DFN Happy Hour No. 43: Good Vibrations

This week the combination of our transistor circuit and the RC circuit gets us dancin’! At least oscillating anyway. Bring on the multivibrator circuits in Round 3 of the DFN Happy Hour with circuits.

Same DFN Time: Friday at 5:30 pm PST
Same DFN Channel: https://www.twitch.tv/deepfriedneurons
Same DFN Folks (for now…): Barb Noren (BarbMakesThings), Tod Kurt (todbot) and Carlyn Maw (carlynorama)

Last Week Redux: May 14, 2021

Post | Video

Beverages Consumed

“Thanks for Existing!” Awards

This Week Preview

Astable Multivibrator circuits combine what we’ve learned about RC circuits and transistors. These days a search for “multivibrator circuit” will wind up on a page with a 555 timer. The 555 timer has a flip-flop (bistable multivibrator) and some comparators, and generally represents the normal way to inject some oscillation into a circuit. I’ve heard it described as the teddy bear of IC’s, so loved and ubiquitous it is. The Evil Mad Scientist 555 wiki page, and their Three Fives kits (through-hole and surface mount) have fabulous information on the 555.

We are not going to be using the 555 this week. We’re going to build a flip flop (bistable multivibrator) and a free running relaxation oscillator (astable multivibrator) by hand with just transistors, resistors and capacitors.

The Flip Flop

Figure 1: Two transistor circuits with switches combine to make a bistable multivibrator circuit

A single flip-flop circuit isn’t super useful on its own to project builders building interactions on human scale. It’s just a device that remembers its position, which a toggle switch or a bank of DIP switches can handle nicely. But let’s say a project needs to remember hundreds, thousands or millions of switch settings. Or loop through different setups at millisecond speeds. And do this while being smaller than a sky scraper. Tiny little flip-flops make all that computer memory and logic possible. They matter to us this week because understanding the bistable multivibrators will help make the astable multivibrator, our real goal, more intelligible. Let’s build up from a circuit we already know, a switch “not” on the base of the transistor similar to our dark detector logic.

Getting Astable

Figure 2: Two transistor circuits with capacitors combine to make an astable multivibrator circuit

Humans epically suck at tapping buttons repeatedly at a perfect intervals for hours on end, so we want to be able to use an RC circuit to do the work. An astable multivibrator or a “relaxation oscillator” gets built up in a very similar way to the bistable, but replacing our switch with a capacitor.

Driving more than a wee little LED might happen for our projects this week, so I wanted to show what that would look like using another NPN transistor (left), and a PNP transistor (right).

Figure 3: Two examples of using an astable multivibrator to drive a larger load than it could handle in line.

Breadboarding the Astable Multivibrator

This DFN episode takes me way back into CRASH Space history. I built a discrete part multivibrator circuit at CRASH Space in March 2010 while preparing for a class at the deeply missed Machine Project. That wall and those bench-tops look squeaky clean because they’re new! I still love this breadboard layout for an astable multivibrator circuit because it looks like a little crab.

2011 Flickr album of astable multivibrator circuits

This layout would be very hard to model in Fritzing with the parts so smashed together so I made a couple of other options to try.

Figure 4: Two options for breadboarding an astable multivibrator

Parts List

  • 2 general purpose NPN Transistors like PN2222 or 2N3904
  • 2 current limiting resistors, 220-1k Ω
    • in the schematic example: 220Ω
    • in the pictures on flicker: 330Ω
  • 2 RC pairs:
    • in the schematic example: 10kΩ and 200uF
    • in the pictures on flicker: 10kΩ and 220uF
  • 2 LEDs

These circuits will blink a few times a second, but the RC pairs don’t have to be the same. Try this astable multivibrator calculator to figure out what values to use for your own. It requires a lot of inputs, but provides reasonable defaults.

One shot pulse generator

Figure 5: Combining a transistor circuit with a switch and a transistor circuit with a capacitor gets leads to a monostable multivibrator circuit.

The bonus circuit on offer this week is the monostable multivibrator which combines one side of bistable with one side of astable. There are several schematics for this circuit depending on the specific application. A very simple one, using a manual trigger but keeping the collector-coupled pair, is shown above.


I make things that do stuff. The best, though, is teaching others to do the same. Founder of @crashspacela Alum of @ITP_NYU

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