Latch triggering

[Kevin Weddle]

I have this latch I want to trigger, but it is very sensitive. So I used a pull down resistor on the input. The input to the latch is the ouput of a gate. This seemed to pull down the voltage so that it would not be so sensitive to trigger with a short duration pulse. Do you think this actually works or am I just in the unknown region of the device? It seem to me that you need both voltage and duration to trigger a latch. If you want to catch a pulse, but not so often, then you can skimp on the voltage and it won't be prone to triggering as easily.

 

[Kevin Weddle]

I want to be certain that the latch is not in the unknown region when I try to trigger it. Can I skimp on the voltage to keep it from triggering?

 

[MP1]

Skimping on the voltage will put it in the unknown region. How have you handled switch bounce? Perhaps you are seeing some switch bounce when it seems very sensitive.

MP

 

[Kevin Weddle]

I don't think switch bounce is the problem. I can reset it and it still triggers because there is a pulse on the set. Does this sound like switch bounce? I am not really sure what switch bounce amounts to. I know that it is an irregularity in the pulse caused by the bouncing contact which would cause a ripple in the pulse.

 

[Kevin Weddle]

I have a pulse applied to a latch when I don't want one. How can I change the pulse so that it does not trigger the latch? Would a series resistor be better than a parallel resistor? The way logic works doesn't leave me with many options. What I need is a weak source and a weak sink. That sounds like a series resistor.

 

[audioguru2]

Kevin,
1) Why does the gate that drives the latch's "set" input have a pulse when you don't want one? The problem is with the inputs to the gate, not a latch that is too sensitive.
2) What is the duration of the unwanted "short duration" pulses?
3) Are you trying to decode the outputs of a ripple-counter, instead of using a synchronous counter?

 

[Kevin Weddle]

Yes, the pulse is of a very short duration. I found that a series resistor won't work because of how easy it is to produce a high input. All it takes, in logic, is to produce a no voltage situation since this will reverse the PN junction.

 

[audioguru2]

But what is causing that short-duration, unwanted pulse?
If we can help you eliminate that pulse, then your problem is solved.

 

[Kevin Weddle]

I found a good solution to my problem. I used a parallel resistor to pull down the voltage. Then I used a parallel capacitor to distort the pulse a little. This ensured that pulse did not have the duration because a capacitor tended to round the sharp edge. Now I intend to find the right capacitor and realize the purpose of the circuit.

 

[audioguru2]

Kevin,
What are the part numbers of the gate and latch?
What supply voltage are you using?
What is the value of your pull-down resistor?
Your pull-down resistor and parallel capacitor may be overloading the gate's output, or may be drawing an out-of-spec current that another gate can't duplicate (in the future if you replace this gate or build another circuit).

 

[venkat2k]

Another solution could be to use a diode in series instead of a capacitor in parallel to the the pull down resistor. The pull down resistor is connectod to the latch trigger and the cathode of a diode such as IN4148 or the like is used where the anode is the source for the trigger.

 

[Kevin Weddle]

A diode would reduce the voltage, but I don't have a low enough current diode. So I would still need a bias resistor to ground to make the diode operate.

 

[Kevin Weddle]

What determines the startup phase of an oscillator? I have the feeling that the startup is in phase with this other oscillator. Could it be that the oscillator startsup in phase with another oscillator just because they are in close proximity and have the same frequency?

 

[hotwaterwizard2]

Woosh there it goes. It just went over my head! ???

 

[MP1]

Kevin, the circuit starts at 0 potential, then the voltage rise of the first oscillation happens when the circuit is powered up. There is no phase control on an oscillator. It starts at zero then rises. This is the starting phase. If you are using multiple oscillators and they start up at different intervals, even within milliseconds, they will be out of phase with each other. If this is a problem, then you would need to add circuitry to your design for phase correction. It is easier to use one oscillator in most cases and use a divide circuit when you need different oscillators in the same circuit with the same phase.

MP

 

[Kevin Weddle]

I tried to start this oscillator by using a battery source. When I connected the battery, the oscillator seemed to be in phase with this other oscillator on the same circuit board. I don't know how exactly they were in phase, but it was close. Does this make much sense to anybody? The only way I know of to get them out of phase is to use an RC circuit to make sure of the phase difference.

 

[audioguru2]

If both oscillators are being powered by the same small battery (9V?), then they might crudely syncronize if the battery voltage jumps up and down due to the oscillators' currents through the internal resistance of the battery. Especially with 555 oscillators, which cause supply-current spikes of 400mA!

 

[Kevin Weddle]

One is being powered by a battery, the other is not. But when I connect the battery I am connecting it to the power supply. I think this is a natural phenomenon.

 

[audioguru2]

You are connecting a battery to a power supply?
Read the warning label on the battery.