Whole circuit oscillating? - Page 1
Forum section: electronics / beginners

author: Dyvyd
author_rank: Newbie
timestamp: July 03, 2024, 12:33:38 pm
content: I have just joined this forum to ask a specific question.I've made a half dozen or more variations of the 555 astable oscillator circuit including the one provided as a morse code generator kit by the ARRL. In this kit there is an output jack for external speakers. Normally to get output to speakers I take the signal from pin 3 of the 555 timer. In the schematic shown for the ARRL kit, pin 3 connects to an onboard speaker, but the output for external speakers (J1) does not directly receive signal from pin 3. Both the onboard speaker and the output jack seem to share only a ground connection? The output to external speakers works fine. How does the output jack get the signal? Does the whole circuit oscillate?

author: Grandchuck
author_rank: Super Contributor
timestamp: July 03, 2024, 02:08:29 pm
content: Is J1 for an external key?

author: RoGeorge
author_rank: Super Contributor
timestamp: July 03, 2024, 02:09:34 pm
content: J1 is for an external Morse Key, not for external speaker/headset. J1 is to connected yet another "button" in parallel with the existing onboard key.

author: jzx
author_rank: Regular Contributor
timestamp: July 03, 2024, 02:10:47 pm
content: J1 seems more like a connector for an external key, at least it is in parallel. If you connect here a speaker, it will be in series with the circuit.Quote from: Dyvyd on July 03, 2024, 12:33:38 pmJ1 is not connected to ground.

author: Dyvyd
author_rank: Newbie
timestamp: July 03, 2024, 06:57:54 pm
content: Thanks for that! That makes sense. I lost the original build instructions many years ago, but I can see that J1 would allow you to bypass the key with a better key than the kit provided. However, thinking it must be an output jack originally-- when I plug it into powered computer speakers, the oscillator tone comes through loud and clear, while it still triggers the onboard speaker also. So why that works is still a mystery to me.

author: Dyvyd
author_rank: Newbie
timestamp: July 03, 2024, 06:59:48 pm
content: Thanks for that! That makes sense. I lost the original build instructions many years ago, but I can see that J1 would allow you to bypass the key with a better key than the kit provided. However, thinking it must be an output jack originally-- when I plug it into powered computer speakers, the oscillator tone comes through loud and clear, while it still triggers the onboard speaker also. So why that works is still a mystery to me.

author: SteveThackery
author_rank: Super Contributor
timestamp: July 03, 2024, 07:20:28 pm
content: Quote from: Dyvyd on July 03, 2024, 06:59:48 pmIt seems obvious to me, so probably I'm missing something. The reason the speaker makes a tone when plugged into J1 (which is actually for an external key) is that the circuit is powered through the speaker. So the varying current drawn by the 555 as it oscillates results in the cone moving in and out, thus making an audible sound.

author: Dyvyd
author_rank: Newbie
timestamp: July 04, 2024, 03:10:52 am
content: I know little of circuit analysis, but this sounds sensible to me. I also note that the onboard speaker and the exterior ones connected via J1 are producing the same pitch.