Missing pulse detector + latching on switch puzzle

[brainwash]

I have a new puzzle - a missing pulse detector with latching on action.

Some background: the circuit should be normally off until the user presses a button.
Requirement in pseudo-code:

Code: [Select]

if user-press
   wait{     
   }until input has been HIGH for two seconds

   output LOW

   wait{     
   }for one second
end if
[default to high output state]
Requirement: the circuit drives the output LOW only if the input has been HIGH for at least two seconds, otherwise the output stays high because it is being pulled up. The output should be driven low only for not more than two seconds.

I've only gotten the missing pulse part going, using one transistor, one diode, one capacitor and a resistor. I'm working on getting the switch-on and auto-shutoff part working but I thought you might have some ideas.

Yeah, I'm trying to avoid 555 and smart logic, it should only be based on BJT, diodes, resistors, capacitors, mosfets, as few as possible.

[fcb]

whats the question? looks like you've created an elegant solution already.

[brainwash]

That was just the solution middle part of the problem, detecting the missing pulse and triggering on it. Since it's reversed logic it's much easier to handle.
The full problem was to start this circuit only on key press (a second input, driven low) and to also have it shut off after one second or so of being activated.
I kind of achieved this goal with a few extra parts (a transistor, a diode, a capacitor, one resistor) but it is very sensitive to the duration and delay of the user input. I'm going the uC route now, it was only an exercise. If anyone is curios to take over I can post the new simulation values and schematic.

The goal was the following: the simple coffee machine I described in another post has a limitation that I have seen on a lot of these 'dumbed' devices. You can not start the coffee preparation unless the water is heated.
While the heater is on the LED is flashing once every two seconds (50%). After the boiler has reached the required temperature the LED stays on and you can press the coffee button. This process takes ~70 seconds at first start so it's really annoying if you want to make a coffee and then forget about it. You have to wait until the flashing is finished to prepare the poison.

Another use of the same LED is to show that it is out of water. In this case the led flashes at about 5 times per second until the water tank is refilled and replaced, after which it starts the heating process.

I wanted the simplest two or three transistor circuit that could accomplish this but I don't really think it is doable in a reliable way.


This is just warming up to tackle the big coffee machine - a Saeco Talea Giro - which I intend to enhance with some features and overrides. The complexity is a bit high though: 48 I/Os (digital, analog, capacitive), serial I/O, IrDa, huge flash memory and a lot of unknowns. It's probably easier to piggyback the existing board and override the controls.

[amspire]

Do a Google search for "transistor monostable". Basically you want a circuit that generates a fixed length pulse if the input voltage (the voltage on the capacitor) drops below the normal value. A 2 transistor monostable can probably do this.