I'm still very short of time, so this blog is just a quick, simple one.

 

Well, it had to happen eventually, here I am doing the astable multivibrator (an oscillator circuit).

 

This is going to be in two parts. First a look at the circuit in the simulator and then I'm going to build one. That second

part will be a trip down memory lane for me because this is the first circuit I ever built and I'm going to have a go at

recreating, in a modern form, what I did back then. More on that in a moment.

 

Here is the schematic as I've simulated it - it's also what I'm going to build later on. It's not as elegant as the beautiful

hand-drawn sketch that I showed in the previous 'Transistors' blog on the Ho Serial Astable, but it will do. I've added

various test meters around the circuit so we can see what is going on.

 

 

The supply I've chosen as being 9V so that I can run it on a PP3 (SF22) battery. The LEDs in the circuit are just a random

selection from those available in the simulator - in the real thing I'm going to use small red LEDs with a forward voltage

of around 2V. The capacitors are 10uF electrolytics and need to be the right way round - I've marked the positive end (the

simulator doesn't have a proper symbol for a polarised capacitor). The voltage rating of the capacitors needs to be greater

than 9V; 16V or higher would be fine. The transistors are 2N2222A parts from CDIL. If you want to build one, you could

probably get it to work with almost any small NPN transistor - just try whatever you have handy and see what happens. I

chose the 2N2222A simply because it was available for a reasonable price in a metal TO18 can and I thought would look nice

in the final build.

 

In case you're wondering what IC1 on the circuit is, it's not a component at all; it's an initial condition. It's just there

to get the oscillator started. Unlike real life, where start-up noise would get the oscillator oscillating, the simulated

version needs a kick otherwise it just sits there with the two sides perfectly balanced.

 

Here are the waveforms that result

 

 

Analysing the circuit purely from the schematic is quite tricky. Where do you start? The symmetry means there is no input as

such, instead each side makes a change and triggers some response on the other side. What helps us is seeing the waveforms.

We can see that what happens at the base of transistor T2 (VM3) is the same as what happens at the base of T1 (VM2) except

that there's a time delay involved. We might also suspect that each follows on from the other. Similarly, the waveforms at

the two collectors (VM1 and VM4) are the same except for a shift in time and align with the previous two. We can also see

where the delay is coming from on the VM2 and VM3 waveforms - the upward slope is characteristic of a capacitor charging and

the circuit is waiting for a certain voltage (somewhere around 800mV in each case) to be reached before operation continues.

 

Perhaps take that and see if you can work out how it operates. (Helpful hint: you can't instantaneously change the voltage

across a capacitor - if you step the voltage on one side, you'll see the same step on the other.)

 

Now for building it. Instead of using a prototyping block or a circuit board, I'm just going to solder the components

together. The original used torch bulbs, electrolytics and resistors which were larger than moderen equivalents, and

transistors that, at that time, were packaged in small metal capsules. This version is about half the size.

 

Here are pictures taken from the front, back, and side. The resistors for the feet are just decoration and can be any value.

The base is a 9V battery box; it's the kind intended for PCB mounting and I've got it upside down, with the circuit board

leads pointing up. The weight of the battery makes for a resaonable base. The arms are a bit short - if I did it again, I

might consider extending them. I decided that it needed a face, so cut a simple shape out of cardboard that just sits on the

LED leads.

 

 

Finally, here it is working. The proof that Super-LED-man can flash his eyes (and that I can't hold a camera still).

 

 

Final question: why is this technically a bad circuit?