I have the following circuit which uses an LM2903 comparator. But Im not sure if it is designed well enough, and not sure about the open collector output is used correct:


enter image description here

I want to modify the above circuit for two types of inputs from two types of pulse outputting devices.

Input to the circuit will be two types of pulse trains coming from one of the two types of devices. The devices will be either powered by 12VDC and which outputs 12VDC pulses or will be powered by 24VDC which outputs 24VDC pulses. Pulse freq. range is 5Hz to 800Hz. I want the device and the comparator circuit to be powered from the same power supply.

And as I tried to clarify the pulse outputs of the devices will go to the circuit as an input.

I want to modify this circuit in a way that:

1-)The output will be 0 to 5V or 6V pulse, for both 24V and 12V cases.

2-)I want the circuit has a large hysteresis in Schmitt trigger configuration and immune to noise(i.e. output as clean as possible).

3-) I also want an LED to blink and indicate whether there is output by not effecting the pulse freq. or introduce noise to the pulse outputs.

I would be glad to have your version or suggestions for this.


To add large amount of hysteresis I had to use the inverting input.

And to use an LED indicator I followed analogsystemsrf's suggestion.

Below is the circuit so far:

enter image description here (click to view the image)

Now I have large hysteresis but LED doesn't go off.

But now I have the following problems and worries?

How can I employ an LED so that I don't introduce noise?

And what do you think about the capacitors in this circuit? Do you recommend additional ones?


With analogsystemsrf's suggestion I modified the circuit again. When I did that, I had to rearrange some resistors to increase hysteresis. I also used 8V zener. Here is the last version:

enter image description here(click to view image)

And here is the LED current(ringing due to LTSPICE?). enter image description here


Try this output network:


simulate this circuit – Schematic created using CircuitLab

| improve this answer | |
  • \$\begingroup\$ If the power supply voltage is 12V or 24V, then D1 would always be on with current go through D2. \$\endgroup\$ – rioraxe Feb 27 '17 at 23:00
  • \$\begingroup\$ Could you see my edit? I would be glad if you can help me. Im stuck at some points. \$\endgroup\$ – floppy380 Feb 28 '17 at 0:10
  • \$\begingroup\$ As 'rioraxe' showed, I had an error. The LED would have remained on. To cure that, the Zener is now on top of LED. For noise control, move your R3 to filtered_VDD at the 1uf on IC's VDD pin. This circuit still draws 10 or 20 ma AT ALL TIMES. And the LED does not blink. Shall we use the other 1/2 of LM2903 for that? And you can increase you input noise-filter cap at least 5x, to 0.047uF, and maybe to 0.1uF. \$\endgroup\$ – analogsystemsrf Feb 28 '17 at 5:31
  • \$\begingroup\$ @analogsystemsrf Do you mean LED still will not blink when there is input? And yes it is possible to use another LM2903 or a transistor if you have a better idea. Thanx \$\endgroup\$ – floppy380 Feb 28 '17 at 9:20
  • \$\begingroup\$ Please see the last version at EDIT 2. I made changes following your suggestions. Is R2 and R3 at the right place? I also reduced R1 to increase hysteresis again after you modify 22k to 560. And also many use 100nF for supply pins why did you suggest 1uF? What do you think about the last circuit? Sometimes I see they use caps across R1 R4 ect. I dont know it is necessary. And would be great if you have a better idea for power consumption as you mentioned. \$\endgroup\$ – floppy380 Feb 28 '17 at 10:00

Not the answer you're looking for? Browse other questions tagged or ask your own question.