# What does this circuit on a t-shirt do?

A t-shirt with this image showed up as a suggested purchase on one of the popular ecommerce sites. What does this circuit represent?

Update: Searching on aliexpress.com for "What Part of Don't you Understand" reveals a variety of similar shirts with topics such as math, physics, hockey, etc., all with complicated and mostly implausible drawings related to the topic. So, it seems the humor is not based on the particular circuit (as I had thought), but on being a "complicated electronics drawing."

• The part I don't understand is why there's no diode across U3. Feb 21 at 3:44
• A more fleshed-out version of this circuit is discussed here: eleccircuit.com/dual-adjustable-power-supply-circuit-diagram Feb 21 at 7:55
• the part I don't understand is that I've been told the 7805 is antiquated and I should be using a buck converter instead Feb 21 at 13:49
• Also I don't understand why it's not drawn symmetrically, with ground/zero in the middle and the negative line at the bottom :) Feb 21 at 13:50
• "You should have that on a T-shirt" :) Feb 21 at 23:10

What does this circuit on a tshirt do?

The positive voltage regulator looks like it'll burn because they have the incorrect pins numbers for the voltage regulator: -

LM7805 image from here

It looks they blindly assumed the positive regulator was the same pin-out at the negative regulator. What an embarrassment for the guy wearing it.

Or maybe, the true meaning is: -

• Strange. Usually people foul it up the other way around since negative regulators are relatively uncommon (and the 7805 has that lovely in-com-out order that looks good in a schematic). Feb 21 at 13:02
• @SpehroPefhany very true and, an interesting known fact about both 7805 and 7905 is that pin 1 is the most positive terminal and pin 2 is the most negative terminal so, even if you replaced a 7805 with a 7905 it wouldn't harm the device. Much worse on the above T-shirt and, they've even made cross-over junctions. Pure amateuristic work. Feb 21 at 13:47
• It's meant to be a dual + and minus 5 volt regulator circuit that takes a feed from a transformer secondary @MarkHarrison Feb 21 at 22:41
• @SpehroPefhany It's uncommon. That's why it's the one you check. The you copy-paste it for the common one which you don't check because it's common. Feb 21 at 22:46
• @Andyaka That little "high/low potential order" snippet is worth ten points on its own! Thanks! Feb 22 at 14:50

"I'm bipolar."

. . . . . .

• +1 - LOL... very good :-) This should be the accepted answer. Feb 21 at 15:22
• "I'm bipolar, and I'm screwed up." -- Possibly not what the designer intended, but that's what it says. Feb 21 at 16:06
• They are also pretty hot 🔥 judging by the usual dissipation of such circuit. Feb 21 at 18:36

By representing vintage regulator ICs in a linear power supply circuit with an irascible title, the message, to me, seems most likely one of excluding the alternative: "I don't want your steenkin' switching power supply". Perhaps the user has been burned by too many noisy or otherwise faulty SMPSs; perhaps they just don't (potential implication: refuse to) understand them, and how to choose an effective unit, or how to filter the noise produced by a mediocre one. "Things used to be so simple."

This interpretation may reflect my own bias, of course: I am quite familiar and comfortable with SMPS, as well as the components and circuit in question, and there are few if any cases where I would strictly prefer the given circuit in my professional work. The message therefore has a certain jarring nature, which complements the irascible tone of the text.

A second level of amusement arises from the apparent hubris, noting the pinning error on the 7805, and lack of diode on the 7905. (Or, at risk of commiting further pedantism: the diodes aren't required as there is no load on the input side of the regulators; an antiparallel diode at the outputs could however prevent polarity reversal in case of an ungrounded overload condition.)

A third level of representation is off-topic, but no less relevant, so I will continue:

There is a social function: messages where one is simultaneously celebrating an achievement (knowledge of linear power supplies), while rejecting adjacent topics (switching power supplies), provides observers a hint at the psychological disposition of the bearer. Such messaging correlates with a preference for team-identity, in-group/out-group dynamics, and other authoritarian traits (as defined by e.g. Altemeyer's research). Or we can interpret the message as a preference for traditional methods over newer alternatives -- literally the "conserve" in conservatism; same correlation.

My bias note, further hints at this interpretation: notice that, to an observer outside either group (linear-prefering, SMPS-prefering), the message seems to only reflect a preference, nothing more; to an observer within either group, however, ones' bias toward or against the identifying subject, predisposes one to interpret it as rejecting the alternative. When an in-group/out-group dynamic exists around that subject, such messaging serves to reinforce the self-identity of the group(s).

These readings are contingent on the interpretation, of course, and these specific traits are not perfectly correlated with such messages, either. Mind that one needs far more (and different types of) messages, or much stronger ones, to confidently assess such traits; as I said, merely a hint, nothing more.

This subject, besides being off-topic, is of course also well outside my (professional) scope, and indeed, I don't expect you [the reader] to take it too seriously; I would invite you to read this post as an applied introduction of the topics in question, not any kind of authoritative description of them. (I'm being wordy to be precise, but this has the downside that being wordy sounds authoritative.) I would invite you to peruse the literature on the topic, including the cited author; and if you have further questions in this direction, consider asking the psych.SE.

It's part of a power supply. AC comes in from the connector on the left, probably from a center-tapped transformer. Bridge rectifier, a few smoothing capacitors, linear regulators, and you end up with + and - 5V rails and indicator LEDs for each one. D2 prevents C3 from discharging back into the regulator when power is removed. D3 is missing in action for some reason. Possibly for an audio application of some kind?

• But that doesn't tell the tale of the T-shirt. The graphic may be a reference to "Bipolar" as in the depicted power supply.
– Nedd
Feb 21 at 2:49
• @Nedd - Although that seems possible (and I hadn't spotted that interpretation - I just thought old, linear power supply...) I found a vendor poking fun at a flyback schematic too. Therefore I'm not convinced that this is anything more than a randomly chosen simple-ish schematic by the T-shirt vendor in the original question. (Of course, if evidence comes to light that the T-shirt design is trying to be unkind to people with bipolar disorder, that would be different.) Feb 21 at 4:40
• @Nedd - Indeed. My thoughts above remain unchanged - just because someone chooses to put a simple linear power supply schematic (which happens to have a bipolar output) on a T-shirt / poster etc. doesn't mean they are offending people with bipolar disorder. The fact that I quickly found a similar design with a flyback schematic, points away from any hidden meaning of the chosen schematic (otherwise they would be sticking with the linear schematic, to get maximum "gag value"). The layout of two-rail schematic better fits the "portrait" orientation of a T-shirt than a simple single-rail. Feb 21 at 5:00
• (It's funny that it has a big mistake in the schematic. I have several gag T-shirts, but I won't be getting that one :) ) Feb 21 at 5:02
• @SamGibson reminding us that he's not just a mod, he's a proper engineer! Feb 21 at 6:07

Y’all are overthinking this. Read as “What part of bipolar don’t you understand?”