Simple Oscillator, but interesting thing happened - Page 1
Forum section: electronics / projects

author: wael
author_rank: Contributor
timestamp: May 20, 2016, 07:19:20 pm
content: Hi,I was building a simple oscillating circuit, attached a circuit diagram for it, the interesting thing that happened is whenever i bring my hand close to one of the capacitors, the LED stops blinking, without even touching it, i wonder why is that, any ideas ?I'm guessing something to do with body capacitance , but how such small capacitance can have such effect ?BTW, the BJT i'm using is C945Please let me know what do you think, also if you have any recommendation regarding the circuit diagram let me know !

author: Dave
author_rank: Super Contributor
timestamp: May 20, 2016, 07:24:58 pm
content: Either you didn't provide the right circuit or that thing doesn't actually oscillate.

author: danadak
author_rank: Super Contributor
timestamp: May 20, 2016, 09:43:13 pm
content: Here are a couple -http://cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circuit.htmlhttp://blog.jongallant.com/2015/01/simple-blinking-led.htmlRegards, Dana.

author: Audioguru
author_rank: Super Contributor
timestamp: May 20, 2016, 11:45:46 pm
content: I think the capacitors charge enough to turn on the transistor then the LED current causes the 5V power supply voltage to collapse, then the transistor and LED turn off and the 5V comes back and it starts over again.

author: uncle_bob
author_rank: Unknown
timestamp: May 21, 2016, 12:03:45 am
content: Quote from: wael on May 20, 2016, 07:19:20 pmHiWell, first off, that circuit (with a good battery) will not oscillate. It will simply turn on slowly and stay on. The only way to get it to turn on and off is to get the battery to the point it's barely working. In that case, you have a chemical oscillator. Heat from your hand could easily effect it.Bob

author: wael
author_rank: Contributor
timestamp: May 21, 2016, 12:22:52 pm
content: It might not be completely turn off, but its brightness does change for sure.Just tested the voltage source and it looks good.Also when i attempt to measure the voltage between base and emitter it stops blinking, not sure why......

author: uncle_bob
author_rank: Unknown
timestamp: May 21, 2016, 12:30:08 pm
content: Quote from: wael on May 21, 2016, 12:22:52 pmHiThe circuit as drawn in the schematic is not an oscillator. If the implementation you have oscillates, there is an error between your circuit and your schematic or at least one of the parts involved is defective in some fashion. Having a circuit oscillate as some frequency (often RF) due to a wiring error is not all that unusual. Troubleshooting an issue like this is one of the most basic reasons to own an oscilloscope.Bob

author: @rt
author_rank: Super Contributor
timestamp: May 21, 2016, 02:43:51 pm
content: Because you capacitively grounded it.Why wouldn’t it oscillate?The transistor base is held high with a resistor, why wouldn’t the capacitors pass the change of the rising edge to pull the base low again?Edit.. because they aren’t a path to groundI still think that’s why though (meat body counterpoise).

author: wael
author_rank: Contributor
timestamp: May 21, 2016, 06:00:39 pm
content: Quote from: uncle_bob on May 21, 2016, 12:30:08 pmHi,I think you are correct , when i shortened the cable between both grounds (between the emitter and the negative terminal of the battery), they were connected via long cable, couldn't get it to blink any more, also found that returning to the old setup, it blinks again , also it blinks when the cable (mentioned above) is either near the capacitor or the led , i'm confused even more, also i don't have an oscilloscopeThanks,Wael

author: uncle_bob
author_rank: Unknown
timestamp: May 21, 2016, 09:02:57 pm
content: Quote from: wael on May 21, 2016, 06:00:39 pmHiYou have a circuit that has additional circuit elements in the form of the wires and cables. At some point (if they are long enough) they become part of the circuit. Put another way - what you have on the schematic is not *all* of what you have wired up. Without test gear of some sort, there really is no way to figure out what's going on. The only real alternative is to wire up the circuit with short / solidly connected leads. Keep everything under 1 cm. The purpose is to reduce the size of the "extra" components you have in the circuit. If you have things like wire wound resistors in the circuit, they are a special case of "don't do that" ... Once it is wired up, put a couple of nice bypass caps from the supply to the ground. Start with 100 uf, then add 10 uf, then 1 uf, then 0.1 uf. (you can also do that in reverse). At some point the strange stuff will go away.Bob

author: wael
author_rank: Contributor
timestamp: May 22, 2016, 03:54:49 pm
content: Quote from: uncle_bob on May 21, 2016, 09:02:57 pmMany Thanks, it turned out that disconnecting the base from everything, and just hooking the led between the collector and emitter, the LED turns on by touching it , and stays on as long as i'm touching it, although it's a BJT transistor ! tried to measure the current between my hand and the base but there was no current on the micro Amp range, i think this explains the proximity thing, but how touching the base turns the BJT on with no current flowing though it ?

author: wael
author_rank: Contributor
timestamp: May 22, 2016, 04:13:24 pm
content: Quote from: wael on May 22, 2016, 03:54:49 pmJust found the issue, there is an AC current between my hand and the base exactly 2.4 uA !! , just hooked a long wire to act as an antenna, the current increased, the led brightness has also increased, it's amazing how it's sensitive to such small currents !!But how it conducts AC, i mean the circuit from antenna to base and then ??Also is it conducting the half of the AC signal only due to the bias ?

author: uncle_bob
author_rank: Unknown
timestamp: May 22, 2016, 09:42:21 pm
content: Quote from: wael on May 22, 2016, 04:13:24 pmHiMost likely what you have is an RF signal between you and the circuit. In order to properly measure it, you would need an RF voltmeter or a spectrum analyzer, or a scope. If you are trying to figure RF out with a normal multimeter, you will just get very confused.Bob

author: wael
author_rank: Contributor
timestamp: May 23, 2016, 06:58:23 am
content: Quote from: uncle_bob on May 22, 2016, 09:42:21 pmCould you please let me know why the circuit diagram posted won't oscillate , i expected the base to be down when the capacitor is charging, and then the capacitor will discharge when it reach a certain voltage through the transistor, and thus the led would blink , could you please explain what is wrong here ?

author: uncle_bob
author_rank: Unknown
timestamp: May 23, 2016, 12:17:42 pm
content: Quote from: wael on May 23, 2016, 06:58:23 amHiIn order to oscillate, a circuit needs to meet certain criteria. There are a lot of ways to look at them and some of it gets a bit involved. One relatively simple way:You need 360 degrees of phase shift around the loop in order to have oscillation. You also need to have gain at that frequency. With the single capacitor you have in your circuit, you only have 90 degrees. You also have no feedback path from the LED back through the capacitor so the inversion in the collector does not contribute to the phase shift.So what happens in your circuit:Power comes on. Transistor base is low.Current flows into the base and the capacitorsVoltage on the base risesLED comes onVoltage continues to riseCircuit hits it's DC equilibrium pointNothing more happens.What happens in an oscillator:At some point AC feedback cuts in and it's not a simple DC circuit anymore.Yes, that's still a bit confusing. Oscillators are a bit weird.What to do to turn your circuit into an oscillator:Add another R/CAdd another gain stage (transistor)Close the loop between the stage you have and the input to the new one.A previous post had a bunch of links to working circuits.Bob

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2016, 10:14:41 pm
content: You know you can edit the quoted text and delete the old messages up there..?So, you were lying to us! You drew an assumed representation of a real system, which as it happens, was not at all representative of it!'Tis the duty of the engineer to make such observations. Of course... it's not always trivial to do, nor is there necessarily a reasonable schematic equivalent that can even be drawn (e.g., microwave circuits).Tim

author: wael
author_rank: Contributor
timestamp: May 24, 2016, 04:43:39 pm
content: Quote from: T3sl4co1l on May 23, 2016, 10:14:41 pmHi,My apologies, just noticed not removing the old messages in the quoted sections could be annoying.I didn't mean to mislead anyone here, i'm not experienced as some of the readers are, the schematic is correct (well i think), it doesn't include all of the details though, the length of the cables are usually not included in the schematics.Regards,Wael

author: uncle_bob
author_rank: Unknown
timestamp: May 25, 2016, 10:14:17 pm
content: Quote from: wael on May 24, 2016, 04:43:39 pmHiNobody ever includes *everything* in the schematic. The issue is having enough experience to realize when a non-schematic element becomes important.Bob