SparkFun Forums

[Mach3Maelstrom]

I have a Schmidt trigger as follows:


(Note that power supplies *are* decoupled near the pins)

The issue is that I want OUT to be 5V upon power-up. I then adjust RV1 until I get 2V in the input. This tends to be around 6.67K (to save everyone the math).

Unfortunately, upon power-up, OUT often remains at -5V. I say often because *sometimes* it will switch to the positive value I want.

The "input" for the Schmidt trigger comes from a different op amp, whose output should be zero (virtual ground), represented by GND in the photo above.

Can anyone offer some insight as to what I'm missing?

[skimask]

Thinking out loud, not exactly sure of what's going on and I'm not an opamp expert, but it might be powering up and not allowing the schimdt trigger to do it's schmidt-ing. (is that a word?)
Try putting a high value resistor from the inverting input to ground rather than straight to ground.
Try putting a cap in parallel with the lower resistor giving it a change to "rise" up to 2v rather than snap directly up there.

[jremington]

With zero volts on the (-) input, the present circuit is stable in either configuration (+/- 5 V on output), so there is no reason for it to "choose" one state over the other upon power up. I think a small cap (say 0.01 uF) from +5 V to the (+) input will do what you want, but it will slow the switching transition.

As it is, this circuit will switch when the (-) input exceeds +2 V and again when the (-) input drops below -2 V. Is that what you intend?

[Mach3Maelstrom]

Thanks for the replies! For more information, I'm building this circuit:


I want the Schmidt trigger to begin by outputting +5V, creating a negative voltage ramp at the other op amp. When I power up the device, I put a 100R resistor in parallel with the capacitor so I can see what the Schmidt trigger outputs.

skimask & jremington - I tried all of your suggestions, but neither worked unfortunately. When I power it up, it *sometimes* starts at 2V and *sometimes* starts at -2V. As jremington states, it's stable in either configuration, but I want to isolate how to ensure it will start outputting +5V... perhaps by way of adjusting the two potentiometers.

If you have any other ideas how to make the Schmidt trigger start by outputting +5V, please let me know. I've been wracking my brains for a solution.

[Garth]

I think that since the two halves of the circuit depend on each other for the level, the only way you'll be able to assure that it starts up the way you want is to use a power-up reset circuit.