I just recently started learning about electrical engineering stuff, and since my final goal of building a Z80 Computer seems a bit far away, for now, I decided I'd make a simple clock I could possibly use for that first.

Since I decided to not order a 555 Timer, I wanted to build something with the same result with the parts I got laying about.

Here's the schematic I used and the circuit I built (hopefully it isn't too awful for something a newcomer created!).

Schematic taken from https://hackaday.com/2011/12/01/multivibrator-in-theory-and-practice/ Astable Multivibrator on a breadboard, it doesn't change from how it is in the picture.

I've gone over the schematic multiple times. The only differences I am aware of are the fact I used the BC549, which is supposedly interchangeable with the 2N3904, and I used some 0 Ohm Resistors due to not having enough wire laying about. Otherwise, all the resistor values, capacitor values, etc. should be exactly the same as those in the schematic.

Any and all help/advice is appreciated!

Edit: The pin order of the Transistors is opposite of what they'd be with a 2N3904. Thank you, Audioguru!

  • 1
    \$\begingroup\$ Double-check your construction. It looks like a resistor lead that should be going to the collector of the transistor on the left is unconnected or connected to the rail right next to the collector rail, and it looks like the capacitor leads connected to that transistor's base and collector are shorted. \$\endgroup\$
    – TimWescott
    Sep 18 at 16:55
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    \$\begingroup\$ The pins on a 2N3904 are EBC. The BC549 pins are the opposite at CBE. See it on their datasheets. The resistor values should not be 100 to one, try 20 to one instead. \$\endgroup\$
    – Audioguru
    Sep 18 at 17:01
  • \$\begingroup\$ @Audioguru Flipping them around fixed it! Thank you! \$\endgroup\$ Sep 18 at 17:06
  • \$\begingroup\$ @PixelBrush I'm really happy that you are setting a high goal for yourself -- the Z80. It's doable. But what a path to take! It may help you a little to read Microprocessor Design Using Verilog HDL by Monte Dalrymple. It focuses in great detail on the Z80 instruction set. It's for laying out an HDL design for FPGA or ASIC, which isn't what you hope to do. But more than half the book is on all the preparation and detailed consideration about the Z80 instructions and hardware behavior -- before ever writing HDL. And all of that would be useful to you. \$\endgroup\$
    – jonk
    Sep 18 at 18:46

Your schematic doesn't show the LEDs but I assume these are in series with R1 and R4 and that would be fine. The breadboard allows you to make few quick checks:

  • Remove the capacitors. Now you have two independent (identical) circuits driving LEDs. The LEDs will be on (assuming a decent transistor gains).

  • Remove resistors R2 and R3 and the respective LEDs should be off.

Now you have verified the transistors are healthy, properly connected and the eight components are doing their job.

Next possibility is that the multivibrator circuit does work but at higher blinking frequency than an eye can resolve. Do you have let's say 10uF (or 22uF) capacitors handy? If not, you may wave the board in front of your eyes trying not to follow the board with your eye and you may notice a stroboscopic effect (persistence of vision) even if the LEDs are flashing at higher frequency than (approximately 16Hz) that human eye+brain can resolve as fastest flashing frequency.


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