Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

pulse delay problem between 2 schmidt trigger segments

Status
Not open for further replies.

Dialtone

New Member
Can anyone steer me in the right direction on why I am having problems delaying a pulse using r/c configuration between two 74ls14 schmidt trigger elements? Typical pulse width is ~1 us and I need a ~250ns delay. I have tried using 100pf cap and 2.2K resistor but can see only small offset from input signal. Stepping resistor to 4.7k produces no output at all so I assume I am dropping voltage too much to trigger second stage. Also tried varying the cap but see no noticeable improvement either. Should I just drop the schmidts and replace IC's with standard hex inverters to get the desired results? Any help would be appreciated.
Dialtone
 

laroche73

New Member
schmitt trigger delay

You may have better luck using an CMOS or HCMOS version of the '14. The input impedance of a 74LS14 is fairly low (< 10K). A 74HC14 (or 74C14/CD4584) has a much higher input impedance, and the nominal switching threshold is also closer to one RC time constant (3.25V/5V = 0.65; 1 TC = 0.632).

Note that the 74HCT14 has TTL level input thresholds, so the low-high switching point would be closer to 1/2 time constant (1.8V/5V = 0.36; 0.5 TC = 0.393). I'd use the HC version for the extra range.
 

Dialtone

New Member
Thanks for the suggestion. After studying data sheet for the 74ls14 closer, I see the it has some fairly tight switching thresholds and my delay calculations were based on a 1 time constant value which will not work for the desired time needed.
Dialtone
 
Status
Not open for further replies.

Latest threads

Top