Transistor astable oscilator - Page 1
Forum section: electronics / beginners

author: Gcsillaz
author_rank: Contributor
timestamp: April 21, 2021, 02:07:12 am
content: Hello everyone!I just build the circuit below(Attached) and would like to know if someone could help me the difference between two components.Where we have the 1.5nF cap I used a 1.5nF CERAMIC and I'm getting a frequency of ~15kHz... on the schematics of the 5 transistor model from jay diddy's he says it should be 100kHzThis difference might be because there is a difference between polyester/mylar/other models?Thanks!

author: MikeK
author_rank: Super Contributor
timestamp: April 21, 2021, 03:13:20 am
content: LTSpice sim does show 100k oscillation frequency. Ceramic cap should be fine. Tolerance of all components will matter a little. Generic ceramic caps are usually 20%, but that's not enough to account for your variance. Voltage is? Recheck component values?

author: Gcsillaz
author_rank: Contributor
timestamp: April 21, 2021, 04:26:35 am
content: Quote from: MikeK on April 21, 2021, 03:13:20 amResistors are 1%... Voltage is 5Vdc ok...I will recheck all resistors

author: Capernicus
author_rank: Frequent Contributor
timestamp: April 21, 2021, 10:37:26 am
content: Oscillators are so hard to read!!! I need more practice.

author: PerranOak
author_rank: Frequent Contributor
timestamp: April 21, 2021, 12:29:56 pm
content: BTW what is the name of this type of oscillator?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 21, 2021, 01:43:24 pm
content: Hi,This circuit is called an emitter-coupled astable multivibrator.The design process is similar to designing an amplifier. In fact if you remove the capacitor it is an amplifier.Here is the model without the coupling capacitor.I have indicated the voltages on each node.The capacitor causes the amplifier to oscillate.Regards,Jay_Diddy_B

author: PerranOak
author_rank: Frequent Contributor
timestamp: April 21, 2021, 02:00:50 pm
content: Great, thank you very much.In amp,mode, input is at the base of Q1, yes?

author: Gcsillaz
author_rank: Contributor
timestamp: April 21, 2021, 02:38:14 pm
content: Quote from: Jay_Diddy_B on April 21, 2021, 01:43:24 pmHello Jay_Diddy_B!First of all thanks for the diagram and explanation!I noticed that I made a rookie mistake using my osciloscope... my time settings had the x10 knob pulled out xDI've fixed the reading but still.... I'm only getting 70kHz :/ Is this enough? Please see attached how I build the circuit on the breadboard and the reading.Obs: DC coupled (centered base-line), 2V/div, 5uS/div - 5Vdc PSU. 3.3kRes substituted by a 3kRes since I did not have 3.3k... (I have also added a 270Ohm in series to see if it my be this the problem but isnt)Thanks in advance!

author: mikerj
author_rank: Super Contributor
timestamp: April 21, 2021, 03:47:37 pm
content: Do you have the middle knob on the timebase control turned all the way to the "Cal" position?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 21, 2021, 03:48:08 pm
content: Hi,the photograph of your breadboard is very useful. The frequency is probably off because of this:I don't know very much about those ceramic capacitors. I used much better ones when I built the circuit.An ESR Meter does not need to be very accurate to be useful. You will get very similar readings at 70 kHz versus 100 kHz. You can try a 1nF capacitor if you have one.Regards,Jay_Diddy_B

author: Gcsillaz
author_rank: Contributor
timestamp: April 21, 2021, 04:03:08 pm
content: Hello Jay,I bought this in bunches from china... that kind of assortment kit with lots of values... Even reading what they were gave me headaches hahahahaOk... I've substitute for 1nF -> 150kHz.Sadly in this kit there is nothing between 1.5nF and 1nF...I have ordered some new parts to build the meter (Specially the 1n5711w which is pretty impossible to find here... I've bought 1N60's hope it works xD)There is a 1.5nF cap in the bunch... I'll see how it behaves! Anyways, thanks a lot for your time helping me!

author: Tom45
author_rank: Frequent Contributor
timestamp: April 21, 2021, 04:09:03 pm
content: Quote from: Gcsillaz on April 21, 2021, 04:03:08 pmIf your kit has values smaller than 1nF, you can put 2 or more in parallel to get a value between 1.5 and 1.

author: Gcsillaz
author_rank: Contributor
timestamp: April 21, 2021, 04:18:32 pm
content: Quote from: Tom45 on April 21, 2021, 04:09:03 pmAbsolutely... I put two 680pF in parallel and got ~98kHz which is pretty good! Thanks for the input Tom45!Also... Sometimes when measuring at higher frequencies the graph seems to be moving from left to right and this makes hard for me to count the divisions xDSomeone knows what that is about?

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 21, 2021, 05:52:48 pm
content: I've found those cheap red/brown ceramic caps to be super drifty and out of tolerance, especially the higher values.

author: Gcsillaz
author_rank: Contributor
timestamp: April 21, 2021, 06:49:52 pm
content: Quote from: BrokenYugo on April 21, 2021, 05:52:48 pmYep..I don't know if it is supposed to work like this but... everytime I measure one of this with my multimeter it never shows the same value... there are discrepancies as far as 30% sometimes...

author: Tom45
author_rank: Frequent Contributor
timestamp: April 22, 2021, 02:55:18 pm
content: Quote from: Gcsillaz on April 21, 2021, 04:18:32 pmThat is almost certainly a problem with your scope's trigger settings.What is the model number of your scope?If you take a closeup photo of your scope's controls, everything to the right of the CRT display, we can probably spot the problem. The one photo that you posted shows only a few divisions for the vertical axis. If you set the vertical scale to make the vertical range larger, that might be enough to get a stable trigger.

author: Moriambar
author_rank: Unknown
timestamp: April 23, 2021, 02:47:29 pm
content: Quote from: Jay_Diddy_B on April 21, 2021, 01:43:24 pmHi, I know this is OT, but where can I find theory for this circuit? I mean the "amplifier" part is ok and I understand it, but I cannot understand why the cap makes it oscillate.Caps always make me dizzy and seem unpredictable to me.

author: PerranOak
author_rank: Frequent Contributor
timestamp: April 23, 2021, 03:14:52 pm
content: QuoteI'm with you there!They are the simplest of components to understand, in theory.However, understanding their in circuit "spooky action" (to rip-off a great man) in your bones is what makes experts, er, well,expert.