I believe that if ever the switch is closed, then LED turns ON (because the of the right NPN transistor) and stays on (because of the left NPN transistor).
I’m wrong, this doesn’t work and I’m not super sure why.
Yes, a few.
The right hand transistor will not stay on because there is nothing to keep it on. You have probably destroyed it anyhow connecting the base directly to 5V without any current limiting resistor.
The left hand one will not come on because the base and emitter are at the same potential so can have no base current. The base will always be 0.6V above the emitter
A transistor is a CURRENT operated device NOT a VOLTAGE operated one like a Mosfet. The collector current is controlled by the base CURRENT and will usually remain at about 0.6VOLTS above the emitter when conducting. Because of this “self biasing” that fault finding on a direct coupled amplifier can be a nightmare. Believe me it is quicker and more economical to replace the lot in this situation.
I think I used to have a collection of transistor latch circuits laying around, I will see if I can find them tomorrow.
In the meantime I respectfully suggest you indulge in some reading on transistor operation. It is all applicable, after all most ICs have thousands of the little buggers inside. It is not all done with mirrors in a box with lots of legs.
Cheers Bob
PS: Re the circuit with the OR gate. To turn it off you would have to remove power or break the connection between the output and one input.
Yep I agree with that
I think what your saying is that my use of HIGH and LOW don’t make sense in when we are talking about current. Sure, I can see that it would be confusing. I should probably use CONNECTED and DISCONNECTED instead.
Ok that’s interesting. I think I understand that.
Suppose I got a 74LS32 quad OR Gate.
I claim that if I connected
A1 to a switch.
Y1 into B1
Then if I tuned that switch ON then Y1 would Latch on.
I’m super confident of that because I ran that exact test before I started to experiment with transistors.
What is the 74LS32 doing makes this work?
edit
Found my answer. From the 74LS32 datasheet have a look at what a single gate looks like.
In my TAFE course many years ago, there was a student who built a basic logic system using just transistors, resistors and diodes; like the diagram you have provided. He then discovered TTL logic chips and built a basic computer system. Then it was microcontrollers and he developed an operating system for the Z80. He almost didn’t pass the course because his project was taking too long. Then it was programmable logic arrays using Altera and Zilink devices; this was back in the 1990’s. In the class he was looked on as a joke, but he really was quite brilliant when he set his mind to it. Considering a modern CPU can have billions of transistors building one from scratch can be quite a challenge.
Love that thread you provided.
I think what I meant is that my super simple but flawed P100 circuit won’t work.
I’m not so bold as to say NO circuit exists. I’m certain it’s possible.
For example, last night I was reading something about an NPN, PNP design and something called an SCR.
I think your thread mentioned those too. Thanks @Jeff105671
Oof. sound warning for not reinventing the wheel.
Thanks for sharing @James46717
final thoughts
This OR gate was a fun experiment.
In then end I’m going to stick to PLAN A which is my trusty SR Latch.
It’s been working flawlessly and having the inverted Q is really handy.
I seem to remember the circuits I have somewhere entail NPN/PNP combinations and are meant to be low current substitutes for SCRs which are sometimes hard to find in the low current low power versions. I think all entailed 2 buttons, one for ON and another for OFF.
Cheers Bob
Setup for scr similar to transistor. Gate current would need limiting as well the resistor required on the K cathode to ground would limit current to reduce the load on your supply. The output at K would be close to your 5V.
BC
Simple. Press button “ON”.
Mosfet will stay ON until button “OFF” is pressed to discharge the Gate/Source capacitor.
Even if power is removed and re-applied Mosfet will remain ON.
This condition will last hours, days and possibly weeks until that capacitor is discharged.
Note this capacitor is internal to the Mosfet. They all have it.
Cheers Bob
Hi Brian
Cannot agree. In this application the SCR is required to provide a short circuit to ground to operate a camera flash so any resistor would defeat the purpose.
The gate is a very high input resistance and the trigger pulse is a differentiated spike provided by the 10n/1k cap/resistor combination. No limiting resistor needed. Main current flow is a spike to ground via the 1k resistor
Cheers Bob
Robert, my apologies I was thinking along the lines that it was required to simulate ttl as such. If we are specifically wanting to flash a camera we could also use a simple opto coupler 2N24 from memory. Current in transistor out on.
In the early 70’s replaced a swag of old reed relays with new on the market opto.
I agree with what you offer.
BC
You appear to want a basic flip-flop. This is simple to build with 2 gates but you can build with 2 transistors - see Flip-flop (electronics) - Wikipedia.
I suggest you find a book on basic digital electronics, this is electronics 101.
I’ve been burned a few times using random diodes where they are not appropriate and realized I needed a very particular one.
Maybe I over learned that lesson. Good to know I can use the NPNs I have already and just supplement it with some PNP.
I’m working my way through some exercise from Art of Electronics.
Sometimes to test my understanding I try stuff things like this.
I never expect them to work, but when they fail I get this “Ah ha!” moment.
Understanding why I failed helps me internalize the problem; something I’ve just read in a book but didn’t really appreciate.
In this case the importance of applying a potential at the base (because IE = IB + IC)
Not quite right.
IE = IB + IC is correct but not quite for the reason I think you think (if you can follow that.
The important quantity is the CURRENT supplied to the base. If course you have to have some sort of potential for this to happen but it is the CURRENT that is important.
Without IB you can have NO IC so if IB = 0 then IC = 0 so the equation is still correct, IE = 0 + 0 which is of course 0.
All very confusing but with a transistor you must think in terms of current. Because the base / emitter junction can be likened to a forward biased diode the voltage measured here (potential) will ALWAYS be 0.6 to 0.7V.
When you have a transistor amplifier and apply a sine wave signal at the base it is always via a resistor so you end up supplying a sine wave current into the base thus controlling the collector current.
All very confusing and you need to read some detailed explanations to get a grasp of this as some understanding is very important.
Amplifiers using Op Amps operate the same. They are fed via a series resistor and you are pushing current into the input. With a simple inverting amplifier this is balanced by a current of the same value and opposite phase applied back from the output. The net result is that you have 2 out of phase currents at the inverting input and the net result is 0V (with the non inverting input referenced to ground). Kirschoff says that the sum of the currents into and out of a junction must equal Zero.
This gets confusing if trouble shooting such an amp with an oscilloscope or any other voltage measuring instrument. If you measure a sine wave at the series resistor input you might measure say 1V P-P. But measured at the inverting pin of the Op Amp you will measure 0V yet the amp is working. This is because the inverting pin is at virtual ground.
I trust you are confused enough with all of this to go back and do some more reading and you haven’t started on electrons and holes yet.
But don’t worry. You really don’t need to get down to electrons and holes but the current into and out of a junction can be important, especially when you measure nothing when you expect to have something there. And the reason for transistor “self biasing” is pretty important.
Keep going, you are doing OK.
Cheers Bob
