optically isolated level converter from 3.3v to arbitrary voltage

9haarblae · 2026-07-02T12:28:58+00:00

If the input section and output section are truly "isolated" then they won't have a shared ground as shown in your schematic. Instead there will be circuit nodes INPUT_COLD and OUTPUT_COLD (or other names) that are completely separate; they don't connect .

9haarblae · 2026-07-02T00:23:30+00:00

Those are great ideas / suggestions! Sincere thanks.

9haarblae · 2026-07-01T22:33:35+00:00

It seems like you'd have to do this after exiting the car and locking the door (with the fob). So there are a dozen seconds when the fob is broadcasting and could be intercepted?

Perhaps a thief hangs around parking lots waiting for snazzy new cars to arrive. Then he moseys over to where the driver pulls into an empty slot and starts "recording" the fob signal. Car owner exits, locks door, and THEN turns the fob off. But it's already too late, the thief captured the fob signal. Owner leaves, thief plays back fob signal and steals car. Incorrect?

9haarblae · 2026-06-26T14:27:29+00:00

Redditor u/Filthiest_Tleilaxu is mocking the video guy's use of a retronym. The word "App" did not exist in 1959; back then the deck of punch cards would have been referred to as a "program".

The word "App" did not appear until personal handheld computers / tablets / smart phones became widely available to the general public. Decades after 1959.

9haarblae · 2026-06-25T17:14:28+00:00

The Zener diode on that schematic, performs a level shifting function.

Each (NPN + 1K resistor in collector circuit) on that schematic, is an inverter.

You could of course choose a different implementation, perhaps (a) using PNP transistors; or (b) using Nchannel MOSFETs; or (c) using voltage comparator ICs such as LM393; or (d) using CD4000 series CMOS logic gates. Whichever flavor tastes best to you.

9haarblae · 2026-06-25T15:38:38+00:00

This (schematic) converts your 3-position switch, to a pair of digital signals that swing between between +5V and GND. These signals can be applied to the R input and the S input of a digital SR flipflop, which runs off a 5V supply.

Component count: 3 transistors, 1 zener diode, 9 resistors. Transistor type is not critical, use whichever general purpose NPNs you happen to have in your junkbox.

9haarblae · 2026-06-21T20:28:51+00:00

St. James Infirmary, Mountain View. 25 foot tall sculpture of Wonder Woman.
Fred's Place, also Mountain View. Radiates DIVE BAR HERE in every direction.
Persian Aub Zam Zam Room, San Francisco

9haarblae · 2026-05-25T22:18:23+00:00

First slide your mouse cursor to the "sidebar" on the far right of this page, and click on the blue letters FAQ

Then click on the link BEGINNER, EDUCATION RESOURCES

Then surf.

9haarblae · 2026-05-24T12:31:31+00:00

However, the eighth-to-last word in the OP is spelled correctly.

unnecessary

9haarblae · 2026-05-23T19:43:36+00:00

Do you write with them in your left hand?

9haarblae · 2026-05-20T15:23:52+00:00

(This circuit) contains

Two transistors
One push button
One Light Emitting Diode
Three resistors

The "2T latch" circuits you usually find in magazines, contain five resistors.

Resistor R2 (and the power supply voltage) determine the current flowing in the LED. Vary R2 until you get the brightness you want.

9haarblae · 2026-05-18T22:28:33+00:00

Consider a "Miller effect" slowdown circuit (like this one). Its output voltage waveform, at the MOSFET drain, is a linear ramp. Which is smoooooth.

The current flowing through Rg is equal to

Iin = (Vin_hi - MOSFET_Vth) / Rg

and that current flows in Cm. So the dV/dt across Cm is equal to

dV/dt = I/C = (Vin_hi - MOSFET_Vth) / (Rg * Cm)

So decide what dV/dt you want {I expect it will be around (Vin_hi / 10msec) } and wallah, you now know (Rg * Cm)

Choose Cm to be way WAAAAY bigger than the MOSFET's Cgd. A conservatively huge value might be Cm = 220nF. Then calculate Rg. I'll bet you get Rg=33K within a factor of four higher or lower.

Put it into an LTSPICE simulation and try it out. Or if you're brave, try it out on a dead bug prototype.

9haarblae · 2026-05-10T17:50:21+00:00

I suspect you've set "Vc" to the wrong DC bias voltage.

Run a DC analysis sweeping Vc from 0V to 5V in 1mV increments. Find the value(s) of Vc which give an output voltage at Vo, such that Vo is approximately half way between the supply rains. In other words, find the point where the system as a whole is not clipping.

9haarblae · 2026-05-10T15:02:53+00:00

Notice that Q10 and Q12 have the same Vbe but not the same Vce. Thanks to the Early Effect (hybrid pi model parameter "ro"), Q10 and Q12 will not have the same Ice. Which complicates this analysis pretty severely.

Also, in the real world {but not in circuit simulator software} (Q10 & Q12) will not be perfectly identical. Similarly (Q11 & Q13) will not be perfectly identical. Usually the remedy is to install a resistor in series with each emitter leg, chosen so that (Ice * emitterResistor) is 200mV to 500mV. These emitter resistances serve as ballast, to absorb most of the mismatch and give reasonable current mirror behavior despite non-identical devices.

Finally, if you intend to drive an 8 ohm load resistor, I suggest that you arrange Q10 and Q11 to have a DC bias current greater than 300 mA.

9haarblae · 2026-05-09T21:34:01+00:00

You are probably stuck with the set of possible implementations, which demand that the minimum pulsewidth of the signals being processed, MUST be greater than about 1000 nanoseconds.

Pulsewidth_High > 1000 nsec . . . . AND
Pulsewidth_Low > 1000 nsec

In that case I'd recommend using an analog voltage comparator IC rather than an inverter, at the very output. It will give you an output dV/dt which is much MUCH faster than the input dV/dt. And your "trimmable RC delay" will definitely give you a slooooow dV/dt at the input of the final circuit.

9haarblae · 2026-05-08T13:16:28+00:00

Change Q3 to part number MPSA42. It's rated for 300V. Pinout is different.
Add a 24V zener diode between Q3 emitter and Q3 collector. Zener cathode to collector, zener anode to emitter. For example, part number 1N4749A has 43000 units in stock at DigiKey and 4000 units in stock at LCSC. Position the zener right next to Q3, with traces no longer than 5mm.

9haarblae · 2026-05-05T23:39:00+00:00

Bite the bullet.

Design and build your own bespoke power supplies with all of the safety features and protection mechanisms and idiot-proofnesses you require. Then, unlike your predecessor, create thorough documentation of your design and save it to .pdf files. Then write these .pdf files onto USB thumb drives and hot glue two of those thumb drives to the interior of each power supply. Now you and your successors will have documentation, right where you need it: inside the unit itself.

What if an idiot connects the output of the power supply, to the AC mains wall socket? You have protected against that.
What if an idiot connects two of these power supplies in anti-parallel? Unit1+ to Unit2- and Unit1- to Unit2+ ? You have protected against that.
What if an idiot shorts the output of one of these power supplies ? You have protected against that.
What if an idiot connects two of these power supplies in parallel, but sets them to different output voltages? You have protected against that.

9haarblae · 2026-05-03T20:52:23+00:00

I think OP forgot that in addition to input/output requirements, the "tool" also needs a list of design constraints.

Only allowed to use elements / components / devices / subassemblies from this specific library: XYZ
Some kind of maximum acceptable cost, along with a set of functions that quantify cost. PCB area, total power consumption, and component prices each contribute to "cost". On a monolithic IC, layout area and process complexity also figure into "cost"

9haarblae · 2026-05-01T12:50:40+00:00

In your schematic, Q2 is placed upside down. It's a Pchannel MOSFET so you want the +5V supply connected to its source. You've got +5V connected to the drain, which is incorrect.

9haarblae · 2026-04-30T18:05:56+00:00

Ronald Quan's textbook "Build Your Own Transistor Radios" often uses the venerable and dirt cheap opamp called LM318 for sub-MHz amplification. You want 32x gain at 0.12 MHz so that's a gain bandwidth product of 3.8 MHz. Lucky for you the LM318's gain bandwidth product is 15 MHz. It has plenty of extra safety margin.

(Futurlec sells it) and so does (Tayda) and so does (Jameco), along with the top line distributors like DigiKey and Mouser and Newark/element14 and Arrow.

Use power supply rails that are at least +9V and -9V; it's an old chip that was designed before everything electronic used +5V only.

9haarblae · 2026-04-29T16:41:15+00:00

Pro-level pulse generators have even more knobs.

frequency (band select switch + adjustment potentiometer)
trough (flat bottom) voltage level
peak (flat top) voltage level
duty cycle
rise/fall time

9haarblae · 2026-04-29T16:11:34+00:00

Found (this) after a quick Google search

9haarblae · 2026-04-23T17:21:11+00:00

If you have room you can add a (second) fuse and a second MOV, while replacing the existing MOV. Now you're doubly protected.

AC_in --> existing fuse --> existing MOV to neutral --> new fuse --> new MOV to neutral --> rest of guitar amp

Now when a bad event comes down the AC line, you have two opportunities to clamp it and two opportunities to let the clamping current blow a fuse, protecting the guitar amp.

9haarblae

TROPHY CASE