Crickets: nature's favorite astable multivibrator

A project log for Hacking Nature’s Musicians

An artistic ecosystem of analog electronic sound generators

Kelly HeatonKelly Heaton 09/25/2018 at 15:420 Comments

What rural soundscape would be complete without crickets?

What is it like to be a cricket?

It's one thing to listen to their rhythmic chirping, but imagine spending your life singing the same tune indefinitely, changing tempo in response to heat, and only shutting up when you're afraid of imminent death. Wait, how is that different from human thought chatter? I digress.

Crickets are an excellent way to explore the basic, fundamental building blocks of electronic music: oscillators. Specifically, crickets taught me about astable multivibrators -- a term which, for those uninitiated in EE lingo, is probably the last thing you would search in your quest for "how to build an electronic cricket." 

An astable multivibrator is a circuit that bounces back and forth between two states (and will do so indefinitely until you disconnect power). It's like a perfect Pong volley -- and therein lies its great strength and weakness. You can use an astable multivibrator to generate a consistently oscillating waveform. This gets acoustically (or visually) boring after awhile and in later logs I will show you how to introduce randomness. But for now, I will focus on my design of boing-but-educational crickets.

My simplest circuit for a chirping cricket is comprised of two astable multivibrators made with discrete transistors: one controls the chirp timbre and the other controls the chirp tempo. For your reference, I uploaded a schematic called "Heaton_transistor cricket.jpg" that you can find in my project files section of this site. 

To understand how the circuit works, it helps to look at a single astable multivibrator wired with LEDs. Check out my video "Transistor astable multivibrator" in which I change the capacitor values in a single astable multivibrator. The first configuration is symmetrical, having two 100 uF electrolytic capacitors. Next, I replace one of these with a 22 uF capacitor and the pulse becomes obviously asymmetric. Last, I remove a 100 uF capacitor and replace it with 47 uF. The pulse remains asymmetric but is visibly different than before. This is a simple, low-cost way to achieve different oscillating patterns, aka tempo.

PS: Wow, my breadboard is dirty from 20 years of use/abuse.

Next, let's look at how to make the quality of a cricket's "voice," or the chirp's timbre. For this, I use an astable multivibrator with smaller electrolytic capacitors, like 2.2 uF, causing the electricity to bounce back and forth fast enough that we perceive a texture instead of a pattern. In the beginning of my next video, "Simple discrete transistor cricket," you will hear a steady, unbroken ringing sound that is created by this circuit design. The resulting "timbre oscillation" (happening on the left side of my breadboard) goes through a simple high-pass filter and transistor amplifier (right side of my breadboard) to one of my custom 2.8KHz piezo electric speakers (seen in the background).

Next, I unplug a base resistor from one of the transistors in this astable multivibrator and you hear silence, because the timbre oscillation stops. I then reconnect the transistor's base to a signal incoming from another, slower astable multivibrator -- the tempo generator, shown when I pan left. Voila, the cricket starts to chirp. Finally, for extra credit, I add a pull-up resistor on the same transistor's base to show you how this simple change affects the sound. Minor adjustments to my design will give you many variations on the theme (==crickets with personality).

Happy chirping!