Need LED circuit

[zachtheterrible]

Can anyone provide me w/ the light flasher circuit that is in the back of the book "Getting started in electronics", by Forest M. Mimms, I think that's his name. I'm away from that book right now, and I really need that circuit.

BTW, is 100 milliamperes equal to .1 amp? If not, what does it equal?

 

[TillEulenspiegel]

Not mimms but this is a good link......
**broken link removed**
And yes 100mA=.1 A

 

[zachtheterrible]

Thx, but I need the circuit that has two transistors. I've seen two transistor circuits, but they have diodes in them, and as i mentioned, i'm away from home (where all my diodes are). I can't use any ic's. I'm going to make this circuit to test a capacitor actually.

 

[Phasor]

Is this what you are looking for?

 

[ljcox]

Phasor's circuit really needs diodes between the emitters and gnd in order to prevent the transistors going into avalanche breakdown during the off periods. This won't do any obvious damage to the transistors, but may degrade their gain.

Len

 

[Roff]

Phasor's circuit really needs diodes between the emitters and gnd in order to prevent the transistors going into avalanche breakdown during the off periods. This won't do any obvious damage to the transistors, but may degrade their gain.

Len

Another alternative is to use a supply voltage that is less than the B-E breakdown voltage. Five volts will work for some transistors (e.g. 2N3904).

 

[Nigel Goodwin]

Phasor's circuit really needs diodes between the emitters and gnd in order to prevent the transistors going into avalanche breakdown during the off periods. This won't do any obvious damage to the transistors, but may degrade their gain.

Transistors commonly go S/C under these conditions, it's crucial to include the diodes to protect the BE junction.

 

[ljcox]

[quote="Nigel Goodwin
Transistors commonly go S/C under these conditions, it's crucial to include the diodes to protect the BE junction.[/quote]
Nigel is right. In the multivibrator circuit shown, it is likely that there will be sufficient energy stored in the capacitors to damage the BE junction.

The transistors would not be damaged with small capacitors, but my understanding is that their gain would be reduced.

Len

 

[audioguru]

Simple multivibrator avalanche breakdown

Hi, guys,
I am glad to see that you understand about the problem with a silicon transistor's reversed-biased base-emitter junction at high voltage and high current.
Have you seen this simple inverter circuit's problems? It blows-up its coupling capacitors! Its discussion is here:
https://www.aaroncake.net/forum/top.../120V+inverter+again&Forum_Title=Power+Supply

 

[Roff]

Re: Simple multivibrator avalanche breakdown

Hi, guys,
I am glad to see that you understand about the problem with a silicon transistor's reversed-biased base-emitter junction at high voltage and high current.
Have you seen this simple inverter circuit's problems? It blows-up its coupling capacitors! Its discussion is here:
https://www.aaroncake.net/forum/top.../120V+inverter+again&Forum_Title=Power+Supply

Deja vù all over again. See

and
https://www.electro-tech-online.com/threads/digital-pulse-analysis.8418/
That piece of crap circuit has been pasted all over the web. Aaron Cake (or whatever his name is) should nuke it from his site, but it will still live forever.

 

[Nigel Goodwin]

[quote="Nigel Goodwin
Transistors commonly go S/C under these conditions, it's crucial to include the diodes to protect the BE junction.

Nigel is right. In the multivibrator circuit shown, it is likely that there will be sufficient energy stored in the capacitors to damage the BE junction.

The transistors would not be damaged with small capacitors, but my understanding is that their gain would be reduced.

Len[/quote]

The circuit showed 10uF, which I would expect to be high enough.

Back in about 1973, at technical college, we were given a project to build (the idea had been kicking about for a long time, but they had never had a class they thought was good enough). It was basically a very simple 'one armed bandit' type device, it used three 7 segment filament displays (back before LED's), using just the three horizontal segments.

Each display had a button underneath it, and pressing the button froze that display - to win you had to stop all three on the same line. A win or lose light lit up accordingly, fed from simple gates.

The project had to be built from only discrete componets, on plain matrix board (using pins), with all components and wiring visible on the top of the board.

The basic design was three astable multi-vibrators feeding three ring of three counters, with bistables used to stop the counters, and diode gates to fed the win/lose lamps.

Because of it's modular nature we were split into teams, each of which built a particular section - mine was the ring of three counters!.

Anyway, the reason for this long story - the team building the astables was run by a guy called Dave, who later became a lecturer at the same college. He didn't fit the diodes, and despite me telling him to, he refused, saying they weren't needed, and simply kept fitting transistors till he found some which managed to survive. From what I remember he had a pile of dead transistors, 30-40 of them!!!.

 

[Phasor]

Ok, point taken re B-E junction. This one also not mimms, but perhaps it will suffice? From 4QD site. They have quite a few other astable designs there, take a look if you're interested.

(I know it doesn't show a flashing light, but you could add it with not much modification)

 

[zachtheterrible]

Yeah, that one will do, thanx very much!! :lol:

 

[ljcox]

Nigel's story reminded me of an event while I was a student.

We had to calculate the sweep time of a Miller Integrator sweep circuit. I calculated 10 mS, but when I measured the one in the lab (built by the lab staff) the sweep was about 1 mS.

But when I looked at the waveform on a CRO in detail, I noticed that the slope was initially correct for a 10 mS sweep, but changed to a much steeper one.

The lab staff had inserted the transistor the wrong way so its collector and emitter were reversed. Although it worked in this mode (transistors will work in reverse but with a much lower gain), the change in slope occurred when the C-B (which was really the B-E junction) voltage reached the avalanche breakdown point of that junction (about 5 Volt) and this altered the sweep slope.

Since the curent was small, the transistor did not fail.

So we reversed the transistor and, hey presto, the sweep was now about 10 mS as calculated.

Len