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help with transistors Answered

could someone tell me if this will succeed in keeping itself on if the button is hit once?



11 years ago

This may work, simply, much like like an SCR.


This is tested, tweaked, and now it works. Needed 10K resistors to help it.


for everyone reading, the above shematic is the correct one, and will work under slight variations fro mthe original schematic.

ok I'll try this one, I'm quite sure it will work (since you tested it).

WOOT IT WORKS! *E-hug* tytytytyty except the off switch turns it on and the on switch turns it off so I just reversed them heh. thanks again.

One is NPN, one is PNP, and I use (lots of) 2N3904 and 2N3906 which are typical small transistors about 1/2 watt. In UK the equivalents start with "BC" and in Japan, "2S", with different numbers found using a "cross-reference" book or search. If they are black plastic, and viewed from above with the flat side facing right, or from the bottom with flat facing right, the top pin is C, the middle B, and the bottom E. In schematics E has the arrow and B is straight from the middle. Various other transistors have pins in BCE or reverse order, you have to test them or look them up to know. The "diode" is an LED. I changed my idea; don't use the 1K resistor, but it might be necessary to use a 10K from there to the negative for the "off" button to work. BTW the one "transistor" circuits might work if you use an SCR or TRIAC which can be found in small lamp dimmers as the transistor. They don't "dim" DC, but they do latch on when you push a button in a DC circuit. They also handle lamps up to and over 100 watts.


11 years ago

Here is a pdf containing your basic two-transistor bistable circuits to implement a set/reset latch, a monostable (pulse) circuit, and an oscillator (which happens to be the same as my Instructable on PCB DRCs, although that's not specifically what it was about.)
Somewhat more complex but essentially similar circuits make up MANY a standard Integrated Circuit, from 555s to flipflops to registers and static RAM.

I'm after a circuit that once a MOMENTARY switch is hit, the thing will keep on untill I cut the power (hit a momentary off switch)

here is a site to test them out: Circuit Simulator

I found that the capacitor does need to be recharged all the time and it defeats the purpose...

You would need at least two transistors to create a flip-flop circuit to toggle the LED on and off.

could you explain what a flip-flop switch does and how to amke it? I'm really just interested in making a circuit (small, easy to make, so I can make multiples) that once a momentary switch is hit, it stays on untill I make it turn off.

but couldnt you just use an NPN and put the cap between the base and the +?

Unlikely, but possible. It depends on the voltage available, the specs of the transisitor, the current of the lamp and the values of the resisitors. In other words, If this were on a test, the answer would be "Not enough information to answer."

The following is your schematic, corrected for using an NPN transistor as a switch. Note the differences and follow the current to understand how it works. Normally, I would place power on the left. I tried to keep the layout similar to your's to allow comparison.

Picture 1.png

thanks, I'll try it out. I made my version last night and it didnt work, all it managed to do was have the Emitter give a boost and make the control voltage higher.

also, if this works, couldnt it be used as a VERY efficient RAM system for computers? I mean if they can fit 51 million transistors into a chip about 2x2 inches, surely it would make great RAM

Place a small capacitor between the base and collector (negative of the battery). To turn off, short the capacitor. BTW: This is actually how dynamic RAM works. Capacitor charged, is on, discharged is off. The problem is that the capacitor has to be refreshed regularly. Static (Nonvolatile) memory works by toggling a transistor switch.

now is the capacitor going to keep charged from the battery?? or do I have to like....charge it form somewhere?

it almost looks like it is always on. Im interested in this because I seem to always have to make latches outa relays... Im gunna make it and see if it worx 4 u.

thats the point, once it turns on, it stays on (great for memory. all you have to do to turn it off is cut the power, it turns off, and when power is re-applied, you need to hit the momentary switch again.

what's a transistor?