The chip looks like a 74LS221 (dual monostable). Look at the datasheet here.
You are right, It makes more sense. If we look at the logic table, B is kept HIGH, and when a transition to LOW occurs on A the output triggers. This is how it is supposed to work.Hi,
Possibly: 2B is high, until Q6 turns on and draws the voltage to ground which held 2B high. 2A is low (because it's input voltage is high) when Q6 is off. it looks like R45 charges C3 and maybe C23, until Q6 goes high and drains the capacitors? 2A doesn't look as though it is low all the time as it is connected to 12V via R26. If it's high going into the internal inverter all the time then it will be low at the output fo the internal inverter, I don't see how that would be useful because 2A and 2B feed an AND gate, so both would need to be high (internally) at the same time at at least some point for the AND gate to go high at some point. If it's always low going into the 2A inverter, and therefore internally high at the inverter's output, then that's a different story.
I would have thought F_1 were an output, but it could equally be something entirely different without being able to see what is connected to it.
I'm probably wrong about several parts of the interpretation above.
The circuit looks wrong, is R19 in the right place?
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the base of the transistor will hold 2A input at .6V ?
If you want a low, then ground the pin.
Is the circuit reversed engineered?
My guess is R19 is suppose to be between the base and the pull up R26.
Also I think Vcc is missing from R31|R44
Yeah, I have to reverse engineering this board.
I double checked again and actually the circuit is a bit different, here the scheme.
View attachment 139263
That transistor is a E B C, you swapped base and collector in your reverse engineering.
- - - Updated - - -
Sorry people for messing up.
Here the correct schema.
Now back to the initial question. Can we say that 2A (pin 9) is kept high and it waits for 2B to go low to trigger the output (1Q) ?
Can you guess if F_1 is an input or output?
Thank you again!
looking at the transistor I thought ztx was the manufacturer and 3704 the part number. Typing that separately on google the only component it finds is the 2N3704 so I thought, that is.
This one has inverted collector and base.
But if you type ztx3704 you can find very obsolete links of the pinout but no datasheet.
So it is my bad I didn't pay much attention, I hope this mistake was reasonable.
The circuit issues a rest pulse on power up after a short period, R45 C3 is the timing for that.
There is an external reset by putting a high on F1.
R53 and C2 dictate how long the rest pulse is.
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