The a-stabile multivibrator circuit made with 2 x BC 547 b is a very good beginners circuit.
You can learn a lot about: R-C frequency relations, how to generate different frequencies in the most elementary way, how to generate certain waveforms that you need/like by playing with resistor and capacitor values, suplly voltage, time relations, etc etc. Important: could be that it sometimes does not want to start, like YT user Micky Bee mentioned. If it does not want to start oscillating go to https://youtu.be/rmrH4MqvQRo
for more info.
When it does not want to start oscillating change the value of the BASE resistors of TS 1 and TS 2 substantially, say: 1 K and 10 K or 10 K and 100 K or 4K7 and 10 K/22K. To create an “inbalance” when the supply voltage is switched to the circuit.
For beginners (& profs…) : (always…) use the advised transistors for first electronic experiments (in general, of course). Reason: they always work (!). So the BC 547 (NPN, Hfe 200-250) the BC 557 (PNP, Hfe = 200-250), the BD 139 (medium power NPN, Hfe=70-150) or the BD 140 (medium power PNP, Hfe = 70-150) or the 2N3055 (Hfe 50-70) or the 2N2219 for HF purposes up to 30 MHz.
When you want success in first experiments: buy/use them and always check their Hfe (= “GE” amplification factor) before soldering them into a circuit.
The circuit surely works with 2 x BD 139 or with one BC 547 B (TS1) and a Darlington of BD139-2N3055 (TS2-TS3).
Sorry to say: sometimes you buy BC 547 b on flea markets with an extreme amplification factor, say 400-500 or so. Throw transistors with unexpected high Hfe’s away, they were/are (in general) dumped on the hobby market by manufacturers, their “steilheid” is too high.
“Steilheid” (Dutch word) means the relation between input current on their bases (tiny) and the current between collector-emitter is too high, thus irrational, these transistors can only act as a “switch”.
Other types of transistors (apart from the mentioned) could (read: could…) give disappointing results in beginners and professional circuits & experiments, sorry to say.
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Regarding all my video’s: I constantly keep them actual, so the original video’s with the most recent information are always on YouTube. That is the source, and search there. When my video’s are reproduced or re-edited on other websites/channels you can not (!) be sure about the original content (=really working electronics) and important adaptations to the circuits.
Be aware of that, I saw on the internet my circuits reproduced in a poor or even not proper way. I can not help that, sorry. Update 3 july 2019.