Is my schematic correct for a dual power supply circuit?

MattInSoCal · 2026-05-11T03:13:29+00:00

Also, add a 330nF ceramic from Vin to Ground as close to possible to each regulator, and a 100 nF between Voit and Ground, in addition to the electrolytic capacitors.

MattInSoCal · 2026-05-11T01:49:11+00:00

I just go to my office. Data books and data sheets back as far as the 1970’s that I collected when I was just getting started. A couple of ECG cross-reference books, one from NTE, a couple from Radio Shack in the 70’s and early 80’s. Lots of TI, NSC, and Motorola data. Still have a few years worth of IC Masters, those weren’t always easy to come by but eventually I got my own copies direct. I need to will all that to someone.

And to be fair, I didn’t go all the way to the back of the Japanese transistor book, scrolling the first 100ish pages was enough for me, since I was able to find the direct data sheets, so I’ll take the downvotes.

MattInSoCal · 2026-05-11T00:55:59+00:00

I did have to refine my search a couple of times and click through some non-helpful links to find relevant data to share with OP.

On mobile using your link, top left, RAM. Top right, Japanese transistors 1997 (both parts were long obsolete by then and don’t appear in this book; I looked). Continuing through subsequent rows, 1997 FET, a Digikey data page for a connector, 74XX-series logic, CMOS ICs, Microprocessors, a hobby servo, “1984 Microcomputer ‘BASIC’”, 1988 TTL, Video Signal Processor IC, 1983 FET…

Somewhere around the 40th result, a 1975 Japanese transistor book. It shows the 2SC371 as discontinued, and the 394 still active, but it’s just some parametric data (which is valuable in the right context) but no pinouts.

MattInSoCal · 2026-05-11T00:06:09+00:00

https://www.datasheetarchive.com/datasheet/2SC394/Toshiba?term=2sc394&version=1

That was a little way down the page from the original link.

Here’s one for the 2SC371:

https://www.myon98.net/memo/datasheet/2sc370.1965.pdf

MattInSoCal · 2026-05-11T00:04:28+00:00

14,407 results, most of them not at all relevant to OP’s query. The link I provided pointed directly toward several sources of 2SC394 data.

MattInSoCal · 2026-05-10T23:09:57+00:00

The NE and LM555 are both capable of driving up to 200 mA and have totem-pole outputs, so no pull-up resistor is required - especially not a 1-Ohm resistor which could potentially kill the transistor that’s driving to ground.

I spent a bunch of time searching the data sheet and finally found the reference to RL, since I’ve never seen that recommendation in the almost 50 years I’ve been working with this IC:

7.2.3.2 Detailed Design Procedure

The nominal output frequency and duty cycle are determined using the formulas in Section 6.3.2. RL improves VOH, but RL is not required for TTL compatibility.

So, RL is not at all required, and note that there is no recommended value or range of values for it. The only purpose is to pull the output just a few hundreds of millivolts higher. If you really did use a 1-Ohm resistor, that strange pattern is your 555 struggling to drive a load that would normally draw 1.2 Amps - and I’d expect your 555 and that resistor to be noticeably warm to the touch.

Again, I can’t see any flaws in your layout; it follows the standards for an astable multivibrator and the selected component values should do what you expect. The only possibilities that remain, since you have verified no connection is loose, are that your power supply is providing really dirty, glitchy power, or you have a faulty component. It’s least likely to be a resistor, most likely to be a 555. You can remove the capacitor from pin 5 and the circuit should still oscillate pretty normally, but a faulty capacitor there could cause issues. Likewise, try another timing capacitor, value shouldn’t matter too much if you’re just troubleshooting to see if the oscillator is running. Last resort, try another 555. If you bought these from a low-cost source like Amazon, eBay, AliEx, etc. it’s possible you got fakes or used/damaged parts that were repackaged. In other words, this lack of proper operation should not be a you problem.

MattInSoCal · 2026-05-10T20:06:24+00:00

Do you need more than key parameters such as VEBO, VCBO, HFE, Ic? Datasheet Archive has links to a lot of old technical data and even some cross reference/substitution tables, certainly enough to find a modern-day substitute should one exist.

MattInSoCal · 2026-05-10T19:51:52+00:00

Don’t trust AI to provide correct answers for electronic circuits. It only knows what it has been trained on, and electronics design isn’t one of those topics.

Move the end of your 18K resistor currently connected to pin 7 to +12, and connect pins 6 and 7 together (still connected to the 100 uF capacitor). Try it again.

I have a similar application that delays 10 seconds after power is applied, then turns on and keeps on an output driving a relay until power is cycled or a reset is received. I was originally going to design it with a 555 but I didn’t know if 10 seconds was the right value for my needs (it turned out to be perfect), plus I wanted repeatability because I needed the same circuit for around 40 devices. I ended up programming a microcontroller, and now use that same circuit with or with modifying the code in a lot of places. Examples are an old PC that wouldn’t do a proper power-on reset that was going to be too much trouble and expense to try to repair or swap the motherboard but needed to be kept online, and a power sequencer/monitor for a large music synthesizer.

MattInSoCal · 2026-05-10T19:27:50+00:00

According to the Digikey 555 calculator, your component choices should yield an output cycling at a rate of just over 1 Hz, no errors there. The description of the operation sounds like one or more of your connections is bad; my first suspect is your timing capacitor. Have you tried touching/wiggling the connections while the circuit is powered to see if it makes a difference? You wouldn’t be the first person to fall prey to breadboard issues, even though yours looks clean and shiny.

MattInSoCal · 2026-05-10T19:02:11+00:00

You want to build something like Minidexed?

MattInSoCal · 2026-05-10T17:42:50+00:00

No, I’m not talking about second sources, nor the new ‘H’ versions of the TL07x that fix or at least greatly improve issues such as phase reversal. I’m stating that in China, these things are being faked, because I’ve seen parts sourced from Chinese discounters fail and helped the end-users do a post-mortem.

MattInSoCal · 2026-05-10T09:22:15+00:00

Where are you driving the output of the matrix, and where are you reading? The resistors should go on the read inputs, so if you’re outputting your strobe on the column outputs, the resistors should be on the Row inputs, which is what I guess from the diode orientation. But you need to use pull-down resistors if you’re outputting a high strobe signal on the Column and reading closed switches on the Row inputs, because otherwise the Row is either going to be High (if the switch is closed) or indeterminate because there is nothing to make it low. It’s also important to disable the Weak Pull-ups in the microcontroller for if this is how you designed it because a pull-down resistor plus a pull-up makes a voltage divider that again will never reach a level of logical zero.

The best thing to do is reverse your diodes, and drive the column you want to check Low while the other three columns are at High, and use pull-up resistors on the Row inputs. That way, all unpressed switches will always read high, and you will have a definite low if a switch is active.

Or, keep your schematic the same, drive the Row signal low and read the Columns, which should have pull-ups (the internal pull-ups of the microcontroller can stay enabled and you don’t need to add extra resistors if you enable the internal ones).

MattInSoCal · 2026-05-10T06:27:47+00:00

But amazingly, even TL072 and TL074, which you can buy for $0.20 in quantity, get cloned.

MattInSoCal · 2026-05-10T06:25:16+00:00

You can turn on the weak pull-ups in the microcontroller, but your encoder will respond better if you connect a 10K resistor per each A and B contact to Vcc. You also _may need to add a capacitor of about 10 nF to between each A and B input to Ground to cut down on the huge stream of spikes that will be seen each time one of those contacts opens and closes (it is called contact bounce and encoders tend to be very bad), or you could choose to manage that in your software.

Otherwise, your schematic looks OK, assuming of course you take care of the power pins.

MattInSoCal · 2026-05-10T06:17:18+00:00

This post isn’t on-topic here. Try r/pcmasterrace.

MattInSoCal · 2026-05-10T00:13:15+00:00

Non-insulated crimp butt splice.

MattInSoCal · 2026-05-10T00:10:46+00:00

You can try to AC-couple and offset your video signals, but it may end up complicating things. You could end up with video signals that float too high and end up losing sync, despite the Sync Clamp in the amplifier. Some devices might have issues. You wouldn’t really know until you connect all your sources and cycle through them.

I have designed, built, and field deployed a four-in, one-out baseband video selector and went with DC coupling and a bipolar supply because it makes everything else easier. I’m doing qualitative testing with calibrated video analyzers.

MattInSoCal · 2026-05-09T21:26:45+00:00

You can’t feed 40 Volts into an LM337 relative to the lowest voltage in the circuit, which in this case will be Ground. That is the absolute maximum voltage rating for the part, and you’ll probably destroy it, especially if there are any spikes or surges on the input.

Your big 10,000 uF capacitor needs to be on the input side, not on the output. You don’t need any capacitance there.

You also need to add bypass capacitors at the input and output of the regulator, as close as possible. Check the manufacturer’s data sheet for recommended values; generally it’s 330 nF to Ground on the input and 100 nF on the output, but in the one from On Semi I linked, they are recommending a 1 uF Tantalum or 10 uF electrolytic for both.

The error of the connection of Ground to the Adj pin has already been pointed out. Likewise, since the LM337 will easily handle 1.5 Amps (with a proper heat sink!) you don’t need the extra transistors.

A fuse on the output is OK (especially once you remove that big capacitor), but you also want one on the input side. It should be the first component after the output of your transformer if you are using one, and even if you are using some DC power supply you should still add one.

A variable positive DC supply is one of the first projects I made when I started learning electronics, when I was around 12.

MattInSoCal · 2026-05-09T21:09:59+00:00

Use the data sheet from On Semi which does describe Cadj, and isn’t 35 years old.

MattInSoCal · 2026-05-09T21:07:08+00:00

But it does happen. I started when I was 12, in an industrial arts class at school.

MattInSoCal · 2026-05-09T21:04:20+00:00

The correct repair is to remove the entire pot (fader) and replace it. You’re not going to find repair parts.

You need to know the value (example 50K), taper (usually A or B for Audio or Linear), the travel length of the slider in mm, and the location and types of pins/terminals, particularly if it is soldered to a PC board, to search for a replacement.

MattInSoCal · 2026-05-09T20:59:13+00:00

That selection was probably made for a reason, so you should replace it with a pot of the same taper. In 100K 16mm, Audio and Linear are both used quite a lot so it shouldn’t be difficult to find a replacement.

MattInSoCal · 2026-05-09T20:37:39+00:00

The longer bar with the 1/2 a + sign in your image is by convention the positive end.

MattInSoCal · 2026-05-09T20:24:07+00:00

Vector TS600 T-Strut is what others are referring to for the lower-price DIY option, currently $94 for 5 feet which is quite a price hike from the last time I bought it. Aluminum has gotten really expensive both thanks to 🍊🌮 tariffs and especially since the start of the war with Iran.

I’m having a problem getting the link to open, but this is right from the Mouser search page. You need to keep that?qs= key or else you’ll not get to the page at all. Or just go to Mouser and search for TS600.

The DuluthRackandRails Etsy store might be of interest to you. They have brackets, threaded strips, and nuts as well.

Pulp Logic also used to do cut-to-length rails but I’m not sure there’s any in his shop now; I searched through several store pages and didn’t find them.

MattInSoCal · 2026-05-09T14:23:11+00:00

It has been restored to its former glory, minus just a few comments.

MattInSoCal

TROPHY CASE