2018, 03:44:53 pmThat 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.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 23, 2018, 06:52:01 pm
content: Quote from: Beamin on September 23, 2018, 03:44:53 pmHere, LMGTFY:etc. etc.

author: Seekonk
author_rank: Super Contributor
timestamp: September 23, 2018, 08:22:40 pm
content: We had a dual relay flip flop that we used for testing circuits by putting horrible spikes on the power line.

author: Beamin
author_rank: Super Contributor
timestamp: September 23, 2018, 11:49:48 pm
content: Quote from: alsetalokin4017 on September 23, 2018, 06:52:01 pmHow does that first one work? The cap is a lower impedance path to ground and while charging the led is off then the cap discharges lighting the led then the cycle repeats? The transistor is chosen because of its voltage drop like a diodes?The second one works because of special properties of that unijunction transistor being able to have the current reversed It acts as a NPN and PNP?I'm finding these schematics online are just people drawings, and not tested in simulators or even in real life just "this seems like a good idea, my last circuit worked , so lets say it will work".As far as hooking the relay to its own contact coil I wanted to add a cap and resistor so you could control the speed, but wouldn't a diode and alternative path need to be added at some point, where the cap charges up and then the current starts getting put to ground instead of just topping off the cap? Fun clicking sounds to be had. Sucks having to relearn this stuff but that's the reality I face every year or so. In some ways its fun because it's novel again!

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 24, 2018, 12:36:15 am
content: More like a buzzing than a clicking, with the relay, although with the right component values you can control the speed somewhat. What's really fun is when you discover the high voltage inductive spike generated when the coil is de-energized. If you don't shunt this with a diode across the coil, you can zap your friends with it instead.

author: janoc
author_rank: Super Contributor
timestamp: September 24, 2018, 07:15:11 pm
content: Quote from: Beamin on September 23, 2018, 11:49:48 pmBy abusing the reverse breakdown voltage of the PN junctions. That one is quite low for these transistors. So once the capacitor charges up, the voltage across the transistor exceeds the breakdown voltage, that will rapidly discharge the cap (and make the LED flash once). The voltage drops below the breakdown voltage again because the discharged capacitor acts as a short at that moment - thus turning the current off. And the cycle repeats.However, it is not the best circuit for something repeatable or robust.Quote from: Beamin on September 23, 2018, 11:49:48 pmUnijunction transistor is a very obscure part, having little in common with regular transistors. The way it works is a bit complex, it is well explained here, for example:https://www.electronics-tutorials.ws/power/unijunction-transistor.htmlBasically a relaxation oscillator - the transistor is off, the capacitor charges up until the turn on voltage is reached, the transistor turns on and discharges the capacitor again. And cycle repeats. Similar circuits have been done also with neon indicator tubes which have a similar behavior.Quote from: Beamin on September 23, 2018, 11:49:48 pmHehe, welcome to the real world. Yep, that's very common, even circuits published in magazines and such.