I played around with this circuit using Crocodile Technology and couldnt
get
I have just set the circuit up again using Crocodile Technology. I have
posted a copy of the schematic in binaries.
Afraid I don't follow. How are you applying the trigger? Are you saying it
is below 0V? How? In what way do the NAND gates "not like
the negative voltage..."?
I am using a 10v input via a push to make switch to apply the trigger, and
using 10v supply voltage to the NAND gate. Unless undestructable components
are selected, the second NAND gate blows in the simulation with the message
"The pin voltage was taken below the negative supply voltage by 9.2604v. The
maximum rating is 1v". When a trigger voltage is applied to the first NAND -
there is a negative voltage on the right side on the cap.
What do you mean by 'substantial'? As you see, my trigger pulses were
around 100 ms. That was from a push button (connected from pin 2 to
0V, with a 10k resistor from pin 2 to Vcc). But that standard circuit
works with any -ve going pulse, from a few microsecs up.
When the input is made high, unless the input remains high for 2 seconds the
trigger isnt activated, after I a closer look, When the circuit is
stabalised (i.e powered up a while) the capacitor charges to show a 10v
charge across it. In the simulation I have used a 440cF cap, and 10k res.
When the input trigger goes high, the cap starts to discharges through the
resistor, when the input trigger goes low again the capacitor start to
charge again up to 10v. However the timer will only trigger the second NAND
gate once the input trigger goes high and the voltage across the capacitor
is drops to 4v.
If the input trigger is operated when the voltage across the cap is about 4v
the the trigger works ok. but if the cap is allow to charge to 10v - then
in takes another 2 second input to bring the voltage back down to 4v to
trigger the circuit. If the cap/resistor values are changed to give a 15min
timer, then the time needed to hold the trigger down would become more than
substantion..
I must be doing somthing wrong. can anyone see it?