When I went out the the work shop the other day I left the circuit drawing in the house. It is 24 degrees my hips hurt me worse than my back it is a big deal to go back to the house to get circuit drawing. Parts all tested good but today I checked parts again they are still good but now I see I have 47K where 470 should be, and 470 where 47K should be. Today I have 1 dual flasher working with resistors in correct place, good capacitors, and LEDs soldered on the correct way. Maybe today I make 4 more flasher PC boards with only the transistors and caps, tracking shows my resistors should be here Friday. I have 11 LED flashers along driveway, front of house, east side of house, garage, work shop, wow they show up good after dark. I should put LED flashers on west side of house to make it look like there is some type security system over there too. My original flasher has been flashing 16 days it will be interesting to see how long it lasts with those worthless Harbor Freight batteries. I bought better battery a few days ago.You can test the transistors in the circuit. When a transistor is turned on by its 47k resistor on its base to +3V then the capacitor turns it off.
make the test by simply shorting the base to 0V and it should turn off.
I don't know how to design the circuit, I only know how to build it. I can experiment with parts to make circuit better or worse. I understand the capacitor charges through the resistor. Larger resistor makes capacitor charge slower. Larger capacitor charges slower than small capacitor. Larger capacitor holds more charge than small capacitor. I have no clue how 2 transistors make the LED flash. I can see when 3v is turned ON circuit does not flash for 3 seconds until capacitor charges through the resistor. Capacitor discharges through LED then the cycle starts over. I have forgotten the formula to calculate how long it takes for resistor to charge capacitor, I can not find it online probably because I don't know the correct search words to find it. I wish I understood transistors better when I was in college we learned tubes, every thing was tubes then.You said you tried connecting LEDs in parallel. They both will light only if they both have exactly the same forward voltage but the forward voltage varies between LEDs even if they have the same color.
You show the schematic that is missing the very important current-limiting resistor. Then the LED and/or BC547 might burn out and the flash duration will be so short that the LED appears to be dimmed, as you found.
I built this circuit, with 3v it blinked every 10 seconds. I was surprised to learn with 4.5v it blinks every 15 seconds. LED is very bright. It needs to blink faster about 3 seconds. I have no 180 ohm resistor so I used two 100 ohm in series.I looked in Google for Capacitor Charging and found this graph that shows that R x C= one time constant which is a 63% charging voltage.
The capacitor must charge to about 0.8V for the transistors to turn on then discharge a little for the transistors to turn off.
The capacitor does not discharge through the LED, instead it discharges through the transistors.
That is interesting, I never thought of that but I know that is right. Speaker construction puts the speaker coil in center of the magnet so it can move both directions. I built several push pull tube amps 45 years ago but never a transistor amp.A resistor in series with the speaker will increase the collector and reverse=base voltages maybe high enough to zap the transistor.
A speaker is driven from AC, not DC. An audio amplifier has two output transistors or Mosfets for them to produce AC with push-pull action. A single transistor or Mosfet drives a speaker with DC pulses which is bad since the cone moves only half of its usual push-pull motion.
These linear MOSFET amps (I.e. not Class D) look good on paper with standard switching mosfets IRF but the bias must be set for each specific mosfet, then, even with perfectly set bias, they sound like crap if you change the volume or room temperature changes enough to make the bias drift.That is interesting, I never thought of that but I know that is right. Speaker construction puts the speaker coil in center of the magnet so it can move both directions. I built several push pull tube amps 45 years ago but never a transistor amp.
I did google search for push pull mosfet amplifiers and found this. It looks like an LM386 will drive 2 mosfets.