transistors - Page 1
Forum section: electronics / beginners

author: queennikki1972
author_rank: Regular Contributor
timestamp: August 14, 2019, 02:28:33 pm
content: Nothing I build with the small black silicone based transistors like the BC547 seem to work. I need a book or something to understand them better. I google the direction of the pin out for emitter base and collector, but none of them seem to work. Are they more heat senstive than say capictors and resistors..? For Reference I purchased bulk from china if that matters. I have "Getting Started in Electronics by Forrest Mims" and I plan on buying "The Art of Electronics" as well.

author: kosine
author_rank: Regular Contributor
timestamp: August 14, 2019, 03:08:14 pm
content: As well as books, I'd suggest having a play with LTspice. You can very quickly rig up some simple BC547 circuits and see how they respond.I wouldn't waste time building and measuring circuits on a breadboard to begin with. Learn using spice, test the results later on.It may also help if you go and learn about Gummel plots:http://www.eng.auburn.edu/~niuguof/multisimdev/_images/grapher_gummel.pngIt's basically two logarithmic current plots. The lower one is the base current, the upper is the collector-emitter current. Pick any base voltage and the plot shows the corresponding currents. The collector-emitter current will flow through anything connected to the collector or emitter (such as resistors, LEDs etc.). From that you can then figure out the collector and emitter voltages.Try some simple LTSpice simulations and you should see that the voltage across the transistor is whatever it needs to be to "take up the slack", so to speak.

author: dcbrown73
author_rank: Regular Contributor
timestamp: August 14, 2019, 04:11:00 pm
content: Here is aquick tutorial on the BC547that explains how it works and example circuit.Good luck!

author: magic
author_rank: Super Contributor
timestamp: August 14, 2019, 05:27:37 pm
content: Freewheeling diode antiparallel with Q1

author: mikerj
author_rank: Super Contributor
timestamp: August 14, 2019, 05:40:35 pm
content: Semiconductors in general tend to be more sensitive to heat damage than passive components, but you have to try fairly hard to damage a bipolar transistor in a leaded package like a BC547. That said you shouldn't need to apply the iron for more than a few seconds per lead to make a solder joint.Chinese components are often suspect, there is a vast market in counterfeit parts that either work poorly or not at all but they tend to target the more expensive parts. Jellybean transistors are so cheap to manufacture there would be little profit in counterfeiting them, but that's not to say it doesn't happen.What circuit have you been trying to construct, do you have a schematic?

author: MarkF
author_rank: Super Contributor
timestamp: August 14, 2019, 06:08:07 pm
content: This my help (NPN Amplifier) :https://www.electronics-tutorials.ws/transistor/tran_2.htmlPinout:

author: ledtester
author_rank: Super Contributor
timestamp: August 14, 2019, 06:13:14 pm
content: What circuits don't work for you?Note that the metronome and light flashers circuits on page 104 of Getting Started in Electronics require an NPN and a PNP transistor.

author: queennikki1972
author_rank: Regular Contributor
timestamp: August 14, 2019, 11:53:02 pm
content: I will try to the circuits again in Getting Started in Electronics..Thank you everyone..

author: Circlotron
author_rank: Super Contributor
timestamp: August 15, 2019, 12:51:05 am
content: Quote from: magic on August 14, 2019, 05:27:37 pmYeah...But if the inductive motor load is in the emitter circuit it doesn't need any clamp diode for over voltage protection. When the transistor turns off, the motor will try and pull the emitter below earth (base) potential, partially turning the transistor back on and clamping the emitter voltage to 0.6V or so below base voltage. Does dissipate a bit in the transistor though, so a reverse (preferably schottky) diode across the motor is the proper way to do it.

author: Audioguru again
author_rank: Regular Contributor
timestamp: August 15, 2019, 01:28:01 am
content: Quote from: queennikki1972 on August 14, 2019, 02:28:33 pmParts from China are frequently counterfeit, fake or defective. Buy name-brand parts from a local genuine electronic parts distributor like Digikey.

author: ledtester
author_rank: Super Contributor
timestamp: August 15, 2019, 01:42:03 am
content: Here's how to perform a "sanity check" on your transistors with the diode test function of your multimeter:https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07One of the "$20 component testers" (which you can get now for ~ $10) is also a convenient tool to have. Search ebay/aliexpress for "component tester".Even the much poo-pooed "hfe transistor tester" port found on many cheap multimeters is useful:When the transistor is good and plugged in correctly you should get a number >50 for a small signal transistor like the BC547.

author: magic
author_rank: Super Contributor
timestamp: August 15, 2019, 05:09:56 am
content: Quote from: Circlotron on August 15, 2019, 12:51:05 amYeah, the 10k resistor to ground saves the day when emitter falls below Ic/β*10kΩ, a few V negative. Actually, it could be a clever way to quickly brake the motor by applying maximum reverse voltage the transistor can withstand if 10k is replaced with higher resistance or simply a 30V zener. What's the pulsed SOA of BC547?

author: Jwillis
author_rank: Super Contributor
timestamp: August 15, 2019, 05:24:12 am
content: Transistors functionality is dependent on application.What are you using it for ? What voltages are applied and how much current is pass through the transistor. For example your BC547 has a maximum voltage rating of 50V and a maximum current that can pass through ("Drawn by the load ") is 100mA.The Hfe (AKA beta) is the gain of the transistor .Many factors can alter the gain but for simplification yours has a "Gain" of 110 to 800. Which is to say that the current passing from collector to emitter is 110 to 800 times the current at the base. Sometimes there will be a letter following the BC547 like "A" ,"B" or "C" This is the hfe classification .This means for example that a BC547A will have a hfe "Gain" of between 110 ~ 220.So in "Common Collector" mode if 1 mA is applied to the base then the transistor will pass current across the collector to the emitter between 110ma - 220 ma which would ,of course, exceed its maximum current capability.As you can see it doesn't take much current at the base to drive current across the collector and emitter. So it won't take much to burn out a transistor quickly.You also need to remember that the current at the emitter is the current at the collector Plus the current at the base . An easy solution is to place a resistor at the base that limits the current to the base.So as in Ohms law Voltage divided by the Resistance equals the current that will pass across the resistor to the base of the transistor.You have to consider that current requires a Load.Load "draws" current.And the transistor can also be considered a load at the base.So by using a limiting resistor, the base will only "draw" as much as the resistor will allow.So if your supply is 10 volts and you place a resistor of say 100K ohms at the base then there will be 0.1 mA drawn across the resistor to the base. So 0.1mA times a "gain" of 110 equals 11mA from the collector. And 0.1mA from the base plus 11mA from the collector equals 11.1 ma at the emitter. No matter how much current is available from your power supply that transistor will only pass 11.1mA.Other modes include Common Base and Common Emitter which have different functions .Playing with transistors can be very interesting. Even blowing them up is educational because you can see limitations and what can happen when limitations are exceeded .Failure can be as informative as success.Just wear eye protection.Without getting in a conflict about components from Ebay .I know from experience that they're just fine and have always matched design parameters.The only time they fail is when I've done something wrong .Digikey is a good source as well ,but the shipping charges is what puts me off.Maybe it's better in the States I don't know.If your interested ,as ledtester mentioned,there are inexpensive transistor testers from Ebay available that are good starter tools for the novice .They will tell you what pins are what and the "gain" .Plus they will test other components such as diodes ,capacitors and resistors .Very handy tools.

author: Circlotron
author_rank: Super Contributor
timestamp: August 15, 2019, 05:57:26 am
content: Quote from: Jwillis on August 15, 2019, 05:24:12 amNot trying to be pedantic, but with a gain of 110-800 a *change* of base current causes a *change* of collector current that is 110-800 times as great as the base current change. Or was that allowed for in your simplification?

author: queennikki1972
author_rank: Regular Contributor
timestamp: August 15, 2019, 12:36:39 pm
content: So I tried the flashing light circuit on page 104 of Getting started in electronics. Unfortunately the only 100k variable resistor i had was used pull from old stuff and was a little flaky. It did flash properly for a minute so it must have been ok. I will order more parts and keep doing the same circuit until i figure out what im doing wrong. I will also try more expensive parts from digikey as well.I do have one of the component testers as well. And the parts test ok before using.

author: Jwillis
author_rank: Super Contributor
timestamp: August 15, 2019, 02:14:22 pm
content: Quote from: Circlotron on August 15, 2019, 05:57:26 amYour absolutely correct.Any knowledge is good knowledge.Thanks for pointing that out.Quote from: queennikki1972 on August 15, 2019, 12:36:39 pmIf your could provide that circuit diagram then someone here good walk you through it and mark the pins to where they go .Even a photo of a sketch is a great help.Some of us don't have that book.Electronics is not an exact science. All components will have a tolerance and as for resistors they can be as much as 5% out for ones with a gold band and 10% for silver band. Even the most common ones from Ebay Have a 1% tolerance .This means that the higher the rated Resistance the more deviation your going to get. For example a 100k 5% resistor can be as high as 5000 ohms out and a 100k 1% resistor can be as high as 1000 ohms out .This is not a problem with simple circuits .There are precision components available but are far to expensive and unnecessary unless you plan on launching a probe to some distant planet.Even Transistors are never perfect.Remember, there is no such thing as a stupid question.It's only stupid when you stop asking questions and think you know it all.

author: queennikki1972
author_rank: Regular Contributor
timestamp: August 15, 2019, 03:05:34 pm
content: For those needing it here the page and its the bottom right circuit.

author: queennikki1972
author_rank: Regular Contributor
timestamp: August 15, 2019, 03:10:22 pm
content: I assume you could use a red led with this circuit? Even though it calls for some odd ball #222 bulb

author: ledtester
author_rank: Super Contributor
timestamp: August 15, 2019, 03:27:28 pm
content: Quote from: queennikki1972 on August 15, 2019, 03:10:22 pmSure - as always, though, put some sort of current limiting resistor in line.

author: Audioguru again
author_rank: Regular Contributor
timestamp: August 15, 2019, 04:12:41 pm
content: In this thread, people have been talking about hFE and beta being over 100 times for a BC547, but the hFE and beta are only used when the transistor is a linear amplifier that always has lots of C to E voltage and not when it is used as a saturated switch. The flashing light circuit uses both transistors as saturated switches and the datasheet for most little transistors shows that the base current should be 1/10th to 1/20th of the collector current.

author: ledtester
author_rank: Super Contributor
timestamp: August 15, 2019, 04:51:42 pm
content: Quote from: queennikki1972 on August 15, 2019, 03:10:22 pmYou have to bear in mind that the book was written in the '80s and the circuit was probably developed by Forrest much early than that as he has been writing about electronics since the early '70s. At that time LEDs were much more expensive, less available and much less efficient -- i.e. not as bright. Indicator lamps were implemented with incandescent or neon bulbs. In this case the circuit is designed to put out a short bright flash, and I imagine he recommends a particular bulb (which has a certain voltage and current rating) to achieve that visual effect.

author: floobydust
author_rank: Super Contributor
timestamp: August 15, 2019, 05:52:58 pm
content: Quote from: queennikki1972 on August 15, 2019, 12:36:39 pmJust a note, there are two resistors missing in Forrest Mim's blinky circuit-it can destroy the transistorswhen you crank the potentiometer to lowest resistance.There is no resistor to limit base current to Q1 when the potentiometer is cranked, and this also can damage Q2 as there is no base resistor to limit its base current either when Q1 is full on.I would add two resistors, like 470R. One in series with the potentiometer that goes to +V, and another in series with the base of the PNP. In the old days, a 222 light bulb acted as a neat flasher lamp when overvoltaged. It is a 2.25V LAMP (two AA batteries) and on 6V or 9V is very bright, for a little while. It only was rated 5hrs lifetime.

author: Zero999
author_rank: Super Contributor
timestamp: August 15, 2019, 09:41:16 pm
content: Some things to note.If you think a potentiometer is bad and don't have a replacement, simply substitute it for a fixed resistor of half the value.If the circuit calls for an incandescent lamp, but you want to use an LED. Add a resistor in parallel, as well as the usual LED and series resistor. The circuit might be designed to work properly with the low resistance of an incandescent lamp and might not reliably work with an LED. In this case it will work, but the LED might not turn fully off, especially if it's a high efficiency green, white or blue one.Finally please do a little image processing on photographed schematics. It makes them clearer, as well as saving bandwidth.

author: Jwillis
author_rank: Super Contributor
timestamp: August 15, 2019, 10:40:56 pm
content: I tried your flasher circuit and all it did was cook the transistors. So I took the liberty of redesigning the circuit to one that I tested and indeed works and it won't cook the transistor.I added notes but feel free to make any changes you like for different effects.I was to lazy to diagnose the problem with the first circuit.

author: ledtester
author_rank: Super Contributor
timestamp: August 15, 2019, 11:31:18 pm
content: For clearer images a PDF of the book is available at:http://www.wbrc.in/ham/getting-started-in-electronics-forrest-m-mims-iii.pdf

author: ledtester
author_rank: Super Contributor
timestamp: August 16, 2019, 12:37:45 am
content: On the subject of transistors, when it comes to learning how to analyze circuits involving semiconductors, for the beginner/hobbyist I highly recommend either of these books - both by Albert Malvino:Transistor Circuit ApproximationsSemiconductor Approximations: An Introduction To Transistors and Integrated CircuitsUsed copies may be found on Amazon for cheap.They are very practical books with good worked examples. The Art of Electronics is excellent but kinda skimpy on worked examples - largely, I believe, because they expect that to be provided for by teaching assistants in a recitation class.

author: fourfathom
author_rank: Super Contributor
timestamp: August 16, 2019, 12:43:00 am
content: Quote from: Jwillis on August 15, 2019, 10:40:56 pmThis circuit has a problem too, in that when the NPN turns on there is no limit on the current from 9V+, through the LED, through the PNP emitter-base junction, through the NPN collector-emitter path, to ground. Perhaps there isn't enough base current into the NPN to drive things hard enough to cause problems, but I would like to seen some current limiting, probably at the PNP base.

author: floobydust
author_rank: Super Contributor
timestamp: August 16, 2019, 12:52:08 am
content: OMG dusted off my 1980 Engineer's Notebook by Forrest Mims III. It was a real hoot building all that stuff, never had so much fun. The lamp flasher was also our high school electronics class "strobe" light project, where we made our own pcb and cut sheet metal for the enclosure. I later turned evil and made it drive a car ignition coil...It looks like it has a few weaknesses, like needing a resistor to limit current through the potentiometer so it does not smoke, the electrolytic capacitor polarity might be arguably flipped, and I would add a second resistor to limit base current for Q2. This sch is the corrections I have in mind.

author: tooki
author_rank: Super Contributor
timestamp: August 16, 2019, 06:53:52 pm
content: Quote from: Zero999 on August 15, 2019, 09:41:16 pmYour example image is very hard to read, whereas the OP's scanned (not photographed!) original is easy to read. So no, please don't do this!

author: Zero999
author_rank: Super Contributor
timestamp: August 16, 2019, 08:14:35 pm
content: Quote from: tooki on August 16, 2019, 06:53:52 pmI admit it wasn't my best attempt, but I find the lack of contrast in the original difficult to read. More importantly I forgot to add the base resistor to Q2.

author: floobydust
author_rank: Super Contributor
timestamp: August 16, 2019, 10:22:55 pm
content: The flasher circuit is also on Wikipediahttps://en.wikipedia.org/wiki/Forrest_Mims#Author

author: james_s
author_rank: Super Contributor
timestamp: August 17, 2019, 12:36:05 am
content: I remember seeing a very similar flasher circuit in those warning lights they used to put on construction barricades. The old ones has an incandescent lamp and used a pair of 6V lantern batteries in parallel.

author: Jwillis
author_rank: Super Contributor
timestamp: August 17, 2019, 02:30:36 am
content: Quote from: fourfathom on August 16, 2019, 12:43:00 amThe current to the LED is limited by the 330-470 ohm resistor.Because the LED has a forward voltage of 1.7V the current it draws is I= (9-1.7)/330= 22ma ,Although that seems high the actual is lower because the battery is not at full potential (8.5V) . That puts the current at around 20mA . Since the LED is on a pulse ,short bursts of slightly higher current isn't hurting the LED .This has been measured and confirmed .The 470ohm resistor would be better limiting the LED to around 15.5mA .This is also the reason that no limiting resistor is require at the base of the PNP transistor.Because only 20mA is being drawn across the transistor the current at the base is very small.About 0.5mA has been measured at most.

author: floobydust
author_rank: Super Contributor
timestamp: August 17, 2019, 04:06:24 am
content: In the circuithttps://www.eevblog.com/forum/beginners/transistors-204336/msg2618643/#msg2618643, I think the off-delay potentiometer needs a current-limiting resistor and I still see a high-current path through the LED from base current (E-B junction) for Q2. If Q1 ever stays on, the LED is roasted.This construction barrier flashers I've seen were the incandescent light bulb with a bi-metallic blinky switch inside and a 6V lantern battery. Newer ones are LED and not as bright.

author: Jwillis
author_rank: Super Contributor
timestamp: August 17, 2019, 04:40:19 am
content: Quote from: floobydust on August 17, 2019, 04:06:24 amIt was a quick thrown together circuit but your right if the resistance gets to low on that pot it would cause the LED to fail .But as I mentioned any value less than 3Meg theirs no visually noticeable delay. So it may be better to have a 3Meg resistor and a 5Meg pot .If a faster flash is required then lowering the value of the cap would do that. I had it oscillating with a 470pF chiclet. Not noticeable to the eye but just wanted to see what was going happening on the scope.Few extra resistors here and there probably won't hurt performance .

author: fourfathom
author_rank: Super Contributor
timestamp: August 17, 2019, 05:13:48 am
content: Quote from: Jwillis on August 17, 2019, 02:30:36 amIt's not the current through the LED that I was commenting on, but the current through the PNP emitter-base junction and NPN collector-emitter path. When the PNP is in saturation, that base current has little to do with the collector current, but is limited only by how much current that NPN transistor is pulling. The NPN transistor also has no current limit other than the base current multiplied by the current gain since there are no resistors in the PNP/NPN chain. Hey, it probably works fine in practice, but since we're critiquing circuits here I thought I would mention this issue.I used LTSpice to test this (with 2n3904 NPN and 2n3906 PNP), just to see how much of an issue this actually is. I had to increase the "off delay" resistor to 20M before it would oscillate, but it did oscillate. As expected, the NPN collector current is influenced by the "flash rate" resistor value, but setting that at 10K gives me 100mA spikes. That's a lot of base current for that PNP, but probably not damaging. That current is also flowing through the LED, since that 330 Ohm resistor is not in that path. Once the NPN comes out of saturation then the 330 resistor comes into play.I changed the "flash rate" resistor to 50k and the PNP base current spike was reduced to around 25mA. The LED current spiked up to 42 mA then stabilized at 16 mA during the flash.I added a 1K resistor between NPN collector and PNP base and then the currents were as you might expect: PNP base current 4 mA, and LED current 16-21 mA.No big deal, I just prefer to not have these type of currents dependent on transistor parameters.Since I can't figure out how to embed your schematic, here's a link to it:https://www.eevblog.com/forum/beginners/transistors-204336/?action=dlattach;attach=811830;image

author: Jwillis
author_rank: Super Contributor
timestamp: August 17, 2019, 08:12:50 am
content: Quote from: fourfathom on August 17, 2019, 05:13:48 amWhen the LED is off only 0.6 volts is passing from emitter to base .During the off state the capacitor is charging. When the capacitor discharges a, negative current flows from the base of the PNP via the NPN .That current is passing through 100K resistor so at the base of the NPN the current is only 0.09mA. thats like 18ma at the base of the PNP.When the base of the PNP goes negative it switches on and most of the current through the LED is also passing through the emitter and collector which lights the LED .Very little current ever passes through the base of the PNP transistor.

author: AVGresponding
author_rank: Super Contributor
timestamp: August 17, 2019, 08:51:38 am
content: Quote from: queennikki1972 on August 15, 2019, 12:36:39 pmThere's nothing unfortunate about salvaged parts! It's a great way of building a nice collection of general purpose components, some of which can be horribly expensive if purchased new.Parts recovered from decent quality dead equipment can often be more reliable than new parts from those cheap ebay Chinese suppliers.If you're really lucky, you'll encounter the kind of dumpsters Dave Jones seems to find on a regular basis xD

author: Psi
author_rank: Super Contributor
timestamp: August 17, 2019, 09:31:57 am
content: When experimenting/learning to use transistors, make sure you have a good quantity of them.They are cheap, so you can get like 100 for $1 on aliexpress, and even on digikey they are cheap.You want to avoid the annoyance of giving up after 2 hours trying to get one transistor to work only to find you accidentally blew it up 1.9 hours ago.So when in doubt, grab a new one

author: fourfathom
author_rank: Super Contributor
timestamp: August 17, 2019, 02:11:14 pm
content: Quote from: Jwillis on August 17, 2019, 08:12:50 amI do understand how this relaxation oscillator works. That 100k resistor is a variable one so I figured that 10K would be a reasonable minimum value. If we limit the range to 50K and up then I agree that the with the likely range of NPN gain the PNP base current (and LED current) won't be too bad. I still don't like relying on NPN gain specs to limit this current, but perhaps that's just excess paranoia on my part.

author: tooki
author_rank: Super Contributor
timestamp: August 17, 2019, 02:30:22 pm
content: Quote from: Zero999 on August 16, 2019, 08:14:35 pmThat’s awful, too. The OP’s scan isn’t perfect, but there has to be a happy medium between it and the mock faxes you create. Crushing the image to 1-bit black and white produces maximum contrast but low detail with tons of jaggies that make reading much harder.

author: Zero999
author_rank: Super Contributor
timestamp: August 17, 2019, 07:04:52 pm
content: I disagree. I find high contrast much easier much easier to read, than fuzzy greyscale.Quote from: queennikki1972 on August 14, 2019, 11:53:02 pmBeware, those books contain numerous errors in the schematics. The author, Forrest Mims, doesn't have a formal training in electronics. Don't get me wrong, I found his books informative and he has some great ideas, but his lack of education does show. It's a shame they didn't get someone with more experience to proofread the books before they were printed.

author: Jwillis
author_rank: Super Contributor
timestamp: August 17, 2019, 09:19:41 pm
content: Quote from: fourfathom on August 17, 2019, 02:11:14 pm4 modes to any BJT .Cut off ,saturation,active and reverse .NPN PNPVE < VB < VC Active Forward (current flows from C to E proportional to B current) Active Reverse (current flows C to E proportional to B current)VE < VB > VC Saturation (Current flows freely from C to E) Cutoff (No current flows from E to C )VE > VB < VC Cutoff (No current flows from C to E) Saturation (Current flows freely from E to C)VE > VB > VC Active Reverse(Current flows from E to C proportional to B current) Active Forward(current flows from E to C proportional to B current)The 10meg resistor provides bias to the NPN .The current and voltage is so low that the PNP never goes into saturation. The Diode limits feedback to the NPN Base .While the capacitor is charging The PNP is in cutoff mode because VE < VB > VC .The PNP must go Active Forward to light the LED( VE > VB > VC).I've never been very good at writing things to make them understandable.This explains all those low marks in English composition.All I can say is try the circuit and put t to the test.

author: fourfathom
author_rank: Super Contributor
timestamp: August 17, 2019, 10:05:36 pm
content: Quote from: Jwillis on August 17, 2019, 09:19:41 pmThat "Flash Rate" resistor also provides bias (or current) to the NPN when the PNP begins to conduct. This is part of the positive feedback loop. The diode clamps the DC voltage at the NPN base (clamps at about -0.6V), protecting the base from reverse breakdown and ensuring that the base drive (from the "Flash Rate" resistor) will be adequately positive. Without that clamp diode the NPN base-emitter junction would clamp and the capacitor C1 would charge negative, limiting the NPN base current. The diode actually enhances the feedback to the NPN base. By the way, I see the PNP going heavily into saturation -- about 0.02V VCE at the start of each flash.I suggest that you simulate it to check the transistor and LED currents. I had mentioned that with my sim I needed to increase the "Off Delay" resistor to 20M in order for the circuit to oscillate. Not today though -- even 5M works. I double-checked that PNP base current again -- still quite high with the 10K "Flash Rate" resistor.

author: mikerj
author_rank: Super Contributor
timestamp: August 17, 2019, 10:06:19 pm
content: Quote from: Jwillis on August 17, 2019, 09:19:41 pmThis is an oscillator, it has positive feedback. Think about what happens to the NPN base current when the PNP transistor switches on. Then consider the resulting current flow through the LED and the BE junction of the PNP.I'm sure the current averaged over the entire on time of the LED is reasonable, but for lower values of the 10k pot the peak current will be much higher than you are trying to set with the 470R resistor.

author: james_s
author_rank: Super Contributor
timestamp: August 17, 2019, 10:58:31 pm
content: Quote from: floobydust on August 17, 2019, 04:06:24 amI've never seen one with a bimetallic self flashing bulb, maybe the really old ones worked that way but all the old ones I've seen used a little circuit with 2 transistors to do the flashing, they also had a CdS cell to switch them off during the day.Here are two such circuits I found from someone who reverse engineered a few of these lights. These are tested working circuits from commercial products so if one wanted to build such a flasher one of these is probably a good starting point.

author: floobydust
author_rank: Super Contributor
timestamp: August 19, 2019, 07:03:35 am
content: It's mostly about a beginner's circuit that mysteriously stops working due to a turn of the potentiometer.That kind of thing annoys me because when you are learning electronics, you already have enough things that can go wrong. It's not easy sometimes. The circuit operation is not explained anywhere, it's just handed to you and when it doesn't work, it can frustrate an experimenter.In those two construction barrier circuits, the tantalum capacitor sees reverse-voltage pulses, and steady reverse voltage during daylight, around -500mV.I was thinking the tantalum cap would fail. But it looks like that is possibly OKAVX tech pdffor up to 3-10% of rated voltage, maybe a volt, but failure rates are not known.I found these flashing light bulbs realizing most people have never seen them before. Some so old they are made in USA and Holland.Trying the KonMari method fromMarie Kondoto clean up my electronics parts bins, I think they are only good for artistic value, like a toy robot nose or something.

author: soldar
author_rank: Super Contributor
timestamp: August 19, 2019, 09:00:13 am
content: Quote from: Zero999 on August 17, 2019, 07:04:52 pmI agree. Please show some consideration to your fellow forum users and use the least bandwidth possible. My recommendations:Use .jpg only for photos and use .png for line drawings.Post the smallest photo, in pixels and in file size, that shows what you want us to see. Do not post a 5 Mbyte file when a 50 KByte file will do just the same. Some of us have limited bandwidth. Learn to edit, process and compress.Line drawings can be made very clear and very small in file size. I am attaching one just as an example. 6KB. No need to use 60 KB or 600 KB or 6 MB.

author: tooki
author_rank: Super Contributor
timestamp: August 19, 2019, 11:12:36 am
content: Yes, but it has to be done well. The lower the resolution, the more critical the preparation becomes, and frankly, for scanned things, it needs to be quite detailed, lest thin lines and letter detail go missing, exactly as in the first example.

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2019, 01:01:30 pm
content: It's subjective to an extent. People differ in what they find clearest and easiest to read. Some like as higher contrast as possible, others prefer different coloured backgrounds.