Schmitt trigger circuit using NPN Bipolar Junction transistors BJTs by electronzap electronics

#schmitt #2N3904 #electronics

[NELSONTHEBEST]

That's great! This schematic of yours is the solution for applicattions in which we just don't wanna a LED turning on (or off) partially, by emitting intermediary brights - as it could be used, for example, in a profissional bargraph V.U., whose LEDs would light 100% only! So, it's easy to understand that we can use a CMOS 4093 (in instead of eight NPN transistors) to drive four independent LEDs, for this purpose.

[ted_van_loon]

I love hysteresis in electronics, how high of a resistor can you use at the ground side before it starts to give problems(to get a lot of hysteresis). and can you directly wire some point of it to a IC input, or how would you do that. since many IC's do have CMOS but no Schmitt triggers which cause input from analog or physical(button for example) sources to cause more than one trigger.

[ted_van_loon]

Hi electrozapdotcom, I looked at the design and decided to throw it in a mental simulation before building it physically(as part of a project which needed hysteresis). and I found a way to improve the design you showed since this design only has a very small offset and might fail with other types of transistors(for example mosfets) if the resistors are not exactly equal or wrong in the right way. while simulating it in my head I found that to make it work better and more stable(with a large offset) you should increase the resistance of the top left resistor in comparison to the top right one.
for example if the bottom resistor is 1K the top left one 2K and the top right one 1K then when we assume mosfets instead of transistors(as I will talk about later why(1)) then when the input is high there will be the left active voltage devider of 2 to 1 which on 6 volts would bring it down to 2V below the mosfets causing the input to need to go below 2V before it switches to the low state. when the input is low we have the right active voltage devider of 1 to one, which on 6 volts would bring it down to 3v before it switches to high. so this scenario would give 1V of offset between the voltage to switch to high and low.

(1) I talked about mosfets since transistors will reduce the resistance on the right devider in this case this is because current will flow from Base to emiter as well instead of only using collector to emiter making it so that in the case where the resistance of base to emiter is really low that the resistance divider on the right side could be seen as having the 2 top resistors in parallel. this works well in most cases with many transistors, however some have a really high base emiter resistance, or even act as a capacitor, and the resistance often isn't as accurate in many transistors, a transistor can be made to act like a diode by adding a resistor between the left lane and the right transistor, however this ofcource often wouldn't really be needed since allowing it to do that can make the offset even bigger, it is just that if you use something like a mosfet or a high resistance base emiter transistor then the left resistor should be bigger than the right one because many resistors often are not exactly equal and accidentally having the right one having a higher resistance on a high resistance base emiter transistor, or having them equal on a mosfet based one can make it not work

[ukejejasper]

Thanks for this circuit. I built it and it works but I have a problem with not knowing which component to adjust in order to adjust the hysteris. Please suggest and thank you in anticipation.

[ingussilins]

Try made NTC resistor thermoregulator circuit.