Latch Logic using Transistors

Hi everyone,

I am trying to complete a circuit of mine, where I need a Latching circuit to keep my output latched once an some signal occurs at the input... And i can reset the system by resetting the supply of the circuit which is 24VDC.
I did some research and using some resources designed one for my purpose..
I did so using 2 transistors --- 1 NPN and 1 PNP
I am using an optocoupler TLP181 at the input just to keep isolation...
I tried doing this on Breadboard... it works almost all the times, though there are sometimes I come across some problems like
1) It automatically latches before providing input(problem solved)
2) It doesnt latch even after providing the correct input.

I need a solution for my second problem where it does not latch even after input being provided..
After providing signal to the base of the NPN, NPN turns on but it shud also turn on the PNP which it doesnot...
Please take a look at the circuit and do let me know where i am going wrong...

Thanks already!
(P.S.- not planning to use an SCR)

What is the "correct input"?
What is the purpose of C4, C6, and C7?
C6 and R15 are not needed.
Change R18 to 10k.
What load do you have on the output?

You say you're not planning to use a SCR but here
you are making one out of piece-parts.

Your NPN wants a DC base shunt.

crutschow said:

What is the "correct input"?
What is the purpose of C4, C6, and C7?
C6 and R15 are not needed.
Change R18 to 10k.
What load do you have on the output?

Click to expand...

Correct Input as in 24VDC which is supposed to be so...
i already tried doing changing the resistor to 10k... doesnt work
My output is a set of 8 leds and another one to the base of a transistor whose collector is connected to the pin 4 of a 555 timer and emitter to ground...thats bcs i need to turn off my timer when this latch signal is generated

Hi,

Is it possible to see the part of the schematic that comes after "latch" (LEDS and transistor + timer), please? Does the timer/the NPN(?) collector have a pull-up resistor? Without seeing it, probably nothing to do with it, have you tried to get it to latch a) only LEDs and b) only BJT + timer reset pin? - To see if there is a clash/wiring issue going on there.

d123 said:

Hi,

Is it possible to see the part of the schematic that comes after "latch" (LEDS and transistor + timer), please? Does the timer/the NPN(?) collector have a pull-up resistor? Without seeing it, probably nothing to do with it, have you tried to get it to latch a) only LEDs and b) only BJT + timer reset pin? - To see if there is a clash/wiring issue going on there.

Click to expand...

?

The opto-Triac, being a Thyristor, will latch on its own if fed from a DC supply and the output current is larger than its holding current limit.

You don't require anything beyond that, other than a buffer transistor to drive the load..