Help with this circuit...

[Drewlio77]

Hi,

I am wondering if anyone can help me. I need a simple circuit, with as few parts as possible. This is what i need it to do.

When the input goes high (over 2.5 Volts) a 1 second positive trigger is output. Also when the input goes low (below 2.5 volts), a 1 second postive pulse is output. So basically anytime the voltage crosses 2.5 volts (rising or falling) a one second postive output (trigger) is output. I would like the 2.5 volts as the threshhold to prevent false triggering since the input volatge will have noise in it due tom the operating enviroment. The voltage input will be 5 Volts when high, andd less than 200 mv when low.

I need this circuit to trigger a relay for 1 one second everytime 5 Volts is applied to the input and whenever it is removed.

I have looked at Op amps and 555 timer circuits. I haven't found anything that does this. Can these be used to handle the circuits I am looking for?

Thanks in advance.

Drewlio77

 

[Dr_Doggy]

You need an edge detection circuit, replace the clock in my diagram with your inputs, and put the output to a 555-oneshot circuit at the trigger pin. You can then adjust the oneshot to stay on for 1s, and add your relay or whatever.

Idk about thresholds, depends on the IC i think, but i think you want to use an op-amp if you need to set that part up.

that part at the end is an EX-OR gate(under my scope)

 

[crutschow]

The circuit by doggy__ can be done with one CD4030 or CD4070 Quad Exclusive-OR ICs. The inverting gate function can be performed by connecting one input of the Exclusive-OR gate to Vdd. The remaining input will generate the invert function.

 

[MikeMl]

Here is a more detailed version of doggy's suggestion. First, it uses only one quad XOR. The first stage is a Schmitt Trigger to clean up the pulse. The second stage is a bidirectional edge detector, and the third stage stretches the pulse to 1 sec.