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I'm sure it is a simple solution but I am new to this so please take it easy. I need to come up with something to convert a 5V digital output signal from a USB-1608G DAQ device with DIO to 24V digital output signal to reset the timer on a watchdog. The watchdog timer will only draw around 7.5mA, just requires around 24V pulse for at least 1mS to reset the WDI period. Any ideas on how to go about this or directions to go in?

I am thinking I will use an enhancment nMOSFET, with a Vth of about 2.5V. I am not sure what the best transistor would be to use and how to really configure the circuit so I can get that 24V output.

Thanks, Dave

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    \$\begingroup\$ Can you share the datasheet of your watchdog device (or the device that contains the watchdog)? \$\endgroup\$ – The Photon Jun 12 at 15:54
  • \$\begingroup\$ brentek.com/sites/default/files/uploads/attachments/products/… \$\endgroup\$ – Dave Jun 12 at 15:55
  • \$\begingroup\$ Looks like it can accept 5V directly, no need to boost it to 24V \$\endgroup\$ – Jeffrey Nichols Jun 12 at 15:57
  • \$\begingroup\$ Yeah that's the problem. I got it figuring that it can and I wouldn't have any issues. But when I test it, the timer will only reset a minimum of 17V. \$\endgroup\$ – Dave Jun 12 at 15:59
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    \$\begingroup\$ @Dave, please include all relevant information in the question post. (Click "edit" to edit your post) \$\endgroup\$ – The Photon Jun 12 at 16:04
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Your device has a minimum high input voltage of just 2.0 V, so you can trigger it directly with your 5 V signal:

enter image description here

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  • \$\begingroup\$ Yes which is the exact reason I got this watchdog. But when I tap the reset pin with a voltage source, the timer will not reset until about 17V. \$\endgroup\$ – Dave Jun 12 at 16:09
  • \$\begingroup\$ Did you connect it to ground before you "tapped" it with the voltage supply? Because of the edge-triggered feature, you might need a clean 0-5 V transition to be sure to reset the WDT correctly. \$\endgroup\$ – The Photon Jun 12 at 16:18
  • \$\begingroup\$ Yes it is grounded and getting a clean 0-5V transition. \$\endgroup\$ – Dave Jun 12 at 17:09
  • \$\begingroup\$ @Dave How does it behave if you run it off a lower voltage like 5-9V instead of 24V? Normally it's pretty clear if the Vih moves with the supply voltage but this datasheet is really brief. You could interpret the datsheet as it can accept between 2-30V as a HI. But it could also be interpreted that the logic HI threshold can vary between 2 and 30V which would mean that it changes with supply voltage. \$\endgroup\$ – DKNguyen Jun 12 at 18:15
  • \$\begingroup\$ I see your edit but yes, I did try that but like I said, even at the lowest supply voltage, the timer still cannot be reset with anything lower then 17V. \$\endgroup\$ – Dave Jun 12 at 18:28

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