# Computing stuff tied to the physical world

## Mystery circuit

In Hardware on Nov 23, 2012 at 00:01

Here’s a little puzzle for you – what does this circuit do?

That’s two BC549 NPN transistors and two resistors. Let me draw the schematic:

Better still, here’s what it shows when hooked up to my Component Tester:

Horizontal scale: -10 V .. +10 V, vertical: -10 mA .. +10 mA.

Bonus question: is this circuit actually useful for anything?

1. Looks to me like a current limiter, which can be used for creating a current source to power components like LED’s etc.

2. It certainly looks like it Ronald… Or you could use it for battery charging, although they’d charge very slowly at 2mA!

Thanks JC, I forgot how much dry-running analogue circuits in my head hurts!

Is it useful for anything? Well it’s kept at least two of us entertained for a few minutes! Does that count?

3. Upon seeing the schematic I was hoping for a negative resistance region like a tunnel diode, but I guess not. Looks like an okay constant current source.

4. looks like a constant-current circuit.. like Ronald said.

Seems like most designs these days are using DC/DC type conversion circuits to accomplish this, as they are more efficient. Curious to see your application.

5. Starting from V=+2V, it’s a linear heating element (rather than the annoying square you get with resistors). As everyone knows, linear actuators are much easier for controller design.

Obviously, it’s a circuit you’re going to use to improve your central heating characteristics.

6. Is it for a current switch source to talk(Tx) to you OT circuit?

7. Despite my profound knowledge of analogue circuits, it still looks like a seagull making a turn to the right (or left, depending on which side is showing) ;-)

Its only use is to shit on your head…

8. Ok, so that was fairly easy for some of you, I guess :)

Tomorrow, I’ll try to explain in a somewhat intuitive fashion how it works. It’s a nice step up from grasping how a single transistor operates, IMO.